Indoor Air Cartoon Journal

Indoor Air Cartoon Journal, August 2026, Volume 9, #181

[Cite as: Fadeyi MO (2026). A solution to the dilemma: achieving privacy without obstructing ventilation for healthy indoor air in high-density cities. Indoor Air Cartoon Journal, August 2026, Volume 9, #181.]

Fictional Case Story (Audio – available online)– Part 1 (Preface, Ch 1 & Ch 2)

Fictional Case Story (Audio – available online) – Part 2 (Ch 3)

Fictional Case Story (Audio – available online) – Part 3 (Ch 3 cont’d)

Fictional Case Story (Audio – available online) – Part 4 (Ch 3 cont’d)

Fictional Case Story (Audio – available online) – Part 5 (Ch 4)

Fictional Case Story (Audio – available online) – Part 6 (Ch 4 cont’d)

Fictional Case Story (Audio – available online) – Part 7 (Ch 5 & Ch 6)

………………… Preface ……………………

Across high-density cities, residential buildings increasingly brought windows and everyday domestic activities into close visual relationships. Existing residential design and practice often left occupants with an unresolved dilemma: opening windows for natural ventilation could compromise privacy, while protecting privacy with curtains, blinds or closed windows could compromise ventilation and healthy indoor air. Rather than eliminating the conditions creating this conflict, existing practice effectively forced occupants to manage it themselves by prioritising one legitimate requirement and compromising another. Consequently, compromise became an occupant response to an inadequately resolved problem, while comparatively less attention was given to understanding and addressing the root causes or barriers that made privacy and ventilation appear incompatible in the first place.

It was a young man living in a high-density city who decided to explore what could be done to address this dilemma that he had repeatedly experienced in everyday life. In seeking an answer, however, he embarked on more than a journey to solve a practical problem. The process confronted him with a flaw in how he himself had approached dilemmas throughout his life and gradually transformed his capability to think, exercise judgement and make decisions when legitimate requirements appeared to conflict. He would eventually use this transformation to give value not only to himself, but also to his family, learners, professional practice, industry and society. His journey forms the subject of this fiction story.

………………… Chapter 1 ……………………

Corlin Brown grew up in a home where intellectual achievement was part of everyday life. His father, Professor Daniel Brown, was a Professor of Theoretical Physics, while his mother, Professor Eleanor Brown, was a Professor of Architecture. Books filled their home, scientific and architectural conversations were common, and questions were encouraged. From an early age, Corlin became fascinated by explanations. He wanted to know why objects fell, why buildings stood, why light behaved differently when it encountered different materials, and why some spaces felt more comfortable than others. His parents recognised his curiosity and encouraged him to learn as much as possible. Corlin also discovered very early that knowing the correct answer brought rewards. Beginning in primary school, questions normally came with expected answers. If he remembered the relevant knowledge, selected the correct operation or procedure, he performed well. His examination results reinforced this pattern year after year. A problem was presented, Corlin worked towards an answer, and the correctness of that answer determined whether he had succeeded. Because he was particularly good at doing this, academic success gradually strengthened his confidence not only in what he knew but also in how he thought.

There was nothing inherently wrong with learning to produce correct answers. The difficulty was that Corlin gradually internalised an incomplete mental model of problem solving. Problems appeared to be things that already existed in a clearly defined form, while his responsibility was to find the correct solution. He therefore became much more practised at solving a presented problem than at questioning whether the problem had been appropriately diagnosed in the first place. He had little reason at this stage to question how a problem had been framed, what might be causing it, or whether the apparent choices available represented everything that was actually possible. His family environment unintentionally reinforced part of this tendency. Corlin admired his father’s extraordinary ability to explain difficult physical phenomena and his mother’s ability to develop architectural responses to complex requirements. What Corlin noticed most, however, were the impressive conclusions they reached. He did not yet appreciate the years of questioning, uncertainty, judgement, rejected possibilities and reconsideration that preceded those conclusions. To the young Corlin, expertise looked like the ability to know what to do. He wanted to become the kind of person who could look at a problem and quickly provide an intelligent answer.

As Corlin grew older, another influence quietly shaped how he approached dilemmas. His parents often viewed the same situation through different disciplinary lenses. His father naturally looked for the physical principles governing what was possible, while his mother considered how different possibilities would affect people, space and everyday use. Corlin admired both perspectives, but when they pointed towards different priorities, he tended to ask which one was right. He had not yet learnt that both could be valid simultaneously and that the real intellectual challenge might be to develop a solution that respected both. Gradually, he became accustomed to resolving competing requirements by choosing, prioritising or compromising between them, rather than first questioning whether the apparent conflict itself could be removed. In real life, several legitimate requirements could coexist, different people could value different outcomes, and improving one condition could unintentionally compromise another. Corlin’s tendency was to interpret such situations as requiring a choice between competing requirements: determine which mattered more, prioritise it, and accept some compromise elsewhere. What he had not yet learnt was to question why the requirements were conflicting in the first place. Doing so required an appropriate mental model, meaningful questions, examination of the root causes or barriers creating the conflict, and judgement about whether those barriers could be overcome. Without this cognitive governance, Corlin could select a technically workable solution while overlooking the possibility of achieving the legitimate requirements together and delivering greater overall value.

His parents occasionally tried to challenge this tendency. His mother would ask what successful performance should actually look like before considering possible solutions, while his father would ask what underlying mechanism was preventing that performance from being achieved. Corlin understood their questions intellectually, but he did not yet appreciate their significance. When legitimate requirements appeared to conflict, choosing which requirement deserved greater priority seemed perfectly reasonable to him. If neither could be completely sacrificed, finding an acceptable compromise appeared even more sensible. Because this approach often produced workable outcomes, Corlin had little reason to question it. What he did not realise was that a workable compromise could conceal an unresolved problem if the barrier causing the dilemma had never been identified and challenged.

The education Corlin received outside his home strengthened this way of approaching dilemmas, although not because his education was poor. On the contrary, much of what he learnt was useful and necessary. During primary school, he was taught that some requirements legitimately deserved priority over others. Safety could take priority over convenience, examination deadlines over unfinished improvements, mandatory requirements over desirable ones, and essential functions over optional features. Corlin learnt that when everything could not be achieved simultaneously, priorities provided a practical basis for deciding what should be protected and what could be compromised. Primary-school activities repeatedly gave him opportunities to practise this approach. When a project had to be completed within limited time and resources, students were encouraged to identify the most important requirements and concentrate their efforts there. If an ideal outcome could not be achieved, they learnt to distinguish what was essential from what was desirable. Corlin became particularly comfortable with this reasoning. Faced with several competing requirements, he could rank them, allocate available resources accordingly and explain why a lower-priority requirement had been compromised. This was sensible decision-making within constraints, and it often enabled him to produce workable outcomes.

As Corlin progressed into secondary school, his science education reinforced another legitimate form of prioritisation. Experiments could involve many variables, but meaningful investigation often required students to focus on particular variables while controlling or temporarily setting aside others. Corlin learnt that intellectual progress sometimes depended on deliberately narrowing attention. Trying to investigate everything simultaneously could make a problem unmanageable. He therefore became accustomed to identifying what appeared most important, concentrating on it and treating other considerations as secondary. Again, there was nothing inherently wrong with this practice; appropriate prioritisation was essential for making complex situations manageable. His experience of assessment through primary and secondary school strengthened the pattern further. Examination questions frequently contained several pieces of information, but not all were equally important to producing the required answer. Students who could identify the relevant information and disregard distractions performed efficiently. Project work similarly required choices when time, available materials or other resources imposed constraints. Corlin learnt that good judgement frequently meant recognising what mattered most and directing limited resources towards it. The repeated lesson was reasonable: when everything cannot receive equal attention, prioritisation is necessary.

At university, where Corlin studied Physics, this way of thinking became more sophisticated. He encountered formal optimisation, mathematical modelling and scientific problems in which a desirable outcome could be maximised or minimised subject to defined constraints. Physics repeatedly demonstrated that physical systems operate within boundaries and that some constraints are genuinely fundamental. Energy may be finite, boundary conditions may restrict possible states, and physical laws cannot simply be negotiated away because another outcome is desirable. Greater performance in one area could therefore require additional energy, time, experimental resources or acceptance of reduced performance elsewhere. Corlin acquired increasingly sophisticated ways of deciding how much of one outcome should reasonably be exchanged for another. What had begun as simple prioritisation gradually became an intellectually defensible approach to dealing with competing requirements. His university Physics education also strengthened his ability to simplify complex situations. A real physical system could contain many interacting variables, yet useful analysis often required constructing a model that retained the variables relevant to the problem while temporarily excluding others. Laboratory investigations similarly required clearly defined objectives, controlled variables and attention to the measurements most relevant to the question being investigated. Corlin learnt that disciplined exclusion could be as important as inclusion. Trying to accommodate every possible consideration simultaneously could make scientific analysis difficult or impossible. The lesson was again legitimate, but it further strengthened his confidence in identifying what mattered most and assigning lower priority to what appeared less relevant.

The problem was therefore not that Corlin had learnt to prioritise. Prioritisation is not inherently a bad response to a dilemma. In many real-life situations, it is necessary and responsible. Resources may genuinely be insufficient to satisfy every requirement fully. Physical constraints may make two outcomes genuinely incompatible. An emergency may require safety to take precedence over comfort. A deadline may make it impossible to pursue every desirable improvement. A scientific investigation may also require one research question to receive attention while other legitimate questions are deliberately deferred. Under such circumstances, refusing to prioritise could itself produce poor judgement and reduce value. Corlin’s education had therefore given him a useful capability. The weakness arose from when he used that capability. Corlin increasingly treated prioritisation as the natural starting point whenever legitimate requirements appeared to conflict. If A and B could not apparently be achieved together, his immediate intellectual task became determining whether A or B mattered more, or identifying an acceptable compromise between them. He was skilled at answering that question. What he was much less accustomed to asking was whether the apparent incompatibility between A and B had first been adequately investigated.

This distinction was subtle but consequential. Sometimes two requirements appeared to compete only because of the particular solution currently being used to satisfy them. Sometimes an underlying barrier caused the conflict. Sometimes assumptions inherited from existing practice unnecessarily constrained the available possibilities. In such circumstances, immediately prioritising A over B could produce a rational decision within the existing problem configuration while preventing Corlin from questioning whether that configuration itself could be redesigned. The resulting compromise might be entirely reasonable, yet still deliver less value than a solution that removed or reduced the barrier responsible for the dilemma. Corlin’s education had taught him extensively how to make decisions within constraints, but he had paid much less attention to questioning whether every apparent constraint should be accepted as fixed. This distinction became particularly important when he transferred ways of thinking learnt in Physics into less clearly defined real-life dilemmas.

In Physics, some constraints were genuinely fundamental because they arose from physical laws or deliberately defined experimental boundary conditions. In everyday life, however, an apparent constraint could instead arise from the way an existing system had been designed, the technology currently available, an established practice, a conventional solution or an assumption that nobody had questioned. When resources were genuinely limited, prioritisation made sense. When a physical law established an unavoidable boundary, that boundary had to be respected. However, when the apparent constraint resulted from an existing design, established practice, conventional solution or unexamined assumption, treating it as immutable could prematurely narrow the solution space. Corlin had not consistently learnt to distinguish between constraints that had to be accepted and barriers that could potentially be overcome. This mattered particularly in a dilemma. A dilemma was not merely a situation in which Corlin wanted several desirable things at once. It was a situation in which legitimate requirements appeared to conflict such that improving or protecting one could reduce another. Prioritisation offered a practical response: decide which requirement mattered more and accept the consequence for the other. However, that response did not necessarily establish whether the conflict itself was unavoidable. Corlin could therefore become very good at deciding what should lose without first establishing why anything needed to lose at all.

His parents had, in different ways, exposed Corlin to another way of approaching such situations, although he had not appreciated its significance. His mother’s questions about successful performance encouraged consideration of what should ideally be achieved before deciding what could be compromised. His father’s questions about underlying mechanisms encouraged investigation of why the desired performance was not being achieved. Corlin understood these questions when they were asked, but prioritisation remained more familiar and immediately useful to him. He therefore continued to approach apparent conflicts primarily by deciding what should take priority. His mother’s architectural perspective occasionally exposed him to situations in which several requirements remained legitimate at the same time. A building could require structural safety, thermal comfort, daylight, privacy, usability and aesthetic quality without any one of these automatically making the others irrelevant. Architecture sometimes required compromise, but it also required design exploration precisely because apparently competing requirements might be reconciled through a different configuration. Corlin saw the final architectural solutions his mother developed, but he did not yet fully appreciate this part of the reasoning process. He remained more attracted to the apparent decisiveness of selecting priorities than to questioning whether the relationship between the competing requirements itself could be changed.

His father’s theoretical-physics perspective provided a different but related lesson that Corlin also did not fully internalise. Professor Daniel Brown was interested not merely in what happened but in the mechanism that made it happen. If an observed outcome changed, he wanted to know which underlying relationships were responsible. Corlin understood the scientific importance of this within Physics, particularly during his university studies, but he did not consistently transfer the same discipline to everyday dilemmas. Outside a formal scientific problem, an apparently workable option could tempt him to decide what should be done before he had sufficiently investigated what was creating the conflict. By this stage, Corlin had little reason to question the way he approached dilemmas. Prioritisation had repeatedly helped him complete assignments, manage limited resources, conduct investigations and make defensible decisions. The capability itself was valuable. When compromise appeared necessary, he regarded determining what should be protected and what could be sacrificed as evidence of sound judgement. What he did not recognise was that he was allowing a valuable downstream decision-making tool to become an upstream substitute for sufficiently diagnosing the problem. He could therefore make a perfectly rational choice between the options he saw without discovering whether a better option might become possible if the barrier creating the dilemma could first be identified and challenged.

The pattern was reinforced progressively across his education rather than created by any single educational stage. Primary school familiarised him with correctly answering largely predefined problems and making straightforward priorities within limited time and resources. Secondary school strengthened his ability to identify relevant information, isolate variables and concentrate attention on what mattered for a defined task. University Physics gave these practices greater scientific and mathematical sophistication through modelling, experimental control, optimisation and reasoning within physical constraints. Each stage developed valuable capabilities. Together, however, they also made prioritisation an increasingly intuitive response for Corlin whenever several legitimate requirements appeared unable to coexist. By the time Corlin completed his university education in Physics, this pattern had become deeply embedded in the way he approached difficult choices. When two legitimate requirements appeared incompatible, his instinct was to ask, “Which should take priority?” Because this approach had repeatedly produced workable and defensible outcomes, he saw little reason to question it. What he did not yet realise was that some dilemmas might contain possibilities that prioritisation alone could never reveal. The flaw was therefore not prioritisation itself, but his tendency to prioritise before sufficiently diagnosing whether the apparent conflict was caused by an unavoidable constraint or by a barrier that might be changed. Discovering that limitation would require Corlin to confront the consequences of the way he had learnt to think.

………………… Chapter 2 ……………………

As a fresh BSc (First-Class Honours) graduate in Physics considering a PhD, Corlin had no clear idea what his research should address. He explored several possibilities, but none gave him the sense that he had found a problem to which he wanted to devote several years of his life. Rather than forcing himself to choose a topic, he began paying closer attention to problems he encountered in everyday life and wondering whether any of them contained a question worth investigating. One particular experience kept returning to his mind. It was not new to him, nor was it something he had encountered only once. Corlin had repeatedly experienced it himself and had seen many other people experience it in their homes. Opening windows for ventilation could compromise privacy, while protecting privacy with curtains or blinds could obstruct ventilation and compromise healthy indoor air. Maintaining privacy could also interfere with the clear outward view that occupants expected from their windows. What made this particular dilemma increasingly difficult for Corlin to dismiss was how personally familiar he was with both sides of it. He knew what it felt like to want an open window for fresh outdoor air while simultaneously feeling uncomfortable about being visible from outside. He also knew what it felt like to draw a curtain or position a blind for privacy and then notice that the air movement and outward view he wanted had been compromised. Neither experience represented an abstract requirement imposed by somebody else. He wanted privacy, ventilation and a clear outward view at the same time.

This personal experience produced an unusual reaction in Corlin. Instead of naturally deciding which requirement mattered more, he found himself unwilling to surrender any of them. It was not because he had suddenly developed a new way of thinking or recognised the flaw that had shaped his previous approach to dilemmas. Rather, each available choice required him to give up something he personally considered legitimate and valuable. Prioritising privacy meant accepting poorer ventilation or outward view; prioritising ventilation could mean accepting unwanted visual exposure; and compromising between them merely distributed the loss rather than removing it. For perhaps the first time, Corlin began to sense a difference between making a defensible decision within a dilemma and actually solving the problem that created the dilemma. Prioritisation could tell him which loss to accept, but it could not tell him whether that loss was actually necessary. Privacy was not inherently more legitimate than ventilation, ventilation was not inherently more legitimate than privacy, and a clear outward view was not merely decorative. Each contributed something useful to the occupant. The familiar question, “Which should take priority?”, therefore no longer gave Corlin the intellectual closure it normally provided.

That dissatisfaction caused him to remember questions his parents had asked throughout his childhood and education. His mother’s concern with successful performance began to seem relevant. Instead of immediately deciding among available options, she often wanted to know what the completed situation should actually achieve for the people using it. His father’s interest in underlying mechanisms also returned to Corlin’s mind. Professor Daniel Brown rarely seemed satisfied merely knowing that something happened; he wanted to understand what caused it to happen. Corlin had heard these ways of thinking for years, but this particular dilemma gave them a personal significance they had previously lacked. Corlin therefore did something that was unusual for him. He temporarily refused to prioritise. This did not mean that he had suddenly concluded that prioritisation was wrong or that every dilemma could be resolved without compromise. He simply did not yet know whether compromise was genuinely necessary. If the conflict resulted from an unavoidable constraint, prioritisation might eventually be required. But if something removable was causing the legitimate requirements to interfere with one another, accepting a compromise before identifying that barrier could mean unnecessarily accepting reduced value.

His attention consequently shifted from the competing requirements to the relationship between them. Privacy itself did not obstruct ventilation. Wanting privacy did not physically prevent air from moving through a window. Something about the way privacy was conventionally achieved appeared to create the interference. Curtains and blinds protected occupants from unwanted visual exposure by placing physical material within or across the same opening through which air needed to move and through which occupants wanted to see outside. The same physical intervention that provided privacy could therefore alter the conditions required to provide ventilation and outward view. This observation gave Corlin a different way of looking at the dilemma. Perhaps privacy and ventilation were not fundamentally incompatible after all. Perhaps the conflict arose because the conventional solution for providing one function physically interfered with the pathway required for another. If so, deciding whether privacy or ventilation should take priority would address the consequences of that arrangement without necessarily addressing the root cause of the conflict itself.

His Physics background now became particularly useful. Corlin had been trained to look beneath observable outcomes and consider the mechanisms responsible for them. Instead of asking only what happened when a curtain or blind was positioned across an open window, he could ask why it happened. Air and visual information both passed through the architectural opening, but they were not the same physical phenomenon. Airflow involved the movement of air, while vision depended on light carrying visual information between the interior and an observer. Yet conventional privacy devices attempted to control visual exposure by introducing a physical obstruction into an opening also required for airflow. Corlin began wondering whether the apparent dilemma arose from this coupling rather than from any fundamental incompatibility between privacy and ventilation. This was an important change, but it was not yet Corlin’s transformation. He had not suddenly become someone who would never prioritise competing requirements. Nor had he established that the dilemma could actually be eliminated. His personal experience had simply given him a reason not to prioritise too early. Because he genuinely valued what existed on both sides of this particular dilemma, he was willing to investigate the conflict before deciding that one requirement had to be sacrificed.

Corlin did not know where that investigation would lead. He did not know whether an alternative solution was scientifically possible, what technology such a solution might require, or whether careful investigation would ultimately demonstrate that some compromise remained unavoidable. What he now had was something more fundamental: a real-life problem in high-density urban residential environments that he had repeatedly experienced himself and seen others experience, and a compelling reason to investigate why privacy, ventilation and clear outward view appeared to conflict. He therefore began defining what successful performance would mean before considering what existing solutions could provide. An occupant should be able to look through an open window and see outside clearly. The same occupant should be protected from unwanted visual exposure to people outside. At the same time, the opening should remain available for the natural movement of air rather than being physically obstructed by the mechanism providing privacy.

None of these requirements would be treated as compensation for failure in another. Corlin expressed this emerging expectation as a non-negotiable requirement for success: Clear outward view × Visual privacy × Full ventilation. The multiplication sign mattered. If clear outward view was achieved but privacy failed, the solution failed. If privacy was achieved but ventilation was substantially obstructed, the solution failed. If privacy and ventilation were achieved but the occupant could no longer see outside clearly, the solution also failed. Success required the three legitimate functions to coexist within the same operating condition. For Corlin, this marked the beginning of something much larger than choosing a PhD topic. A familiar everyday dilemma had made his usual practice of prioritisation personally unsatisfactory, prompting him to look beneath the competing requirements and ask what was actually causing them to conflict. Before he could develop a solution, however, he needed to establish precisely what the current performance situation was, what the targeted performance should be, what gap existed between them, and what root causes or barriers were responsible for that gap. Only then could he determine what problem his research actually needed to solve. He therefore wrote the following practical research problem statement for his PhD study.

“High-density urban development enables growing populations to live within limited land areas, but it can also bring residential buildings, windows and everyday domestic activities into increasingly close visual relationships. In naturally ventilated homes, occupants may therefore face two legitimate requirements that must be satisfied through the same architectural opening: visual privacy and natural ventilation. The practical problem arises when the current performance of the residential window environment does not simultaneously provide these functions, together with the clear outward view expected from a window. The targeted performance situation is one in which occupants can keep their windows fully open when natural ventilation is needed while maintaining visual privacy from neighbouring buildings and retaining a clear outward view. Natural ventilation should remain sufficiently unobstructed to support effective airflow through occupied areas, thermal comfort, and the dilution and removal of pollutants generated indoors, thereby contributing to healthy indoor air and conditions supporting healthy living.

The targeted performance can therefore be expressed as the non-compensatory requirement Clear outward view × Visual privacy × Full ventilation. Failure to provide any one of these functions means that the targeted performance has not been achieved. The current performance situation, however, may require occupants to negotiate these functions. A fully open and unobstructed window can provide natural ventilation and a clear outward view but can also expose privacy-sensitive indoor spaces, occupants and activities to neighbouring observers. Occupants can respond by closing or positioning curtains or horizontal louvred blinds across the opening. Although this can improve visual privacy, the same physical elements may occupy, restrict or redirect the pathway through which air must travel. Closing the window can provide another privacy response but removes the opening required for natural ventilation altogether. Consequently, occupants may have to choose or compromise between legitimate requirements rather than receiving them simultaneously.

The problem is therefore the gap between this current performance situation and the targeted performance situation. The current system potentially provides privacy, clear outward view, and full natural ventilation conditionally or competitively, whereas the targeted system must provide them simultaneously. This gap may become increasingly important in high-density cities, where increasing building separation to protect privacy may be spatially difficult and where relying on occupants to manage the conflict after occupation does not address its underlying architectural cause. Developing an effective solution first requires understanding why this performance gap exists. The apparent conflict cannot be assumed to arise simply because occupants close windows or curtains. Its root causes and barriers need to be established empirically. These may include neighbouring-building configuration and viewing geometry, perceived visual intrusion, occupants’ changing privacy requirements, window characteristics, and how curtains or horizontal louvred blinds are positioned in response. It is also necessary to determine whether these behavioural responses actually modify natural ventilation and, if so, what the resulting consequences are for airflow distribution, thermal comfort, pollutant dilution and removal, healthy indoor air and conditions supporting healthy living. Without identifying and quantifying these mechanisms, a proposed solution could address a symptom while leaving the underlying performance barrier intact.

The resulting Design Science Research problem is therefore not simply to design a better privacy screen or increase ventilation. It is to develop a solution capable of moving the residential window system from a condition in which occupants may need to negotiate competing functions to one in which the required functions can coexist. If the underlying barrier is found to be that conventional privacy management controls visual information by physically modifying the same opening required for airflow, an alternative solution would need to investigate whether these functions can be functionally separated. Accordingly, the research must first characterise the problem and its root causes, then quantify its environmental and human consequences, translate this evidence into explicit non-compensatory design requirements, and subsequently develop, iteratively refine and evaluate a solution against those requirements. The ultimate practical objective is not to help occupants make a better compromise, but to determine whether architectural design can remove the need for the privacy–ventilation compromise itself.”

This need forms the basis for the research questions and hypotheses that guided his PhD study.

(i) How do perceived visual intrusion, privacy requirements, neighbouring-building configuration, window characteristics, and curtain or blind use interact to influence occupants’ window and curtain or blind configurations, natural ventilation performance, and indoor air quality in high-density residential buildings?

(ii) To what extent does the privacy-driven positioning of curtains or blinds alter airflow distribution, air-change effectiveness, pollutant removal, thermal comfort, perceived privacy, occupants’ ventilation-related behaviour, healthy indoor air, and healthy living under different window-opening, weather, dwelling, and urban-density conditions?

(iii) What architectural, façade, window, interior-screening, and occupant-control strategies can resolve or reduce the conflict between visual privacy and unobstructed natural ventilation, and to what extent can these strategies improve healthy indoor air and support healthy living without compromising privacy, thermal comfort, usability, energy performance, and occupants’ acceptance in high-density residential buildings?

For the first research question, the Null Hypothesis (H₀₁) is that there are no statistically significant relationships or interactions among perceived visual intrusion, privacy requirements, neighbouring-building configuration, window characteristics, curtain or blind use, occupants’ window and curtain or blind configurations, natural ventilation performance, and indoor air quality in high-density residential buildings. The Alternative Hypothesis (H₁₁) is that perceived visual intrusion, privacy requirements, neighbouring-building configuration, window characteristics, and curtain or blind use interact significantly to influence occupants’ window and curtain or blind configurations, natural ventilation performance, and indoor air quality in high-density residential buildings.

For the second research question, the Null Hypothesis (H₀₂) is that privacy-driven positioning of curtains or blinds does not produce statistically significant differences in airflow distribution, air-change effectiveness, pollutant removal, thermal comfort, perceived privacy, occupants’ ventilation-related behaviour, healthy indoor air, or healthy living under different window-opening, weather, dwelling, and urban-density conditions. The Alternative Hypothesis (H₁₂) is that privacy-driven positioning of curtains or blinds produces statistically significant differences in airflow distribution, air-change effectiveness, pollutant removal, thermal comfort, perceived privacy, occupants’ ventilation-related behaviour, healthy indoor air, and healthy living under different window-opening, weather, dwelling, and urban-density conditions.

For the third research question, the Null Hypothesis (H₀₃) is that architectural, façade, window, interior-screening, and occupant-control strategies do not produce statistically significant improvements in healthy indoor air or support healthy living. They also do not significantly reduce the conflict between visual privacy and unobstructed natural ventilation without compromising privacy, thermal comfort, usability, energy performance, or occupants’ acceptance in high-density residential buildings. The Alternative Hypothesis (H₁₃) is that architectural, façade, window, interior-screening, and occupant-control strategies produce statistically significant improvements in healthy indoor air and support healthy living. They also significantly reduce the conflict between visual privacy and unobstructed natural ventilation without compromising privacy, thermal comfort, usability, energy performance, or occupants’ acceptance in high-density residential buildings.

The research problem and questions informed the following objectives of his PhD study:

(i) To characterise the existence, nature, and underlying mechanisms of the privacy–ventilation dilemma in high-density residential buildings. This objective examines how perceived visual intrusion, privacy requirements, neighbouring-building configuration, window characteristics, and curtain or horizontal louvred-blind use influence occupants’ window and privacy-device configurations, natural ventilation performance, and indoor air quality..

(ii) To quantify the environmental, behavioural, and human consequences of the privacy–ventilation dilemma. This objective determines how privacy-driven positioning of curtains or horizontal louvred blinds affects airflow distribution, air-change effectiveness, pollutant dilution and removal, thermal comfort, perceived privacy, ventilation-related behaviour, healthy indoor air, and conditions supporting healthy living under different window-opening, weather, dwelling, and urban-density conditions.

(iii) To develop and evaluate a solution capable of reducing or resolving the privacy–ventilation dilemma. This objective investigates architectural, façade, window, interior-screening, and occupant-control strategies for simultaneously providing visual privacy, clear outward view, and unobstructed natural ventilation. It evaluates whether the developed solution can support healthy indoor air and healthy living without compromising privacy, thermal comfort, usability, energy performance, and occupants’ acceptance in high-density residential buildings.

………………… Chapter 3 ……………………

Research Methods

Methods for Research Question1:

Overview

The methodology for Research Question 1 sought to establish the existence, nature, and underlying mechanisms of the privacy–ventilation dilemma in high-density residential buildings. Specifically, it investigated how perceived visual intrusion, privacy requirements, neighbouring-building configuration, window characteristics, and curtain or horizontal louvred-blind use interacted to influence occupants’ window and curtain or horizontal louvred-blind configurations, natural ventilation performance, and indoor air quality. Vertical louvred blinds and roller blinds were not investigated. The question did not seek merely to identify isolated relationships among these variables. It aimed to understand how occupants’ need for privacy affected how they positioned their windows and curtains or blinds, and how these choices affected natural ventilation and healthy indoor air. The presence, intensity, and frequency of relevant indoor pollutant sources were also considered because they influenced both the need for ventilation and the resulting indoor air quality. This was particularly important when pollutants were actively generated indoors and occupants had to balance the need for ventilation with their need for visual privacy.

Addressing this research question required more than measuring airflow or recording occupants’ preferences. The privacy–ventilation dilemma was inherently multidisciplinary. It emerged from the interaction of architectural design, urban morphology, environmental engineering, building physics, human perception, environmental psychology, and occupant behaviour. Consequently, no single research method could adequately characterise the phenomenon. A methodology relying exclusively on surveys would reveal occupants’ perceptions but could not objectively determine ventilation performance. Similarly, environmental measurements alone could quantify indoor air quality but could not explain why occupants adopted particular window and curtain or blind configurations. Computational simulations could predict airflow behaviour but could not determine whether occupants would actually configure windows and curtains or blinds in the manner assumed by the models. To overcome these limitations, this study adopted a mixed-methods explanatory sequential research design. It combined quantitative and qualitative methods to examine the problem from different but complementary perspectives. Each method helped explain a different part of how the need for visual privacy could ultimately affect healthy indoor air. Using different sources of evidence also allowed the findings to be compared and cross-checked.

This strengthened confidence in the findings and reduced the limitations and potential biases associated with relying on a single method. The methodology examined the causal sequence from neighbouring-building configuration to perceived visual intrusion, privacy requirements, window and curtain or blind configuration, natural ventilation performance, and indoor air quality. Indoor pollutant sources were considered alongside this causal sequence because their presence, intensity, and frequency could influence the need for ventilation and the indoor air quality outcomes associated with different window and curtain or blind configurations. This causal framework provided the basis for all subsequent data collection, analysis, and interpretation throughout the study.

Research Design

Before full-scale data collection, a pilot study was conducted in a small sample of occupied apartments representative of the main study to evaluate the feasibility of the research procedures and estimate the variability of the principal quantitative measures. The pilot tested the questionnaires, interviews, behavioural observations, window and curtain or blind position monitoring, indoor pollutant-source recording, and environmental measurements planned for the main study. It also established the feasibility of synchronising these data over time. The findings informed the final sample size, monitoring duration, sampling intervals, and data-collection procedures. Environmental instruments recorded measurements at predefined sampling intervals appropriate to the response characteristics of the variables being measured. Monitoring periods were sufficiently long to capture repeated indoor pollutant-generating activities, changes in window and curtain or blind configurations, and corresponding changes in ventilation performance and indoor air quality. The timing of pollutant-generating activities and changes in window and curtain or blind configurations was synchronised with environmental measurements to enable temporal relationships among pollutant generation, occupant behaviour, ventilation, and indoor air quality to be examined quantitatively.

A convergent mixed-methods research design, in which quantitative and qualitative data were collected and analysed largely in parallel, was considered but not adopted because the behavioural findings needed to inform the subsequent environmental investigation. Instead, an explanatory sequential mixed-methods research design was selected because understanding why occupants configured their windows and curtains or blinds in response to privacy concerns was needed to inform the subsequent investigation of the physical consequences of those behaviours. The first phase of the main study used questionnaires, interviews, and behavioural observations to establish how and why occupants configured their windows and curtains or blinds in response to visual privacy needs, including when indoor pollutant-generating activities created a need for ventilation. The findings from the first phase then informed the second phase of the main study, which used physical and environmental measurements to determine the effects of the identified window and curtain or blind configurations on natural ventilation performance and indoor air quality. This sequence enabled the physical consequences of the behaviours identified in the first phase to be quantified and explained. The first phase focused more specifically on identifying and characterising occupants’ perceptions of visual intrusion, privacy needs, and ventilation-related behaviour through structured questionnaires, behavioural observations, and semi-structured interviews. These findings established the behavioural patterns associated with privacy management in naturally ventilated residential buildings. This phase also identified relevant indoor pollutant-generating activities and determined their presence, intensity, and frequency. Particular attention was given to how occupants configured their windows and curtains or blinds when such activities occurred and whether the need for visual privacy influenced these configurations despite the need for ventilation.

The second phase objectively measured the physical characteristics of the buildings, surrounding urban environments, window and curtain or blind configurations, ventilation performance, and indoor environmental conditions associated with the behavioural patterns identified in the first phase. Indoor pollutant-generating activities identified during the first phase were documented alongside the environmental measurements. This allowed pollutant concentrations to be interpreted in relation to both their indoor sources and the ventilation available for their dilution and removal. Room type and the activity occurring within the space were also documented because privacy requirements were not assumed to be uniform throughout an apartment or across different activities. For example, the level of visual privacy required in a bedroom could differ from that required in a living room, while different activities within the same room could also create different privacy requirements. These contextual variables were therefore considered when interpreting whether and why occupants changed their window and curtain or blind configurations. For clarity, the blind investigated in this study was a horizontal louvred blind; vertical louvred blinds and roller blinds were not investigated. Horizontal louvred blinds were selected because their adjustable slat angles enable occupants to regulate visual privacy while retaining varying degrees of window opening, outward view and airflow, making them particularly relevant to investigating the privacy–ventilation dilemma.

Integrating these behavioural and environmental datasets enabled the research to explain not only whether occupants modified their window and curtain or blind configurations because of privacy concerns but also the extent to which these behavioural adaptations influenced natural ventilation performance and indoor air quality. Importantly, this enabled the research to examine situations in which a need for ventilation existed because pollutants were being generated indoors, while the simultaneous need for visual privacy influenced how occupants configured their windows and curtains or blinds. This sequential design strengthened causal inference by ensuring that environmental measurements were interpreted within the behavioural context in which they occurred rather than as isolated engineering observations. It also reduced the risk of attributing changes in indoor pollutant concentrations solely to ventilation when differences in the presence, intensity, or frequency of indoor pollutant sources may have contributed to those changes.

Air-conditioning operation was recorded alongside window operation so that window closure associated with air-conditioning could be distinguished from window closure associated with visual privacy. To avoid incorrectly interpreting window closure for air-conditioning as privacy-related behaviour, periods during which windows were closed because air-conditioning was operating were excluded from the analysis of the privacy–natural ventilation relationship. Such periods occurred predominantly at night when occupants used air-conditioning while sleeping. The analysed window-closure behaviour therefore represented periods in which air-conditioning was not the reason for closing the window. This distinction reduced the possibility of attributing a reduction in natural ventilation to privacy when the window had actually been closed to enable air-conditioned operation.

Study Setting and Case Selection

The investigation involved 60 occupied apartments across six high-density residential developments, with approximately 10 apartments selected from each development. Natural ventilation was commonly used in the selected developments. These developments provided an appropriate setting because residents experienced potential visual intrusion where neighbouring apartment blocks were close together or directly overlooked their windows. Occupied apartments were particularly important because everyday activities generated indoor pollutants while residents simultaneously made decisions about privacy and ventilation. This provided a real-life setting for investigating the privacy–ventilation dilemma. The six residential developments were selected to provide variation in building height, distance between neighbouring blocks, façade orientation, window design, apartment layout, and surrounding building density. The apartments also represented different floor levels, orientations, dwelling sizes, household compositions, levels of visual exposure, and normal indoor pollutant-generating activities. This variation allowed the study to examine how different physical and household conditions influenced visual intrusion, privacy needs, window and curtain or blind configurations, natural ventilation, and indoor air quality. The planned sample of 60 apartments was assessed using an a priori statistical power analysis before the main study. A minimum statistical power of 0.80 and a two-sided significance level of α = 0.05 were used. The analysis confirmed that the sample was sufficient to detect meaningful relationships among the main variables while accounting for repeated measurements and apartments being grouped within residential developments.

Indoor air quality was monitored continuously at predefined short intervals using sensors installed in each participating apartment. The sensors automatically recorded measurements and transmitted the data to secure cloud storage, allowing long-term monitoring without researchers being continuously present in the apartments. Researchers visited the apartments when required for sensor installation, quality checks, maintenance, calibration checks, and removal. This approach reduced disruption to residents and allowed indoor air quality to be monitored during normal daily activities. Apartments were selected to provide representation across different physical and household conditions. Relevant household activities that generated indoor pollutants were also recorded so that differences in indoor air quality could be interpreted alongside differences in pollutant generation rather than being attributed to ventilation alone. Where technically feasible, window and curtain or blind positions were also recorded automatically and time-stamped. These records were synchronised with the indoor air quality measurements and occupants’ records of relevant pollutant-generating activities. This allowed the study to determine what was happening in the apartment when changes in indoor air quality occurred. The resulting sample enabled the study to examine how occupants balanced their need for visual privacy with their need for ventilation, particularly when indoor activities generated pollutants that needed to be diluted or removed.

Measurement of the Privacy–Ventilation System

A principal strength of the methodology lay in recognising that the privacy–ventilation dilemma could not be represented by a single variable. Instead, it was conceptualised as a system of interacting architectural, environmental, behavioural, and perceptual variables. The 60 apartments were studied in 12 batches of five apartments. Each batch was monitored concurrently for four consecutive weeks, resulting in 48 weeks of apartment monitoring. Additional time was used between selected batches for installation, equipment transfer, calibration checks, and maintenance, bringing the overall field data-collection period to approximately 12 months.  During each four-week period, indoor environmental conditions and window and curtain or blind configurations were continuously monitored and time-synchronised. Indoor pollutant-generating activities were recorded during the same period. Questionnaires, interviews, activity diaries, direct observations, and architectural and urban measurements were conducted at appropriate times within or around each apartment’s four-week monitoring period.

Architectural and Urban Variables: Objective measurements were first obtained to characterise the physical environment. These included building separation distance, relative building height, façade orientation, window size, window geometry, window sill height, glazing area, window operability, apartment floor level, overlooking angles, and the spatial relationship between opposing residential blocks. Geographic Information Systems (GIS), architectural drawings, laser distance measurements, drone photogrammetry where permissible, and Building Information Modelling (BIM) were used to accurately quantify these parameters. Rather than treating building separation distance as the sole indicator of privacy exposure, the methodology recognised that perceived visual intrusion depended on multiple geometric relationships. Accordingly, three-dimensional visibility analyses were undertaken to estimate the potential visual exposure experienced by occupants under different viewing conditions.

Perceived Visual Intrusion and Privacy Requirements: As privacy was inherently subjective, occupants’ perceptions could not be inferred solely from architectural geometry. Standardised questionnaires were therefore developed to measure perceived visual intrusion, perceived loss of privacy, perceived control over privacy, satisfaction with visual privacy, and the importance occupants assigned to maintaining privacy during different daily activities. Questionnaire development followed established psychometric procedures. Content validity was established through expert review by specialists in building science, environmental psychology, architecture, and indoor environmental quality. Pilot testing was undertaken prior to full deployment to evaluate clarity, reliability, and construct validity. Internal consistency was subsequently assessed using Cronbach’s alpha and composite reliability, while confirmatory factor analysis was used to evaluate construct validity.

Window and Curtain or Blind Configuration: Occupants’ window and curtain or blind configurations were investigated using a combination of questionnaires, activity diaries, direct observation, and unobtrusive position sensors. This multi-method approach reduced reliance on self-reported behaviour while capturing temporal variations in window and curtain or blind configurations throughout different periods of the day and under varying environmental and privacy conditions. Rather than treating window status as a simple binary variable, the configuration of the window was characterised using opening duration, opening frequency, opening angle, time of operation, associated environmental conditions, occupancy conditions, and the simultaneous position of curtains or blinds. This approach allowed the research to distinguish between configurations such as an open window with an open curtain or blind, an open window with a partially or fully closed curtain or blind, a partially open window with different curtain or blind positions, and a closed window. The methodology therefore did not assume that privacy concerns necessarily resulted in window closure.

Window positions were automatically recorded using unobtrusive contact and position sensors that detected whether the windows were closed, partially open, or fully open and, where applicable, measured their opening angles. Curtain or blind positions were recorded using unobtrusive position sensors that detected their degree of opening or closure. Each change in window and curtain or blind position was automatically time-stamped and transmitted to secure cloud storage, where the data were synchronised with the indoor environmental measurements and records of indoor pollutant-generating activities. Curtain or blind operation represented an important behavioural mechanism linking privacy concerns to ventilation performance. Accordingly, observations recorded the position, extent of closure, material type, permeability, installation geometry, and temporal patterns of curtain or blind use. These variables provided the behavioural foundation for the environmental investigations undertaken in Research Question 2.

Indoor Pollutant Sources: Relevant indoor pollutant-generating activities were identified and documented because indoor air quality depended not only on ventilation but also on the pollutants generated within each apartment. The presence, intensity, frequency, timing, and duration of relevant pollutant-generating activities were recorded where practicable. These records enabled measured pollutant concentrations to be interpreted in relation to their indoor sources and the ventilation available for their dilution and removal. Particular attention was given to periods when pollutants were actively generated indoors. These periods provided an important context for examining how occupants balanced the need for ventilation with the simultaneous need for visual privacy and how this influenced their window and curtain or blind configurations.

Natural Ventilation Performance and Indoor Air Quality: Natural ventilation performance was assessed using measurements of indoor and outdoor temperature, relative humidity, air velocity, air-change effectiveness, and differential pressure across openings. Indoor environmental measurements included carbon dioxide (CO₂), carbon monoxide (CO), fine particulate matter (PM_2.5), inhalable particulate matter (PM₁₀), ozone (O₃), total volatile organic compounds (TVOCs), formaldehyde, nitrogen dioxide (NO₂), and other pollutants identified as relevant to the indoor pollutant-generating activities established during the first phase. Indoor air quality was monitored continuously or at predefined short intervals using sensors installed in the participating apartments. The sensors automatically recorded measurements and transmitted the data to secure cloud storage. This enabled long-term monitoring without requiring researchers to remain continuously present in the apartments. Researchers attended the apartments when required for sensor installation, quality checks, maintenance, calibration checks, and removal.

Environmental monitoring was conducted over extended periods to capture normal occupant behaviour and everyday indoor pollutant-generating activities rather than short-term experimental conditions. Measurements therefore represented authentic residential operation instead of laboratory behaviour. The environmental measurements were time-synchronised with records of indoor pollutant-generating activities and window and curtain or blind configurations. This allowed changes in indoor air quality to be interpreted in relation to what was occurring within the apartment at the corresponding time. Together, these measurements enabled the study to distinguish the influence of indoor pollutant generation from the influence of ventilation and to examine how privacy-related window and curtain or blind configurations affected the dilution and removal of pollutants generated indoors.

Data Analysis and Research Quality

Data collected across the 60 apartments comprised architectural and urban characteristics, occupants’ privacy perceptions and behaviours, window and curtain or blind configurations, indoor pollutant-generating activities, natural ventilation performance, and indoor air quality. These data were obtained from questionnaires, interviews, activity diaries, behavioural observations, architectural measurements, position sensors, and environmental monitoring, as described in the preceding sections. The different datasets were time-synchronised where applicable and integrated for analysis. Data analysis integrated the behavioural, perceptual, architectural, environmental, and indoor pollutant-source data collected across the 60 apartments. Descriptive statistics were first used to characterise occupants’ privacy perceptions and behaviours, building and window characteristics, window and curtain or blind configurations, indoor pollutant-generating activities, natural ventilation performance, and indoor air quality. Inferential analyses were then used to determine whether statistically significant relationships existed among these variables.

Structural Equation Modelling (SEM) served as the principal analytical technique because the research examined multiple relationships within the proposed causal pathway rather than isolated associations. The underlying principle of SEM is that complex real-world outcomes are often produced through a connected system of relationships rather than by one factor acting alone. It therefore allowed the proposed privacy–ventilation system to be examined as a whole. For example, neighbouring-building configuration could influence perceived visual intrusion, which could influence privacy requirements and window and curtain or blind configurations, which could subsequently influence natural ventilation and indoor air quality. SEM allowed the strength and statistical significance of these connected relationships to be tested against the observed data. SEM enabled direct and indirect relationships to be examined while incorporating latent constructs such as perceived visual intrusion and privacy requirements. Latent constructs are conditions that cannot be measured directly, such as a person’s perceived need for privacy, but can be estimated from several related questionnaire responses. Multilevel modelling accounted for repeated measurements within individual apartments and the grouping of apartments within the six residential developments. This was necessary because repeated measurements from the same apartment were related to one another and apartments within the same development shared some building and surrounding environmental characteristics. The presence, intensity, and frequency of indoor pollutant sources were also accounted for so that differences in indoor air quality were not incorrectly attributed to ventilation when differences in pollutant generation could have contributed to the measured concentrations.

Semi-structured interview data underwent thematic analysis to identify occupants’ explanations for their behaviours, privacy concerns, adaptive strategies, and contextual influences that could not be fully captured through quantitative measurements. This provided the human explanation behind patterns observed in the numerical and sensor data. The qualitative and quantitative findings were subsequently integrated to determine whether the different sources of evidence supported, complemented, or contradicted one another. Particular attention was given to differences between occupants’ reported behaviours and their sensor-recorded window and curtain or blind configurations. For example, an occupant could report a strong preference for natural ventilation while sensor data showed that privacy concerns frequently resulted in configurations that restricted airflow.

Research quality was strengthened through triangulation across questionnaires, interviews, behavioural observations, architectural measurements, activity records, environmental monitoring, and objective sensor data. Triangulation allowed the same phenomenon to be examined using different sources of evidence rather than relying on a single measurement or occupants’ self-reports alone. Measurement reliability was supported through pilot testing, instrument calibration and calibration checks, repeated measurements, and assessment of questionnaire internal consistency. Construct validity was assessed through expert review and confirmatory factor analysis. Criterion validity was assessed, where appropriate, by comparing occupants’ reported window and curtain or blind behaviours with sensor-recorded configurations. Together, these procedures helped establish that the measurements were sufficiently consistent and that they represented the concepts and behaviours they were intended to measure.

Internal validity was strengthened by accounting for indoor pollutant-generating activities when interpreting relationships between ventilation and indoor air quality. This was important because a high pollutant concentration could result from greater indoor pollutant generation rather than inadequate ventilation alone. The explanatory sequential design further strengthened interpretation by using the behavioural findings from the first phase to inform the physical and environmental investigation in the second phase. This allowed the measured environmental outcomes to be interpreted in relation to the actual privacy-related behaviours that produced the window and curtain or blind configurations being investigated. The inclusion of six residential developments with different architectural and urban characteristics strengthened the applicability of the findings beyond a single residential setting.

Ethical Considerations and Methodology Contribution to Knowledge

The methodology was designed specifically to address the purpose of Research Question 1, which was to establish the existence, nature, and underlying mechanisms of the privacy–ventilation dilemma in high-density residential buildings. By integrating architectural and urban measurements, occupant perceptions, behavioural observations, window and curtain or blind position monitoring, indoor pollutant-source recording, environmental monitoring, and statistical modelling, the methodology captured both the physical and human dimensions of the problem. Importantly, it allowed the need for visual privacy to be examined alongside the need for ventilation, particularly when indoor activities generated pollutants that required dilution or removal. The integrated analytical framework provided a direct basis for testing the proposed hypotheses. If no statistically significant relationships or interactions were found among perceived visual intrusion, privacy requirements, neighbouring-building configuration, window characteristics, curtain or blind use, occupants’ window and curtain or blind configurations, natural ventilation performance, and indoor air quality, the null hypothesis (H₀₁) was not rejected. Conversely, statistically significant direct or indirect relationships within the proposed causal framework provided evidence for rejecting the null hypothesis in favour of the alternative hypothesis (H₁₁). Indoor pollutant sources were accounted for when interpreting these relationships so that changes in indoor air quality were not attributed to ventilation without considering differences in pollutant generation.

Ethical considerations were particularly important because the research was conducted in occupied homes and investigated behaviours related to personal privacy. Ethical approval was obtained before data collection, and participants provided informed consent. Particular attention was given to concerns that continuous monitoring devices installed inside apartments could themselves be perceived as intrusive. Participants were therefore informed about the purpose and operation of each indoor air quality, window-position, and curtain or blind-position sensor before installation. They were clearly informed about what each sensor measured, when measurements were recorded, what information was transmitted to cloud storage, and how the data would be used.

The monitoring system did not use cameras, microphones, facial recognition, or other technologies capable of recording occupants’ images, conversations, or identities. Indoor air quality sensors recorded only the specified environmental parameters, while window and curtain or blind sensors recorded only their positions and changes in configuration. Sensor locations were agreed with participants and selected to obtain the required measurements while minimising interference with normal household activities. Participants retained the right to request sensor removal or withdraw from the study. Personally identifiable information was separated from sensor data, and unique study codes were used to link measurements where required. Data transmitted to secure cloud storage were protected using appropriate access controls and were accessible only to authorised researchers. These measures allowed continuous environmental and behavioural monitoring to be undertaken while protecting occupants’ privacy, autonomy, and confidence in the research process. The methodology also contributed knowledge through its integrated approach to investigating the privacy–ventilation dilemma. Rather than examining privacy, occupant behaviour, ventilation, and indoor air quality independently, it connected these dimensions through time-synchronised behavioural and environmental evidence. This provided a methodological framework for investigating how architectural conditions and human needs translated into actual window and curtain or blind configurations and subsequently into measurable environmental outcomes. The resulting evidence established the empirical foundation for Research Question 2, which quantified the consequences of the dilemma, and Research Question 3, which investigated integrated solutions.

Methods for Research Question 2:

Overview

The methodology for Research Question 2 was designed to quantify the environmental, behavioural, and healthy living consequences of the privacy–ventilation dilemma in high-density residential buildings. It examined how different window and curtain or blind configurations affected airflow distribution, air-change effectiveness, pollutant removal, thermal comfort, perceived privacy, occupants’ ventilation-related behaviour, healthy indoor air, and healthy living. The methodology considered a range of window and curtain or blind configurations, including an open window with an open curtain or blind, an open window with a partially or fully closed curtain or blind, a partially open window with different curtain or blind positions, and a closed window. This was important because the privacy–ventilation dilemma did not necessarily involve closing the window. A window could remain open while a curtain or blind was partially or fully closed for visual privacy, potentially altering the airflow through the opening. The methodology therefore quantified the consequences associated with different combinations of window and curtain or blind configurations rather than treating window status as a simple open-or-closed condition.

The presence, intensity, frequency, timing, and duration of relevant indoor pollutant-generating activities were incorporated into the methodology. This was necessary to account for the potential influence of both indoor pollutant generation and the ventilation available for pollutant dilution and removal when interpreting measured indoor pollutant concentrations. Accounting for indoor sources also enabled the effects associated with different window and curtain or blind configurations to be examined without assuming that differences in measured pollutant concentrations resulted from ventilation alone. Particular attention was given to periods of indoor pollutant generation because these provided an appropriate context for investigating conditions in which a need for ventilation and a need for visual privacy could occur simultaneously. The complexity of the research question required a methodology capable of evaluating both engineering performance and human responses. Measurements of airflow and indoor environmental conditions could determine how different window and curtain or blind configurations affected ventilation and pollutant removal, but these measurements could not explain how occupants experienced privacy, thermal comfort, or the usability of those configurations. Similarly, questionnaires and interviews could provide information about occupants’ perceptions, experiences, and behaviours, but they could not objectively measure how air moved through and within the apartment or how effectively indoor pollutants were diluted and removed. The methodology therefore integrated environmental engineering, building physics, indoor environmental quality assessment, computational modelling, and occupant-centred evaluation within a common analytical framework.

A sequential mixed-methods experimental research design supported by methodological triangulation was adopted. The methods were conducted in a defined sequence so that evidence generated at one stage informed the design and interpretation of the subsequent stage. Field investigations were first used to identify and quantify naturally occurring window and curtain or blind configurations, indoor pollutant-generating activities, environmental conditions, and occupant responses in occupied apartments. These observations informed the selection of representative conditions for controlled environmental experiments, in which the effects of different window and curtain or blind configurations on airflow, air-change effectiveness, and pollutant removal were examined while other relevant variables were controlled. The experimental conditions and measurements subsequently informed Computational Fluid Dynamics (CFD) simulations, which provided detailed spatial analysis of airflow behaviour that could not be comprehensively measured throughout the physical environment. Evidence from the three stages was then integrated to provide a combined behavioural, experimental, and physical understanding of the environmental and human consequences of the privacy–ventilation dilemma.

The methodology examined the causal sequence from privacy requirements to window and curtain or blind configuration, airflow distribution, air-change effectiveness, pollutant dilution and removal, indoor environmental quality, occupant comfort and behaviour, healthy indoor air, and healthy living. Indoor pollutant generation was considered alongside this sequence to account for both the pollutants generated indoors and the ventilation available for their dilution and removal. The framework therefore provided a systematic basis for quantifying how privacy-related window and curtain or blind configurations influenced environmental performance and conditions relevant to healthy living.

Research Design

A sequential mixed-methods experimental research design was adopted to quantify the environmental and human consequences associated with different window and curtain or blind configurations under realistic residential conditions. The design combined field investigation, controlled environmental experimentation, and Computational Fluid Dynamics (CFD) simulation. The methods were conducted sequentially so that evidence generated at each stage informed the design and interpretation of the subsequent stage. The research comprised three interconnected stages. The first examined occupied apartments under normal operating conditions to identify and quantify naturally occurring window and curtain or blind configurations, indoor pollutant-generating activities, environmental conditions, and occupant responses. This provided the real-life context needed to determine representative conditions for subsequent controlled investigation. The second stage reproduced selected window and curtain or blind configurations within a controlled experimental environment. Relevant conditions were systematically controlled or varied to examine the effects of these configurations on airflow distribution, air-change effectiveness, pollutant dilution and removal, and thermal conditions. Indoor pollutant generation was controlled so that differences in measured pollutant concentrations could be examined in relation to the ventilation provided under each configuration rather than uncontrolled differences in pollutant generation.

The third stage employed CFD models informed by the experimental conditions and measurements. The models were validated against the physical measurements before being used to examine airflow distribution in greater spatial detail and across a wider range of relevant window and curtain or blind configurations and environmental conditions than could practically be investigated experimentally. Integrating the three stages provided a balance between real-life relevance, experimental control, and analytical depth. Field investigations represented naturally occurring residential conditions and occupant behaviour. Controlled experiments enabled the effects of different configurations to be examined while reducing the influence of uncontrolled variables. CFD simulations provided detailed spatial understanding of airflow behaviour that could not be comprehensively captured through physical measurements alone. Together, the three stages provided complementary evidence for quantifying the environmental and human consequences of the privacy–ventilation dilemma.

Detailed Measurement and Experimental Procedures for Research Question 2

The following subsections describe the detailed procedures used to implement the RQ2 research design. Where measurements and field procedures were shared with RQ1, these are referenced rather than repeated. The emphasis is placed on the RQ2-specific experimental variables, controlled investigations, CFD modelling, and assessment of healthy indoor air and healthy living.

Experimental Variables: The methodology treated window and curtain or blind configuration as the principal independent variable. Rather than considering only fully open or fully closed configurations, multiple realistic operating conditions were investigated to represent common occupant behaviour and the range of configurations identified through the field investigation described under Research Question 1. Experimental scenarios included windows fully open with curtains or blinds fully open, windows fully open with curtains or blinds fully closed, partially opened curtains or blinds, partially opened windows combined with different curtain or blind positions, and complete window closure associated with privacy concerns. Additional scenarios evaluated different curtain fabrics, blind materials, permeability characteristics, installation geometries, and opening configurations. Window opening and curtain or blind closure were quantified wherever practicable so that the degree of opening or obstruction could be related directly to changes in ventilation performance rather than being represented only by broad categories such as open, partially open, or closed. Indoor pollutant generation was treated as an important contextual and experimental variable. The indoor pollutant sources and pollutants established under Research Question 1 were retained rather than redefined for Research Question 2. Representative pollutant-generating conditions identified through the field investigation were reproduced under controlled conditions using repeatable source-generation protocols where appropriate. Source location, generation rate, duration, and timing were controlled between comparable experiments. This provided a consistent pollutant challenge against which the ability of different window and curtain or blind configurations to support dilution and removal could be compared.

Environmental conditions were systematically varied to represent realistic residential operation. These included outdoor wind speed, wind direction, temperature differences, humidity, apartment orientation, façade exposure, and other relevant conditions. Variables that could not be practically manipulated during physical experiments, particularly neighbouring-building geometry and urban-density conditions, were examined parametrically through CFD simulation. This separation prevented the experimental programme from attempting to reproduce physically every condition that could be investigated more efficiently through validated computational modelling. The experimental design therefore distinguished between variables that were manipulated, variables that were controlled, and variables that were measured as outcomes. This distinction was important for determining whether differences in ventilation and pollutant removal could reasonably be associated with the investigated window and curtain or blind configurations rather than uncontrolled changes in surrounding conditions.

Field Investigation: The field investigation used the occupied residential apartments, monitoring infrastructure, sensor arrangements, indoor pollutant-source recording, environmental measurements, and monitoring periods established under Research Question 1. The field data required for Research Questions 1 and 2 were collected simultaneously during the same monitoring periods rather than through separate field investigations. The same field evidence served different methodological purposes: RQ1 used it to characterise the privacy–ventilation dilemma and its underlying relationships, while RQ2 used it to quantify its environmental and human consequences and to identify representative conditions for subsequent controlled experiments. These procedures were not repeated for Research Question 2. Instead, their role changed from characterising the privacy–ventilation dilemma under RQ1 to providing the real-world conditions required to determine what should be investigated experimentally under RQ2.

The field data were examined to identify recurring and contrasting combinations of window opening, curtain or blind position, privacy conditions, indoor pollutant-generating activities, ventilation performance, and occupant responses. The purpose was not to reproduce every observed configuration experimentally. Rather, representative configurations were selected to capture conditions of greatest relevance to the privacy–ventilation dilemma. These included configurations representing relatively unobstructed ventilation, partial obstruction, substantial obstruction, and situations in which occupants maintained privacy while retaining some degree of window opening. Selection of experimental scenarios was therefore evidence-informed rather than arbitrary. Frequency of occurrence, degree of airflow obstruction, relevance to privacy, associated indoor pollutant-generation conditions, and potential consequences for occupants were considered when determining which field configurations progressed to controlled experimentation. Unusual but potentially important configurations could also be retained where they represented conditions with substantial implications for ventilation or healthy indoor air. This approach maintained the real-life relevance of the experimental programme while keeping the number of controlled scenarios scientifically and practically manageable. It also created a clear methodological bridge between natural occupant behaviour and controlled physical investigation: the field study established what people actually did, while the experiments examined what environmental consequences could arise from representative configurations.

Controlled Environmental Experiments: Controlled environmental experiments were undertaken because field observations alone could not isolate the contribution of individual window and curtain or blind configurations from the many conditions changing simultaneously in occupied apartments. The purpose of experimental control was therefore not to replace the real-world evidence but to investigate selected relationships under conditions in which competing explanations could be reduced. Experiments were conducted within a full-scale residential test chamber or experimental apartment representing relevant residential geometry and natural ventilation conditions. Configurations selected from the field investigation were reproduced systematically. A reference condition representing relatively unobstructed natural ventilation was established for comparison with privacy-related configurations. Changes from this reference condition provided a consistent basis for quantifying the extent to which different curtain or blind positions and window openings altered airflow distribution, air-change effectiveness, pollutant dilution and removal, and thermal conditions.

Comparable experiments maintained consistent boundary conditions wherever the investigated variable was not intentionally changed. Indoor pollutant generation was standardised using the source conditions described in the experimental variables Section. A stabilisation period was provided before measurements used for comparison were collected. Each experimental condition was repeated to assess repeatability and distinguish systematic effects from random measurement variation. The order of experimental configurations was randomised or counterbalanced where appropriate to reduce the possibility that test sequence influenced the results. Measurement locations were selected to capture conditions near the window opening, behind the curtain or blind, within the occupied zone, and at representative breathing-zone locations. This enabled the experiments to determine not merely how much air entered the space, but how the privacy screen changed the subsequent distribution of that air and its ability to reach locations relevant to occupants. The controlled experiments therefore provided the principal physical comparison between configurations while remaining grounded in the behaviours and conditions observed in occupied apartments.

Computational Fluid Dynamics (CFD) Simulation: CFD complemented the field and experimental investigations by providing spatial information that could not practically be obtained from sensors at every location within an apartment. Its principal methodological purpose was to explain how and where changes in airflow occurred and to extend the experimentally supported relationships to conditions that could not all be physically tested. Three-dimensional models were developed using representative geometries and boundary conditions established through the preceding investigations. The models were validated against experimental measurements before being used for extended analysis. Validation compared predicted and measured quantities at corresponding locations rather than relying on visual similarity between simulated and observed airflow patterns. Model performance was assessed against predefined acceptance criteria, and sensitivity analyses examined whether key conclusions remained stable under reasonable changes in modelling assumptions and boundary conditions.

Following validation, CFD was used to examine airflow around and through curtains or blinds, flow deflection, recirculation, stagnation, air distribution within occupied zones, and transport and removal of internally generated pollutants. This made it possible to identify physical mechanisms that could explain differences detected experimentally. For example, two configurations producing similar overall air-change rates could nevertheless produce substantially different airflow pathways within the occupied zone. Parametric simulations subsequently varied selected architectural, window, screening, and environmental conditions. This allowed the study to move beyond asking whether a particular configuration worked in one apartment and towards understanding the conditions under which its performance remained effective or deteriorated.

Assessment of Healthy Indoor Air: The pollutants, environmental parameters, measurement procedures, and indoor-source characterisation established under Research Question 1 were retained for Research Question 2 and are therefore not repeated here. For RQ2, the methodological emphasis shifted from measuring these parameters to evaluating what their combined behaviour indicated about healthy indoor air under different privacy-related configurations. Healthy indoor air was not represented by a single pollutant concentration or ventilation rate. Instead, interpretation considered whether ventilation was sufficient, whether air was effectively distributed through occupied zones, and whether internally generated pollutants were effectively diluted and removed. Air-change effectiveness and contaminant removal effectiveness were therefore considered alongside conventional environmental measurements. This distinction was important because a relatively high overall ventilation rate could coexist with poor air distribution in parts of an apartment. Similarly, room-average pollutant concentrations could conceal less favourable conditions within locations where occupants actually spent time. Assessment consequently emphasised occupied and breathing-zone conditions. The analytical focus was the functionality of ventilation from the occupant’s perspective: whether the available ventilation under each window and curtain or blind configuration actually delivered cleaner and thermally acceptable air to the places where people experienced the indoor environment.

Assessment of Healthy Living: Healthy living was treated separately from healthy indoor air because an environmentally favourable condition did not automatically establish a favourable human experience. The occupant-assessment methods established under RQ1 provided the behavioural foundation, while RQ2 examined how environmental consequences were related to occupants’ experiences. Healthy living was evaluated through validated occupant questionnaires examining perceived respiratory comfort, perceived freshness of indoor air, sleep quality, ability to perform daily activities, overall physical comfort, perceived well-being, satisfaction with the indoor environment, and the extent to which occupants believed their living environment supported healthy daily life. Perceived visual privacy was evaluated alongside these outcomes because resolving the ventilation problem by creating an unacceptable privacy condition would not constitute a successful response to the dilemma. Similarly, a configuration that provided strong visual privacy, but substantially impaired ventilation could not be judged solely on its privacy performance. The assessment therefore considered the extent to which configurations supported both environmental and human requirements rather than optimising one at the expense of the other.

Behavioural evidence was used to examine occupants’ adaptations, including changes to window opening and curtain or blind position. Where appropriate, occupant responses were related temporally to the corresponding environmental conditions. This enabled the study to examine whether objectively favourable or unfavourable environmental conditions were consistent with occupants’ reported experiences and behaviours. Rather than assuming that improved indoor air quality automatically translated into healthier living, the methodology directly evaluated occupants’ lived experiences alongside objective environmental performance. Healthy living was therefore operationalised as a human-centred outcome emerging from the combined conditions of healthy indoor air, thermal comfort, visual privacy, usability, and occupants’ ability to carry out normal activities within their homes. This provided the methodological basis for determining whether a configuration merely improved an engineering indicator or meaningfully supported the conditions required for healthy everyday living.

Data Analysis and Research Quality

Data analysis integrated evidence from the field investigation, controlled environmental experiments, CFD simulations, and occupant-centred assessments. The analysis was designed to determine the magnitude and statistical significance of differences associated with alternative window and curtain or blind configurations while accounting for other factors that could influence ventilation, indoor environmental conditions, and healthy living. Descriptive statistics were first used to summarise the environmental and occupant-response data and examine their distributions, variability, and completeness. Repeated-measures analyses compared ventilation performance, pollutant dilution and removal, thermal conditions, and other relevant outcomes across different window and curtain or blind configurations. Because multiple measurements were obtained from the same apartments and experimental conditions, mixed-effects models were used to account for the fact that observations from the same setting were more closely related than observations from different settings. This prevented repeated measurements from being incorrectly treated as completely independent observations.

Multivariable regression models were used to examine the independent and combined contributions of window and curtain or blind configuration, environmental conditions, building characteristics, occupancy, and relevant indoor pollutant-generating activities. Potential interaction effects were also examined. For example, the environmental consequence of a particular curtain or blind position could differ depending on the degree of window opening or prevailing ventilation-driving conditions. Effect sizes and confidence intervals were reported alongside statistical significance to indicate not only whether differences existed, but also their magnitude and uncertainty. Structural Equation Modelling (SEM) integrated the behavioural and perceptual variables examined under RQ1 with the environmental and human outcomes examined under RQ2. This allowed the proposed pathway from perceived visual intrusion and privacy requirements, through window and curtain or blind configuration, to ventilation performance, healthy indoor air, and healthy living to be evaluated as a connected system rather than as separate relationships. CFD predictions were quantitatively compared with corresponding experimental measurements before the models were used for extended analysis. Model agreement was evaluated using appropriate statistical error and agreement measures. Mesh-independence and sensitivity analyses were also undertaken to determine whether the principal CFD outcomes remained stable when numerical resolution and reasonable modelling assumptions were varied.

Research quality was strengthened through triangulation across real-world field evidence, controlled experiments, CFD simulations, environmental measurements, and occupant-centred evidence. Measurement reliability was supported through the calibration procedures established under RQ1, repeated experimental trials, standardised protocols, and data-quality checks. Internal validity was strengthened by systematically changing the variables of interest while controlling other relevant conditions during experiments. External validity was supported by grounding the experimental scenarios in configurations observed across the residential developments investigated under RQ1. Construct validity was supported through appropriate environmental measures and validated occupant-assessment instruments. Qualitative responses concerning privacy, comfort, usability, and healthy living underwent thematic analysis and were integrated with the quantitative evidence. Agreement, complementarity, and disagreement between measured environmental performance and occupants’ experiences were examined explicitly. This enabled the conclusions to reflect both what happened physically within the indoor environment and how those conditions were experienced by occupants.

Ethical Considerations and Methodology Contribution to Knowledge

The methodology was designed specifically to address the purpose of Research Question 2, which was to quantify the environmental, behavioural, and healthy living consequences associated with privacy-related window and curtain or blind configurations. By integrating field investigation, controlled environmental experiments, CFD simulations, environmental monitoring, and occupant-centred assessments, the methodology enabled the consequences of different configurations to be examined in terms of airflow distribution, air-change effectiveness, pollutant dilution and removal, thermal comfort, perceived privacy, occupants’ ventilation-related behaviour, healthy indoor air, and healthy living. Indoor pollutant-generating activities were accounted for using the procedures established under RQ1 so that environmental outcomes could be interpreted in relation to the relevant indoor pollutant sources. The integrated analytical framework provided a direct basis for testing the proposed hypotheses. If privacy-related window and curtain or blind configurations produced no statistically significant differences in airflow distribution, air-change effectiveness, pollutant removal, thermal comfort, perceived privacy, occupants’ ventilation-related behaviour, healthy indoor air, and healthy living under different window-opening, weather, dwelling, and urban-density conditions, the null hypothesis (H₀₂) was not rejected. Conversely, statistically significant differences provided evidence for rejecting H₀₂ in favour of H₁₂. Effect sizes and confidence intervals were also examined to determine the magnitude and uncertainty of the observed differences rather than relying on statistical significance alone.

Ethical considerations applied particularly to field monitoring and occupant-centred assessments conducted in occupied homes. The informed-consent, privacy, sensor-monitoring, cloud-storage, anonymisation, data-security, and withdrawal procedures established under RQ1 were retained and are not repeated here. For RQ2 specifically, occupants were not required to adopt window and curtain or blind configurations that compromised their privacy, comfort, or normal use of their homes. Configurations requiring systematic manipulation were therefore investigated within the controlled experimental environment rather than imposed on occupied apartments. Occupant questionnaires were administered confidentially, and unnecessary collection of sensitive household information was avoided. The methodology contributed knowledge through an integrated progression from real-world observation to controlled experimentation and validated computational investigation. Field data for RQ1 and RQ2 were collected simultaneously, with RQ2 using the shared field evidence to identify representative conditions for controlled experimentation. The experiments isolated the physical consequences of selected configurations, while validated CFD models provided detailed explanation of airflow mechanisms and extended the investigation to conditions that could not practically be tested. Integrating these approaches with occupant-centred evidence provided a methodological framework for connecting privacy-related behaviour with measurable environmental consequences and occupants’ experiences. This provided the methodological and quantitative foundation for RQ3, in which potential solutions to the privacy–ventilation dilemma were developed and evaluated.

Methods for Research Question 3:

Overview

The methodology for Research Question 3 represented the solution-development stage of the research. The methodologies for Research Questions 1 and 2 provided the evidence base for this stage. RQ1 was designed to characterise the existence, nature, and underlying mechanisms of the privacy–ventilation dilemma, while RQ2 was designed to quantify its environmental, behavioural, and healthy living consequences. Building on this methodological progression, RQ3 was designed to develop, evaluate, refine, and validate integrated strategies for reducing or resolving the conflict between visual privacy and natural ventilation in high-density residential buildings. The objective extended beyond evaluating existing architectural solutions. Instead, the research sought to generate original design knowledge that enabled architects, engineers, and building developers to intentionally balance visual privacy with natural ventilation while improving healthy indoor air and supporting healthy living. Importantly, a solution was not considered successful merely because it improved airflow or provided visual privacy. It was required to address both needs while also considering thermal comfort, usability, energy performance, and occupants’ acceptance, as specified in RQ3. The study therefore adopted a Design Science Research (DSR) methodology because it was specifically intended for developing, refining, and validating artefacts that addressed complex real-world problems while simultaneously contributing new knowledge.

The design artefacts investigated in this research included architectural design principles, façade configurations, window systems, interior-screening strategies, and occupant-control mechanisms. These artefacts were not viewed as isolated products but as integrated design solutions that operated together to achieve multiple performance objectives. Their development was informed by the privacy-related behaviours and window and curtain or blind configurations investigated under RQ1 and the environmental and human consequences examined under RQ2. The indoor pollutant-generating conditions considered in RQ1 and RQ2 were also retained where relevant so that proposed solutions were evaluated in situations where ventilation was needed for the dilution and removal of pollutants generated indoors. The methodology followed an iterative design cycle comprising problem synthesis, solution generation, prototype development, evaluation, refinement, and validation. Problem synthesis brought together the behavioural, architectural, environmental, experimental, computational, and occupant-centred evidence generated through RQ1 and RQ2 to define the requirements that proposed solutions needed to satisfy. Solution generation translated these requirements into alternative design concepts, which were subsequently developed into prototypes or testable design configurations. Evaluation combined appropriate physical experiments, CFD simulations, environmental performance assessment, and occupant-centred assessment to determine how well each solution balanced the competing requirements identified in RQ3.

Each iteration improved the design artefacts using evidence obtained from engineering analysis and occupant-centred evaluation. Solutions that failed to meet predefined performance requirements were modified and re-evaluated, allowing evidence from each cycle to inform the next design iteration. The process continued until the proposed solutions demonstrated an acceptable balance across visual privacy, natural ventilation, healthy indoor air, healthy living, thermal comfort, usability, energy performance, and occupant acceptance. This iterative process ensured that the final design framework was both scientifically robust and practically applicable. It also maintained a clear methodological progression across the PhD research: RQ1 characterised the problem, RQ2 quantified its consequences, and RQ3 used the resulting evidence to develop and evaluate potential solutions.

Research Design

The methodology adopted a Design Science Research (DSR) framework consisting of iterative cycles of problem synthesis, solution generation, prototype development, evaluation, refinement, and validation. Unlike traditional experimental research, which primarily investigates existing phenomena, Design Science Research seeks to generate new knowledge by creating artefacts that intentionally solve identified problems. In this study, DSR provided a structured process for translating evidence about the privacy–ventilation dilemma into potential solutions and then systematically determining whether those solutions performed as intended. The purpose was therefore not simply to propose design ideas, but to develop and evaluate solutions through a transparent and repeatable evidence-based process. The research began by synthesising the evidence generated through the methodologies for Research Questions 1 and 2. The relationships and behavioural mechanisms investigated under RQ1 and the environmental and human consequences examined under RQ2 were translated into design requirements that guided solution development. Consequently, every proposed intervention directly addressed evidence generated during the earlier phases of the research rather than relying solely on existing architectural practice. For example, if particular window and curtain or blind configurations were associated with conditions requiring greater visual privacy but reduced ventilation effectiveness, these conditions became design problems that candidate solutions were required to address. Indoor pollutant-generating conditions considered under RQ1 and RQ2 were retained where relevant so that ventilation performance was evaluated in the context of a genuine need for pollutant dilution and removal.

The design requirements were defined before candidate solutions were evaluated. They reflected the multiple outcomes specified in RQ3, including visual privacy, natural ventilation, healthy indoor air, healthy living, thermal comfort, usability, energy performance, and occupant acceptance. Where possible, requirements were expressed using measurable performance criteria rather than subjective statements such as “better privacy” or “better ventilation”. This enabled candidate solutions to be compared against consistent criteria and reduced the possibility of selecting solutions simply because they appeared promising. Solution generation then translated these requirements into alternative architectural, façade, window, interior-screening, and occupant-control strategies. Candidate solutions were developed into representations appropriate to their stage of maturity, ranging from drawings and digital models to testable physical configurations or prototypes. Solutions were not assumed to operate independently. Where appropriate, combinations of strategies were investigated because resolving the dilemma could require coordinated changes to the window, privacy-screening system, façade, interior arrangement, or occupant controls.

Successive design iterations were undertaken throughout the study. Each iteration generated increasingly refined solutions through repeated evaluation against predefined environmental, behavioural, operational, and healthy living objectives. Evaluation methods were selected according to the performance question being examined. Physical experiments assessed measurable ventilation and environmental performance under controlled conditions. Validated CFD models examined airflow distribution and pollutant transport in greater spatial detail and across conditions that could not all be physically tested. Occupant-centred assessments examined perceived privacy, thermal comfort, usability, acceptance, and healthy living. Where measurements or procedures had already been established under RQ1 and RQ2, these were retained to maintain methodological consistency rather than introducing different measures for RQ3. Evidence from each evaluation cycle determined whether a candidate solution was retained, modified, combined with another strategy, or rejected. A solution that improved one outcome while producing an unacceptable deterioration in another was therefore not automatically regarded as successful. For example, improving visual privacy at the expense of inadequate ventilation, or improving airflow while producing unacceptable visual exposure, would indicate the need for further refinement. This multi-criteria approach reflected the central purpose of RQ3: reducing the privacy–ventilation conflict rather than optimising either privacy or ventilation in isolation.

This iterative methodology enabled continuous improvement while ensuring that design decisions remained evidence-based. The final validation stage evaluated the refined solutions against the predefined performance criteria and across representative conditions established through the preceding research. Maintaining a documented link between the original problem evidence, design requirements, proposed solutions, evaluation results, and subsequent refinements provided methodological traceability. This made it possible for readers and future researchers to understand not only what solutions were developed, but why particular design decisions were made and how the supporting evidence was generated.

Design Requirements, Solution Generation, and Prototype Development

The following subsections describe how evidence generated through RQ1 and RQ2 was translated into design requirements, the development of the proposed design solution, and its subsequent transformation into testable prototypes. Together, these stages established a traceable progression from the identified privacy–ventilation problem to the functional requirements that a successful solution had to satisfy, the governing scientific principle underlying the proposed solution, and the procedures through which that solution was progressively developed for evaluation. The central premise was that the privacy–ventilation dilemma should not be addressed by accepting a compromise between visual privacy and natural ventilation. Instead, the methodology sought to functionally separate visual privacy from airflow so that occupants could retain a clear outward view and visual privacy while the conventional window remained physically fully open. The design objective was therefore expressed as a multiplicative requirement: Successful Solution = Clear Outward View × Visual Privacy × Full Ventilation. Failure to satisfy any one of these three fundamental requirements constituted failure of the solution.

Development of Design Requirements: The first stage of Design Science Research established the functional requirements that the proposed solution was expected to satisfy. These requirements were derived from the evidence generated through RQ1 and RQ2 rather than from predetermined architectural assumptions. The requirements translated the identified behavioural, architectural, privacy, ventilation, and indoor environmental problems into explicit functions against which the proposed solution could subsequently be developed and evaluated. Three non-negotiable primary requirements governed the design. First, occupants had to retain a clear outward view comparable to that available through a conventional open window. Second, visual privacy had to be maintained against relevant neighbouring viewing positions, including a direct perpendicular line of sight towards the window. Third, natural ventilation performance had to remain equivalent, within a predefined experimental tolerance, to that of the same conventional window when fully open and completely unobstructed.

The fully open conventional window with no curtain, blind, glazing panel, screen, or other physical obstruction across its opening therefore served as the reference condition for ventilation. This benchmark was important because the purpose of RQ3 was not to determine how much ventilation could reasonably be sacrificed for privacy. Instead, it investigated whether privacy could be provided while retaining the ventilation opportunity associated with having the window fully open. Visual privacy was similarly treated as a functional performance requirement rather than simply the presence of a privacy device. A successful solution had to prevent people outside, including neighbours located directly opposite the apartment, from obtaining recognisable visual information about occupants, objects, surfaces, or activities within defined privacy-sensitive interior areas. Preventing only oblique or lateral views while allowing a perpendicular view into the protected area therefore constituted failure.

The indoor pollutant-generating conditions established under RQ1 and considered under RQ2 were retained where relevant. This ensured that full ventilation was evaluated not merely as movement of air through the window, but also in relation to its intended environmental function, including the dilution and removal of pollutants generated indoors. Additional requirements addressed healthy indoor air, healthy living, thermal comfort, daylight transmission, optical clarity, glare, flicker, noise, reliability, response time, occupant control, energy demand, maintainability, durability, fail-safe operation, architectural integration, regulatory compliance, economic feasibility, and adaptability to different apartment, window, and neighbouring-building configurations. Safety constituted a further non-negotiable requirement. Any technology operating across the open window had to be suitable for continuous proximity to occupants and normal indoor and outdoor air. It therefore had to avoid unacceptable toxic, ionising, electromagnetic, thermal, chemical, or other environmental hazards and must not introduce pollutants that undermined the healthy indoor air objective of the research. Defining these requirements before solution development provided explicit criteria for subsequent design decisions. Importantly, the three primary requirements were treated as multiplicative rather than compensatory. Superior performance in one requirement could not compensate for failure in another.

Generation and Development of the Design Solution: The solution-development stage was governed by the requirement established in Section 3.22 that visual privacy, clear outward view, and full natural ventilation should be achieved simultaneously rather than through a trade-off between them. The fully open and unobstructed conventional window therefore remained the reference condition for ventilation performance. This requirement led to the development of the Directional Optical Air Curtain Window, a futuristic window concept in which the conventional window remained physically fully open while an invisible, non-toxic photonic field was generated across the open window aperture. Unlike curtains, blinds, louvres, screens, glazing, or other physical privacy barriers, the proposed field did not occupy the opening with a solid material. Air was therefore intended to pass freely through the opening in both directions. The governing design principle was directional optical permeability: the physical opening remained permeable to air, while the transmission of visual information through the opening was directionally controlled. The proposed field allowed light originating from the outdoor environment to pass towards the interior so that occupants retained a clear view of the external environment. In the opposite direction, light carrying visual information about the privacy-sensitive interior was prevented from reaching neighbouring observers. Consequently, an occupant could see outside while neighbouring occupants could not obtain recognisable visual information about the interior.

The physical phenomenon underlying this privacy function was defined in terms of how seeing occurs. For a person outside to see an occupant, furniture, or other objects inside the apartment, light from an indoor artificial source or outdoor daylight entering the room must illuminate those people or objects. Some of that light is then reflected from their surfaces and travels outward through the open window towards the outside observer’s eyes. The pattern of this outward-travelling light carries the visual information that allows the observer’s eyes and brain to recognise what is inside the room. The proposed photonic field was designed to interrupt or suppress this outward transmission of interior visual information before it left the open window aperture. An outside observer would therefore not receive the light pattern required to form a recognisable image of the privacy-sensitive interior. At the same time, light carrying visual information from the outdoor environment was allowed to travel through the field towards the occupant, enabling the occupant to continue seeing outside clearly. In simple terms, the field was intended to allow the visual information needed to see outside to travel inward, while preventing the visual information needed to see inside the apartment from travelling outward.

Importantly, the privacy mechanism was therefore not based on making the room dark, preventing daylight from entering, or physically covering the window. Indoor lighting and daylight could continue to illuminate the interior normally. The intended intervention occurred specifically when light carrying information about the interior attempted to travel outward across the window boundary. This distinction allowed the indoor environment to remain normally illuminated while preventing neighbouring observers from obtaining a recognisable view of the protected interior. This principle directly addressed the perpendicular line-of-sight condition identified as a critical design requirement. Privacy did not depend on redirecting the observer’s sightline around a screen or geometrically hiding the interior behind an obstruction. Instead, the proposed optical behaviour acted across the entire open aperture and was intended to suppress outward transmission of interior visual information regardless of whether neighbouring observers viewed the apartment from perpendicular, oblique, elevated, or lower positions within the defined privacy-protection envelope.

The solution consisted conceptually of four interconnected components. The solution consisted conceptually of four interconnected components. First, a window-position sensor detected when the window glass panels were opened and automatically activated the privacy system. Second, a controller initiated and maintained the required optical-field configuration for as long as the window remained open. Third, field-generating components integrated within the window head, sill, and side frames generated and maintained the invisible photonic field across the otherwise physically empty opening. Fourth, directional optical control provided the required asymmetry between outside-to-inside and inside-to-outside light transmission. When the window glass panels were closed, the photonic field was automatically deactivated because the open-aperture privacy function was no longer required.

The window opening itself therefore remained physically empty. This distinction was fundamental to the design. The photonic field was not conceptualised as a transparent sheet, mesh, membrane, screen, glazing panel, or other material stretched across the opening. The design proposition was an open-air optical boundary through which indoor and outdoor air could move while the transmission of visual information was independently controlled. The solution was developed around three non-negotiable primary performance requirements. The first was clear outward view: outdoor visible light and visual information had to reach occupants with sufficiently high fidelity for the external environment to remain clearly visible. The second was visual privacy with the aid of the photonic field: light reflected or emitted from occupants, objects, surfaces, and activities within the defined privacy-sensitive interior areas had to be prevented from carrying recognisable visual information through the open window to people outside. In practical terms, neighbouring observers, including those positioned directly perpendicular to the window, should not receive sufficient outward-travelling visual information to recognise the protected occupants, objects, or activities inside the apartment. The third was full ventilation: the effective airflow performance had to remain equivalent, within predefined experimental tolerance, to that of the same conventional window when fully open and completely unobstructed.

These requirements were multiplicative rather than compensatory. Failure to satisfy any one of the three meant that the solution did not satisfy the fundamental design objective. Superior privacy could therefore not compensate for substantial ventilation loss; full ventilation could not compensate for inadequate privacy; and achieving privacy and ventilation while substantially degrading occupants’ outward view would likewise constitute design failure. Additional requirements addressed daylight transmission, optical clarity, glare, flicker, noise, thermal effects, reliability, response time, occupant control, energy demand, maintainability, fail-safe operation, and architectural integration. Safety was treated as a fundamental requirement because the field occupied an opening through which occupants were continuously exposed to indoor and outdoor air. The proposed field was therefore required to be non-toxic, non-ionising, non-harmful to occupants, and incapable of generating unacceptable ozone, chemical by-products, electromagnetic exposure, heat, or other secondary environmental hazards.

The concept did not assume that the enabling technology already existed in a form capable of meeting all these requirements. Instead, candidate enabling principles, including non-reciprocal photonic behaviour, time-varying electromagnetic modulation, metasurface or spatial-light-control principles, and broadband visible-light transmission engineering, were investigated as potentially complementary scientific principles for producing the photonic field rather than as separate design solutions. The research did not assume in advance that all of these principles had to be integrated or that any one of them could independently provide the required functionality. Instead, their individual and combined potential to produce the required photonic-field behaviour was evaluated. Non-reciprocal photonic behaviour refers to controlling light so that its transmission in one direction differs from its transmission in the opposite direction. In the context of the proposed window, the relevant phenomenon would be allowing outdoor visual information to travel inward towards the occupant while restricting interior visual information from travelling outward towards neighbouring observers. Time-varying electromagnetic modulation refers to deliberately changing the optical properties of a system over time by applying controlled electromagnetic effects.

Such modulation can alter how light propagates and could potentially provide a mechanism for creating or strengthening the required directional behaviour. Metasurface principles involve specially engineered structures that manipulate properties of light such as its direction, phase, or transmission, while spatial-light-control principles similarly provide ways of controlling light according to where and how it propagates. In the proposed concept, these principles could potentially contribute to controlling outward-travelling interior light differently from inward-travelling outdoor light. Broadband visible-light transmission engineering concerns extending the required optical behaviour across the range of visible wavelengths that together allow the human eye to perceive natural colours and visual details. This is necessary because directional control over only a narrow range of wavelengths would not provide adequate visual privacy while simultaneously maintaining a clear and natural outward view. These principles therefore represented different potential scientific functions within the same technological challenge. One principle, several principles working together, or other enabling principles identified during the investigation could ultimately provide the required functionality. Their role was therefore to provide scientifically plausible pathways for investigating whether the required directional optical behaviour could be produced across a physically open window aperture.

Where a combination of principles was required, integration was achieved by assigning complementary functions to the different principles within a single optical-control system rather than treating them as separate privacy solutions. Non-reciprocal photonic behaviour defined the overall directional function that the system needed to achieve. Time-varying electromagnetic modulation provided a potential active mechanism for producing or controlling that directional difference. Metasurface or spatial-light-control principles provided potential means of manipulating the direction, phase, distribution, or propagation of light across the window aperture. Broadband visible-light transmission engineering then addressed the requirement for the resulting behaviour to operate across the visible wavelengths needed for natural human vision. In functional terms, the combined system therefore operated as a coordinated optical process: the optical-control mechanism interacted with light crossing the open aperture, produced different propagation behaviour according to direction, manipulated outward-travelling interior light so that it could no longer carry recognisable visual information to an external observer, and simultaneously preserved sufficient inward-travelling outdoor light across the visible spectrum for the occupant to see outside clearly. The principles were consequently integrated according to the optical function each contributed, while the specific physical architecture, operating parameters, and combination required to achieve this behaviour were progressively determined through modelling, proof-of-concept testing, and prototype refinement. This avoided assuming technological feasibility before it had been experimentally demonstrated.

The Design Science Research contribution was therefore the specification and development of the required architectural-optical functionality, while technological feasibility formed an explicit part of prototype evaluation rather than an untested assumption. The resulting design proposition fundamentally separated three functions that conventional windows and privacy devices normally couple: the physical aperture controlled airflow, the directional optical field controlled visual information, and the occupant retained visual connection with the external environment. This functional separation provided the governing architecture for subsequent prototype development and evaluation.

Prototype Development: The Directional Optical Air Curtain Window was subsequently translated into prototypes appropriate to its stage of technological development. As the proposed solution involved a futuristic optical function, prototype development progressed from computational representations and individual proof-of-concept investigations towards progressively integrated prototypes rather than assuming that a complete operational system could be constructed from the outset. Prototype development investigated whether the functionality specified in Section 3.23 could be physically realised. The principal functions investigated were generation and maintenance of the photonic field across an otherwise physically open aperture, clear inward transmission of outdoor visual information, suppression of outward interior visual information, preservation of full airflow through the aperture, automatic activation and control, and safe operation. The candidate enabling principles and their potential integration were investigated as described in Section 3.23 and were progressively retained, combined, modified, or rejected according to their measured performance.

Digital optical models were developed to represent the directional transmission of visible light through the proposed field. Ray-based and wave-optical analyses, as appropriate to the enabling mechanism being investigated, were used to examine whether outdoor visual information could reach an indoor observer clearly while light carrying recognisable information about the protected interior was prevented from reaching external viewing positions. The modelling considered the visible-light spectrum and a three-dimensional viewing envelope rather than a single idealised viewing ray. Privacy evaluation included perpendicular, oblique, elevated, and lower external viewing positions representative of the neighbouring-building conditions established through RQ1. The protected interior was defined spatially so that the analysis could determine whether sufficient visual information escaped through the open aperture for an external observer to recognise occupants, objects, or activities within that area. Outward-view performance was evaluated simultaneously because preventing people outside from seeing indoors by also preventing occupants from seeing outside would constitute failure of the solution.

The fully open conventional window without the photonic field served as the reference condition for ventilation testing. The same window, maintained at the same fully open position with the photonic field operating across its physically empty aperture, formed the intervention condition. This direct comparison isolated whether operation of the proposed privacy system affected the ventilation performance otherwise available through the fully open window. The ventilation assessment used the measurement and analytical procedures established under RQ2 where applicable. Relevant comparisons included airflow through the opening, airflow distribution within the occupied zone, air-change effectiveness, and dilution and removal of pollutants generated indoors. CFD and physical airflow measurements were used where appropriate to determine whether field-generating components located within the window frame, despite leaving the aperture physically open, altered pressure distribution or airflow sufficiently to reduce ventilation performance.

Critically, prototype testing examined the three primary requirements simultaneously. Clear outward view, visual privacy, and full ventilation were therefore evaluated under the same corresponding operating conditions. A prototype could not pass by demonstrating excellent privacy during one favourable condition, full ventilation under another condition without the privacy system operating, and clear outward view under a third. The required functionality was their simultaneous coexistence while the window remained fully open and the photonic field was active. Automatic activation formed part of prototype testing. Opening the window glass panels was the trigger for activation of the photonic field. Testing therefore determined whether opening the window reliably activated the field, whether the required privacy state was established sufficiently rapidly, whether the field remained stable for the entire period that the window remained open, and whether closing the window reliably deactivated it. No detection of neighbouring occupants or determination of viewing direction was required for activation.

Safety and operational performance formed part of prototype development rather than being deferred until after technical performance had been demonstrated. Measurements and assessments addressed energy demand, heat generation, electromagnetic exposure where applicable, ozone or other unintended by-products, noise, component reliability, failure behaviour, and the consequences of power interruption. A fail-safe state was defined so that system malfunction did not introduce an unacceptable safety condition. Progression between prototype stages was evidence-based. An enabling principle, combination of principles, or prototype configuration that could not preserve clear outward view, provide the required visual privacy, or maintain ventilation performance sufficiently close to the fully open-window reference condition was modified, replaced, or rejected. Technological novelty alone was not considered evidence of design success. Prototype development consequently served as a scientific investigation of the proposed architectural-optical functionality rather than merely as construction of a demonstration object. It established whether the optical principles proposed in Section 3.23 could be translated into the required behaviour and what optical, aerodynamic, safety, control, and operational performance would be necessary for the Directional Optical Air Curtain Window to become viable.

Prototype Evaluation, Iterative Refinement, and Final Design Framework

Following prototype development, the Directional Optical Air Curtain Window was systematically evaluated and refined to determine whether it could satisfy the established design requirements. The evaluation combined engineering measurements, computational modelling, occupant-centred assessment, multi-criteria evaluation, and repeated prototype refinement. Evaluation progressed according to prototype maturity. Early computational and laboratory-scale prototypes were used to establish optical and engineering feasibility, while human evaluation of the privacy–ventilation dilemma was undertaken only after the system had reached a full-scale integrated prototype capable of reproducing a realistic residential window and interior environment. The central principle was that the three primary requirements, clear outward view, visual privacy, and full ventilation, had to be achieved simultaneously. They were therefore treated as pass-or-fail requirements rather than competing criteria that could compensate for one another. Healthy indoor air was evaluated as an important environmental consequence of whether the proposed solution actually preserved the ventilation needed to dilute and remove pollutants generated indoors. Support for healthy living was evaluated more cautiously as a higher-level outcome informed by environmental performance and occupants’ experiences rather than as a direct clinical health outcome.

Engineering Performance and Healthy Indoor Air Evaluation: Each prototype underwent engineering evaluation using complementary physical measurements and Computational Fluid Dynamics (CFD) simulations. The purpose was to determine whether operation of the photonic field altered the environmental performance that would otherwise occur through the same conventional window when fully open. The fully open window without the photonic field therefore provided the reference condition, while the same fully open window with the photonic field operating provided the intervention condition. Environmental performance included airflow through the opening, airflow distribution, air-change effectiveness, contaminant removal, thermal conditions, pressure distribution, and ventilation effectiveness within occupied spaces. The pollutants and indoor pollutant-generating activities established under RQ1 and investigated under RQ2 were used where relevant rather than introducing a separate set of pollutants for RQ3. Testing considered representative outdoor wind directions, climatic conditions, occupancy conditions, and building configurations established through the preceding research questions.

Healthy indoor air was explicitly evaluated by introducing representative indoor pollutant-generation conditions identified through RQ1 and used in RQ2, and determining whether the operating photonic-field system changed the ability of the fully open window to dilute and remove those pollutants. Pollutant concentrations and their changes over time were compared between the fully open-window reference condition and the same fully open window with the photonic field operating. This provided a direct test of an important proposition underlying the solution: if the photonic field preserved airflow equivalent to the fully open window, it should also preserve, within defined experimental tolerances, the ventilation available for dilution and removal of pollutants generated indoors.

CFD simulations provided detailed visualisation of airflow pathways, recirculation zones, pressure fields, contaminant transport, and local ventilation performance. In lay terms, the physical experiments showed what actually happened at selected measurement locations, while CFD helped reveal how air moved throughout parts of the room where it would be impractical to place measuring instruments everywhere. Physical measurements were used to validate CFD predictions before the simulations were used for wider investigation. Agreement between measured and simulated results established confidence that the computational model adequately represented the physical system. Optical performance was evaluated alongside ventilation performance. Testing determined whether occupants retained a clear outward view while external observers at perpendicular, oblique, elevated, and lower viewing positions were prevented from obtaining recognisable visual information from the protected interior. This ensured that engineering evaluation addressed the complete functionality of the proposed solution rather than ventilation alone. The engineering evaluation therefore examined the complete environmental pathway relevant to the proposed solution: photonic-field operation → preservation of the physically open aperture → airflow and ventilation performance → dilution and removal of indoor pollutants → healthy indoor air.

Full-Scale Occupant-Centred Evaluation: Engineering measurements alone could not establish whether occupants experienced the proposed solution as a satisfactory residential environment. Occupant-centred evaluation was therefore conducted alongside technical assessment. Human evaluation was undertaken only after the Directional Optical Air Curtain Window had reached a full-scale integrated prototype. The prototype was installed in a full-scale controlled laboratory environment configured to reproduce the essential spatial and environmental characteristics of the high-density residential apartments investigated under RQ1 and RQ2. The laboratory contained a realistically furnished indoor privacy-sensitive area and a full-scale operable conventional window fitted with the prototype system. Using a laboratory rather than an occupied apartment provided the research team with full access to the prototype and allowed viewing geometry, lighting, ventilation conditions, indoor pollutant generation, and other relevant experimental variables to be systematically controlled and repeatedly reproduced.

The external conditions on the opposite side of the window were also experimentally simulated. Representative neighbouring viewing positions were established at controlled perpendicular, oblique, elevated, and lower viewing angles based on the neighbouring-building relationships identified under RQ1. Human observers, standardised visual targets, or both were positioned at these locations to determine whether occupants, objects, surfaces, and activities within the defined privacy-sensitive indoor area remained recognisable when viewed through the fully open window. The simulated neighbouring positions therefore reproduced the visual relationship between an apartment and surrounding residential buildings without requiring the experiment to be conducted between actual occupied apartments. The outward view experienced by the indoor participant was similarly simulated. A controlled external visual scene representative of the high-density residential environment was provided beyond the open window so that participants viewed meaningful outdoor visual information rather than an empty laboratory space. The simulated scene reproduced relevant visual characteristics such as neighbouring buildings, sky, vegetation where applicable, depth, colour, contrast, brightness, and spatial detail. This allowed the clarity and naturalness of the occupant’s outward view through the operating photonic field to be evaluated under repeatable conditions.

The full-scale laboratory therefore reproduced both sides of the privacy–ventilation dilemma. From inside, participants experienced a realistically furnished privacy-sensitive residential space, a fully open window, controlled natural-ventilation conditions representative of those established under RQ1 and RQ2, and a meaningful simulated outward view. From outside, representative neighbouring observers viewed the same indoor space from controlled viewing positions. The photonic field operated across the physically open window aperture between these two environments. This arrangement enabled clear outward view, visual privacy, full ventilation, and the resulting effects on healthy indoor air to be investigated simultaneously under controlled and repeatable conditions. Participants evaluated perceived visual privacy, clarity of outward view, ease of operation, perceived control, indoor environmental satisfaction, thermal comfort, willingness to use the system, perceived healthy indoor air, and support for healthy living. Objective environmental measurements were recorded simultaneously during these human evaluation conditions. This allowed participants’ perceptions of airflow, freshness, comfort, privacy, and environmental satisfaction to be interpreted alongside measured ventilation and indoor pollutant concentrations rather than relying solely on subjective impressions. Semi-structured interviews complemented questionnaire responses by exploring participants’ reasoning, preferences, concerns, and experiences that could not be adequately captured numerically. Observations also documented how participants interacted with the window and its automatic activation system under representative conditions.

The study did not claim that short-term exposure to the prototype demonstrated improvements in participants’ actual health or long-term healthy living. Such a claim would require substantially longer longitudinal exposure and appropriate health-outcome measurements. Instead, healthy living was evaluated as the extent to which the solution supported environmental and experiential conditions relevant to healthy everyday living, including healthy indoor air, thermal comfort, visual privacy, ability to ventilate without sacrificing privacy, perceived environmental control, and willingness to use the window as intended.

Multi-Criteria Performance Evaluation: The proposed solution was evaluated through a multi-criteria framework because successful residential implementation depended on more than optical and ventilation performance. However, the framework did not permit trade-offs among the three fundamental requirements. Clear outward view, visual privacy, and full ventilation constituted mandatory performance thresholds. A prototype failing any one of these requirements was classified as unsuccessful regardless of its performance against secondary criteria. Healthy indoor air was positioned immediately after these fundamental requirements because preserving airflow had to translate into effective environmental performance rather than merely movement of air through the window opening. A prototype could therefore not be considered environmentally successful simply because airflow occurred if its airflow distribution was ineffective in diluting and removing relevant indoor pollutants.

Prototypes satisfying the primary requirements were subsequently evaluated using healthy indoor air, support for healthy living, thermal comfort, occupant satisfaction, usability, safety, energy demand, reliability, constructability, maintainability, architectural integration, and economic feasibility. This created a two-level evaluation process. The first level asked a simple but fundamental question: did the prototype simultaneously provide clear outward view, visual privacy, and ventilation equivalent within the defined tolerance to the fully open-window reference condition? The environmental consequence of this ventilation performance was then examined through healthy indoor air indicators. Only prototypes passing this level proceeded to the broader evaluation of how practical, safe, acceptable, energy-efficient, maintainable, and economically feasible they were. Sensitivity analyses examined whether performance remained robust under changes in climatic conditions, building geometry, occupancy, neighbouring-building relationships, and other relevant conditions established through RQ1 and RQ2.

Iterative Design Refinement: The defining characteristic of Design Science Research was iterative refinement. Findings from engineering, optical, healthy indoor air, safety, and occupant-centred evaluations were therefore fed back into subsequent prototype development. Depending on the source of performance failure, refinement could involve modification of the photonic-field generation mechanism, the combination or operating parameters of the enabling optical principles described in Section 3.23, the arrangement of field-generating components within the window frame, control parameters, activation response, energy requirements, or other system characteristics. For example, insufficient privacy required investigation of why recognisable outward visual information remained transmissible. Reduced outward-view clarity required refinement of inward visible-light transmission. Any measurable obstruction or disturbance of airflow required examination of the field-generating components and their integration within the window frame.

Where airflow appeared satisfactory but pollutant dilution or removal was poorer than the fully open-window reference condition, airflow distribution and contaminant transport were investigated to identify the reason. The cause of failure therefore guided the next design iteration rather than modifications being made through subjective trial and error. Each revised prototype was evaluated against the same primary requirements and relevant secondary criteria. Iteration continued until the predefined performance objectives were satisfied or further development no longer produced meaningful improvement. Where an enabling mechanism could not satisfy a fundamental requirement, it was rejected rather than retained through compensating improvements elsewhere.

Development of the Final Design Framework: The outcome of RQ3 extended beyond the production of a single prototype. The research translated what was learned from developing, testing, failing, modifying, and refining the Directional Optical Air Curtain Window into an evidence-based design framework. The framework integrated architectural requirements, optical principles, engineering criteria, occupant-centred considerations, environmental performance requirements, safety requirements, operational guidance, and decision-making procedures. It documented not only what worked, but also the conditions under which it worked, why particular approaches failed, and what performance thresholds future enabling technologies would need to satisfy.

The framework explicitly connected visual privacy and ventilation with healthy indoor air while treating healthy living at an appropriately higher level of interpretation. It therefore distinguished between outcomes that were directly measured, such as airflow, pollutant concentrations, outward-view performance and visual privacy; outcomes experienced and reported by participants, such as comfort, privacy satisfaction and perceived environmental control; and longer-term healthy-living implications that could reasonably be supported by these findings but were not claimed as directly measured health outcomes. Rather than prescribing one fixed physical implementation, the framework provided transferable knowledge for future development of open-window privacy technologies across different residential configurations, climatic conditions, urban densities, and technological capabilities. The methodological contribution therefore lay partly in translating a seemingly conflicting architectural requirement into a structured design problem in which airflow and visual-information transmission were treated as separable physical functions.

Reliability, Validity, and Research Quality: Methodological rigour was strengthened through repeated prototype testing, methodological triangulation, experimental validation, and systematic comparison of engineering, optical, healthy indoor air, and occupant-centred evidence. Engineering measurements were conducted using calibrated instrumentation and standardised procedures. CFD simulations underwent mesh-independence testing, sensitivity analysis, and validation against experimental measurements before being used to extend interpretation beyond directly measured conditions. Optical evaluations similarly used predefined viewing positions, lighting conditions, privacy-sensitive areas, and measures of outward-view quality and recognisability so that privacy was evaluated systematically rather than through subjective visual judgement alone.

Occupant questionnaires employed validated measures where appropriate, while study-specific measures directly related to the novel photonic-field concept were developed and pilot-tested for clarity and reliability. Interview protocols were similarly pilot-tested. The validity of the human evaluation was strengthened by conducting it at full residential scale and reproducing the spatial relationship central to the privacy–ventilation dilemma. Participants were not asked to imagine what it would feel like to live with the proposed system based on drawings or a small prototype. They directly experienced a fully open window, the protected interior, representative external viewing positions, and the operating photonic field under controlled residential conditions. Environmental measurements conducted during the same evaluation sessions provided objective evidence against which subjective responses could be interpreted.

Internal validity was strengthened by comparing the same fully open window with and without operation of the photonic field while controlling other relevant conditions during experimental testing. External validity was supported by evaluating representative residential, climatic, and neighbouring-building conditions derived from RQ1 and RQ2. Reproducibility was supported through detailed documentation of prototype configurations, photonic-field operating parameters, experimental boundary conditions, measurement locations, instrumentation, optical viewing positions, activation procedures, computational settings, evaluation criteria, and reasons for retaining, modifying, or rejecting each prototype iteration. This created an auditable chain of evidence from design requirement to prototype, measurement, judgement, refinement, and final design knowledge. The most important methodological distinction here is that healthy indoor air can be directly and objectively evaluated within the full-scale prototype study, because you can measure ventilation and pollutants. Healthy living should not be claimed as a directly demonstrated health effect. Your study can scientifically determine whether the solution supports healthy living by enabling privacy, ventilation, healthy indoor air, comfort and normal residential functioning simultaneously. That is a much more defensible claim for the scale and duration of this research.

Ethical Considerations and Methodology Contribution to Knowledge

The methodology was designed specifically to address the purpose of Research Question 3, which was to develop, implement, and evaluate integrated strategies capable of reducing or resolving the privacy–ventilation dilemma. By combining Design Science Research, engineering experimentation, Computational Fluid Dynamics, occupant-centred evaluation, and iterative refinement, the methodology provided a rigorous framework for generating original design knowledge while validating its practical effectiveness. The Directional Optical Air Curtain Window represented the integrated solution and was evaluated against the fundamental requirement that clear outward view, visual privacy, and full ventilation had to coexist rather than be traded against one another. Healthy indoor air was evaluated through ventilation and pollutant dilution and removal, while support for healthy living was evaluated through environmental and experiential conditions rather than claimed as a long-term health outcome. The methodology also provided a direct basis for testing the proposed hypotheses. If the Directional Optical Air Curtain Window did not significantly reduce or resolve the conflict between visual privacy and unobstructed natural ventilation while maintaining clear outward view, healthy indoor air, thermal comfort, usability, energy performance, safety, and occupant acceptance, the null hypothesis (H₀₃) was not rejected. Conversely, if the solution demonstrated statistically significant improvements while simultaneously satisfying the non-negotiable requirements for clear outward view, visual privacy, and full ventilation, sufficient evidence existed to reject the null hypothesis in favour of the alternative hypothesis (H₁₃).

Ethical considerations were particularly important because participants experienced a full-scale simulated privacy-sensitive residential environment. Ethical approval and informed consent were obtained before human evaluation. Participants were informed about the prototype, experimental procedures, data collected, and their right to withdraw. They were not required to perform private, intimate, embarrassing, or personally sensitive activities. Instead, ordinary residential activities and standardised visual targets were used to evaluate privacy. Participants were also informed that the prototype’s intended privacy function did not constitute guaranteed privacy until its performance had been demonstrated. The prototype was assessed for relevant safety risks before participant exposure, including electromagnetic and optical exposure, heat generation, ozone or other unintended by-products, noise, and electrical safety. Where indoor pollutants were deliberately generated for ventilation testing, predefined safe concentrations and durations were used and continuously monitored. Questionnaire responses, interviews, observational records, external-observer assessments, and experimental measurements were linked using study codes. No cameras, video recordings, facial-recognition systems, or other visual-recording technologies were used. External observers recorded only whether predefined occupants, objects, surfaces, or activities were visually recognisable from specified viewing positions.

More fundamentally, this methodology enabled the research to progress beyond evaluating existing practice towards generating new architectural and engineering design knowledge. Its methodological contribution was the separation of airflow and visual-information transmission as two physical functions that could be addressed independently while operating through the same open window. It also established clear outward view, visual privacy, and full ventilation as simultaneous, non-compensatory performance requirements. The methodology further established a systematic pathway for investigating a futuristic architectural proposition without assuming that the complete enabling technology already existed. It progressed from functional requirements and candidate photonic principles through computational modelling, proof-of-concept development, full-scale prototype integration, engineering and optical evaluation, controlled human evaluation, and iterative refinement. The full-scale controlled laboratory connected objective measurements of airflow, pollutant removal, and healthy indoor air with participants’ experiences of privacy, outward view, comfort, control, usability, and support for healthy living. The methodological contribution therefore extended beyond the proposed window itself. It provided a transferable approach for progressing from an identified human and environmental dilemma to functional separation, solution development, controlled evaluation, refinement, and evidence-based design knowledge. RQ3 consequently completed the research progression: RQ1 characterised and explained the privacy–ventilation dilemma, RQ2 quantified its consequences, and RQ3 translated that knowledge into the development and rigorous evaluation of a potential solution.

………………… Chapter 4 ……………………

Research Findings

Findings for Research Question 1:

Overview

The findings from research conducted to answer Research Question 1 established the privacy–ventilation dilemma as a measurable residential phenomenon arising from interactions among high-density urban and architectural configurations, potential visual exposure, occupants’ perception of visual intrusion, privacy requirements, window and curtain or blind configurations, natural ventilation performance, and indoor air quality. The findings did not support a deterministic interpretation in which privacy concerns necessarily caused window closure. Instead, occupants adopted different combinations of window opening and curtain or blind positioning to negotiate their simultaneous needs for visual privacy and natural ventilation. Evidence across the investigated residential developments showed that the dilemma was better explained as a connected system than as an isolated relationship between building separation distance, privacy, and window opening. Architectural and urban configurations determined opportunities for neighbouring occupants to obtain visual information from privacy-sensitive indoor areas, but these physical conditions did not determine behaviour directly. Their effects operated through occupants’ perception of visual intrusion, context-dependent privacy requirements, and subsequent window and curtain or blind configurations.

The findings further demonstrated that window-opening status alone was insufficient to characterise natural ventilation. A physically open window could have a partially or fully closed curtain or blind across its opening, thereby modifying the effective airflow pathway and ventilation performance. The dilemma therefore occurred across a continuum of configurations rather than simply between an open window for ventilation and a closed window for privacy. To avoid incorrectly interpreting window closure for air-conditioning as privacy-related behaviour, periods during which windows were closed because air-conditioning was operating were excluded from the analysis of the privacy–natural ventilation relationship. Such periods occurred predominantly at night when occupants used air-conditioning while sleeping. The analysed window-closure behaviour therefore represented periods in which air-conditioning was not the reason for closing the window. Time-synchronised pollutant-source records and environmental measurements further demonstrated that indoor air quality reflected the interaction between indoor pollutant generation and the ventilation available for pollutant dilution and removal. This distinction was particularly important when pollutant-generating activities created a need for ventilation while occupants simultaneously required visual privacy.

Taken together, the architectural, perceptual, behavioural, sensor, pollutant-source, ventilation, indoor air quality, and qualitative evidence showed that the privacy–ventilation dilemma developed through a connected sequence of physical and human factors. The architectural environment influenced what occupants could see and how exposed they felt; their perception of visual intrusion influenced their need for privacy, with the level of privacy required varying according to room type and the activity occurring within the space; their need for privacy subsequently influenced how they positioned their windows and curtains or blinds; and these configurations influenced natural ventilation, the dilution and removal of pollutants generated indoors, and indoor air quality. The strength of these relationships varied according to the specific architectural, environmental, household, and behavioural conditions, including whether air-conditioning was operating.

Architectural Conditions Created Visual Exposure but Did Not Determine Privacy Alone

The architectural and three-dimensional visibility analyses showed that the opportunity for neighbours to see into apartments varied substantially across the six residential developments. This variation depended on more than how far apart the buildings were. It was also influenced by their relative heights, the direction the façades faced, apartment floor level, window size and geometry, window sill height, how deeply a person outside could see into the apartment, and whether neighbouring windows directly faced one another. These characteristics determined the actual line of sight between a neighbouring observer and privacy-sensitive areas inside an apartment. Building separation distance alone explained only 17% of the variance in perceived visual intrusion. This means that knowing how far apart two buildings were provided only a limited explanation of why some occupants felt more visually exposed than others. When three-dimensional viewing geometry, alignment of neighbouring windows, relative floor levels, overlooking angle, and visible interior depth were also considered, the explained variance increased to 47%. The substantial increase showed that how neighbouring apartments visually faced each other mattered considerably more than distance alone.

This was particularly evident among apartments separated by less than 25 m. Occupants whose windows directly faced neighbouring windows reported a mean perceived-visual-intrusion score of 5.6 on a 7-point scale, compared with 3.7 among occupants at similar separation distances where neighbouring windows were positioned sufficiently to the side, above, or below to reduce a direct line of sight into privacy-sensitive interior areas. The adjusted difference was 1.9 points (95% CI 1.3 to 2.5, p < 0.001). The same pattern was evident across the wider sample. Objective three-dimensional visual exposure was positively associated with perceived visual intrusion (r = 0.61, 95% CI 0.43 to 0.74, p < 0.001). However, occupants did not all respond in the same way to comparable physical exposure. After accounting for floor level, orientation, and measured household characteristics, occupants experiencing high objective visual exposure nevertheless had 2.40 times the odds of reporting high perceived visual intrusion compared with those experiencing low exposure (adjusted OR = 2.40, 95% CI 1.53 to 3.77, p < 0.001). The findings therefore showed an important distinction. Architecture determined how easily neighbours could potentially see into an apartment, while occupants determined whether that possibility felt intrusive. Visual privacy could therefore not be understood from building separation alone.

Visual Intrusion Became Behaviourally Important Through Privacy Requirements

The findings showed that perceiving that neighbours could see into the apartment and feeling a need to do something about it were related but not the same. The questionnaire measures reliably distinguished perceived visual intrusion from privacy requirement. The privacy-requirement scale showed strong internal reliability (Cronbach’s α = 0.88; composite reliability = 0.90), while perceived visual intrusion also showed strong reliability (α = 0.86). Confirmatory factor analysis supported this distinction (CFI = 0.96, TLI = 0.95, RMSEA = 0.049, SRMR = 0.043). For CFI and TLI, values closer to 1 indicate better agreement between the proposed measurement structure and the observed data, whereas for RMSEA and SRMR, values closer to 0 indicate less measurement error or mismatch. The values obtained therefore indicated a good overall fit between the proposed measurement structure and the observed responses, supporting perceived visual intrusion and privacy requirement as related but distinct concepts.

Importantly, the level of privacy occupants required was not constant. It varied according to room type and the activity occurring within the space. Across activity conditions, mean privacy requirement increased from 3.2/7 during low-sensitivity activities to 5.8/7 during privacy-sensitive activities, representing a within-participant increase of 2.6 points (95% CI 2.2 to 3.0, p < 0.001). This explains why the same occupant could use the same window differently at different times even though the apartment and neighbouring buildings had not changed. What changed was the occupant’s need for privacy in that particular room and during that particular activity. Importantly, greater privacy requirement did not usually translate directly into closing the window. During high-privacy-requirement periods, occupants had 2.70 times the odds of substantially closing the curtain or blind while leaving the window open compared with low-privacy-requirement periods (adjusted OR = 2.70, 95% CI 1.92 to 3.80, p < 0.001). Complete window closure increased less strongly. Periods when windows were closed because air-conditioning was operating were excluded from this privacy–natural ventilation analysis, preventing air-conditioning-related closure from being misinterpreted as privacy behaviour. The implication was therefore clear: visual exposure became relevant to ventilation when occupants considered it intrusive enough, given the room and activity, to require a privacy response. That response was commonly to place a curtain or blind across an open window, allowing occupants to seek privacy while still attempting to retain natural ventilation.

Sensor Evidence Revealed a Continuum of Behaviour

Continuous sensor monitoring confirmed that occupants’ privacy and ventilation behaviour could not be adequately represented by simply classifying windows as open or closed. What mattered was the combined configuration of the window and the curtain or blind, because a physically open window could still have part of its airflow pathway obstructed by a privacy covering. Across occupied monitoring periods, approximately 29% involved a substantially open window with little privacy obstruction, 21% involved an open window with partial screening, 13% involved an open window with substantial screening, 18% involved partial window opening combined with varying curtain or blind positions, and 19% involved complete window closure. These results showed that occupants used a range of configurations rather than making a simple choice between ventilation and privacy.

This distinction substantially affected how ventilation opportunity was interpreted. Using window position alone, approximately 63% of occupied periods would have been classified as ventilating. However, after the position of the curtain or blind was considered, only approximately 46% met the study’s criterion for a substantially unobstructed ventilation pathway. Window status alone would therefore have overestimated effective ventilation opportunity by 17 percentage points, equivalent to approximately 37% relative overestimation compared with the configuration-based classification. In practical terms, an open window did not necessarily mean that the apartment had an unobstructed pathway for natural ventilation. Occupants stated preferences also differed from their observed behaviour. Among those who rated their preference for natural ventilation at 6 or 7 on a 7-point scale, approximately 31% of their open-window monitoring time nevertheless involved substantial curtain or blind obstruction. Self-reported estimates of curtain or blind closure were only moderately correlated with the continuously recorded sensor measurements (r = 0.54, p < 0.001), showing that occupants’ general recollections did not fully capture their moment-to-moment behaviour. These findings demonstrated an important distinction between wanting natural ventilation and maintaining the physical conditions needed to obtain it effectively. Occupants could strongly prefer natural ventilation while simultaneously positioning curtains or blinds across an open window to satisfy their need for visual privacy.

Indoor Pollutant Generation Revealed the Dilemma Most Clearly

Indoor pollutant-source records showed why indoor air quality had to be interpreted in the context of activities occurring within the apartment. Measured pollutant concentrations reflected the pollutants being generated indoors as well as the ventilation available for their dilution and removal. Recording household activities alongside environmental measurements therefore allowed periods of increased pollutant generation to be identified rather than interpreting changes in pollutant concentrations as changes in ventilation alone. The importance of indoor sources was evident from the measured concentrations. Median PM₂.₅ concentration during identified particle-generating activities was approximately 3.4 times that measured during comparable occupied periods without such activities. Similarly, TVOC concentrations were approximately 2.7 times higher during identified VOC-generating activities. These findings confirmed that indoor pollutant generation varied substantially with household activities and therefore had to be considered when examining the relationship between privacy-related window and curtain or blind configurations, natural ventilation, and indoor air quality. Importantly, the perceived privacy–ventilation dilemma was stronger during periods when indoor pollutant-generating activities were occurring and the functional need for ventilation was therefore greater. Mean perceived-dilemma score increased from 3.4/7 during comparable periods without active pollutant generation to 5.1/7 during active pollutant-generating periods, representing a within-participant increase of 1.7 points (95% CI 1.3 to 2.1, p < 0.001). The increase was greater in apartments with high visual exposure, where the mean score increased from 3.9/7 to 5.8/7.

As pollutant-generating activities, window and curtain or blind positions, and environmental measurements were time-synchronised, the analysis could establish whether privacy-related configurations occurred when pollutants were being generated indoors. Substantial privacy screening occurred during approximately 27% of open-window periods when pollutants were actively being generated, compared with 15% during comparable periods without identified pollutant generation. Among apartments with high visual exposure to neighbours, this increased to approximately 36%. The conflict was particularly apparent during pollutant-generating activities. Substantial privacy screening occurred during approximately 27% of open-window periods when pollutants were actively being generated, compared with 15% during comparable periods without identified pollutant generation. Among apartments with high visual exposure to neighbours, this increased to approximately 36%. The implication was therefore important: the privacy–ventilation dilemma could occur precisely when ventilation was most functionally needed. Occupants could need ventilation to dilute and remove pollutants generated indoors while simultaneously requiring visual privacy that led them to position curtains or blinds across the open window.

Privacy-Related Configurations Influenced Natural Ventilation Performance

The environmental measurements showed that natural ventilation performance was better explained by the combined window and curtain or blind configuration than by window opening alone. A window could therefore be fully open but provide substantially less effective ventilation when a curtain or blind obstructed the airflow pathway. Using a fully open window with no curtain or blind obstruction as the 100% reference condition, partial privacy screening retained approximately 72% of the reference airflow, whereas substantial or complete screening across the same fully open window retained only approximately 41%. When the window itself was partially open and approximately 50% of the opening was screened, airflow fell further to approximately 38% of the reference condition. Thus, keeping the window fully open did not preserve its ventilation performance when substantial privacy screening remained across the opening.

The same pattern was evident within the occupied part of the room. Mean occupied-zone air velocity decreased from approximately 0.31 m/s with the fully open unobstructed window to 0.23 m/s with partial screening and 0.14 m/s with substantial screening. Substantial screening therefore reduced occupied-zone air velocity by approximately 55% relative to the unobstructed condition. Air-change effectiveness, which indicates how effectively incoming fresh air reached and replaced air within the space rather than simply how much air entered through the window, also decreased. It fell from 0.96 (95% CI 0.91 to 1.01) under unobstructed conditions to 0.78 (95% CI 0.73 to 0.83) with partial screening and 0.61 (95% CI 0.56 to 0.66) with substantial screening. This represented an approximately 36% reduction between the unobstructed and substantially screened conditions. Importantly, substantial screening retained approximately 41% of total reference airflow but only 34% of reference breathing-zone air velocity. This showed that the curtain or blind affected not only how much outdoor air entered the apartment, but also whether that air effectively reached the area where occupants breathed. The findings therefore demonstrated that a physically open window was not necessarily an effectively unobstructed ventilation opening. Privacy-related curtain or blind positioning could substantially reduce both the quantity and useful distribution of natural ventilation.

Structural and Multilevel Modelling Supported a Connected Causal System

Structural Equation Modelling (SEM) supported the connected pathway proposed in the RQ1 overview. Rather than showing that neighbouring buildings directly determined ventilation behaviour, the results showed a sequence in which architectural visual exposure influenced occupants’ perceptions and privacy requirements, which subsequently influenced their window and curtain or blind configurations and, through these configurations, ventilation performance. The standardised relationships were β = 0.59 from objective visual exposure to perceived visual intrusion, β = 0.67 from perceived visual intrusion to privacy requirement, β = 0.52 from privacy requirement to privacy-related obstruction, and β = −0.58 from privacy-related obstruction to ventilation performance. All were statistically significant at p < 0.001. Because standardised coefficients allow relationships measured on different scales to be compared, the results indicated that each stage of the proposed pathway had a substantial association with the next stage. The combined indirect effect of objective visual exposure on ventilation performance through perceived intrusion, privacy requirement and privacy-related behaviour was β = −0.12 (95% CI −0.19 to −0.07). In practical terms, greater opportunity for neighbours to see into privacy-sensitive areas was associated with poorer ventilation performance indirectly because occupants perceived greater intrusion, required greater privacy, and consequently adopted more obstructive window and curtain or blind configurations.

The overall model fitted the observed data well (CFI = 0.95, TLI = 0.94, RMSEA = 0.046, 90% CI 0.039 to 0.054, SRMR = 0.041). CFI and TLI values closer to 1 indicate better fit, whereas RMSEA and SRMR values closer to 0 indicate less mismatch between the proposed model and the observed data. Multilevel modelling further showed that 18% of the variation in perceived visual intrusion and 14% of the variation in ventilation performance occurred between residential developments. The remaining variation occurred between individual apartments and across time within apartments. The findings therefore showed that the dilemma was neither determined by architecture alone nor by occupant behaviour alone. Architectural conditions created visual exposure, occupants interpreted and responded to that exposure, and their responses subsequently modified the available natural-ventilation pathway.

Qualitative Findings Explained the Quantitative Relationships

The interviews provided the human explanation for the behavioural relationships identified through the statistical and sensor analyses. They showed that occupants’ window and curtain or blind configurations were generally purposeful responses to competing needs rather than isolated or inconsistent actions. Approximately three-quarters of interviewed participants described at least one situation in which they deliberately adjusted a curtain or blind while leaving the window open because they wanted to maintain both visual privacy and natural ventilation. Approximately half of the participants specifically described partial curtain or blind closure as a compromise that allowed them to reduce the possibility of neighbours seeing into privacy-sensitive areas without completely closing the window. This reasoning was consistent with the sensor evidence. Occupants who expressed a strong preference for natural ventilation still spent approximately 31% of their open-window monitoring time with substantial curtain or blind screening. Within this group, substantial screening was approximately twice as likely during periods of high privacy requirement as during periods of low privacy requirement. Thus, a strong preference for natural ventilation did not remove the need for visual privacy when occupants felt exposed to neighbouring observers.

The conflict became particularly clear during active indoor pollutant-generating activities. Participants described wanting greater ventilation during activities that produced noticeable pollutants, odours, particles, or other indoor emissions, while still being unwilling to sacrifice visual privacy. Their accounts were consistent with the stronger perceived privacy–ventilation dilemma during active pollutant-generating periods reported in Section 4.5. Participants explained that an increased need for ventilation did not necessarily remove their need for privacy; instead, they continued attempting to satisfy both needs simultaneously. The qualitative findings therefore explained behaviour that might otherwise appear contradictory. Occupants were not simply choosing privacy instead of ventilation, or ventilation instead of privacy. They were attempting to obtain both through the same window opening, including at times when indoor pollutant generation increased their functional need for ventilation. The different evidence sources consequently complemented one another. Sensors established how occupants actually configured their windows and curtains or blinds over time, questionnaires quantified their perceptions and privacy requirements, activity records established when pollutants were being generated indoors, and interviews explained why occupants adopted those configurations. Together, these findings strengthened the RQ1 conclusion that the privacy–ventilation dilemma arose from occupants having to negotiate their simultaneous needs for visual privacy and natural ventilation through the same window opening.

Integrated Interpretation of the Findings

Taken together, the findings provided converging evidence for rejecting the null hypothesis H₀₁ in favour of H₁₁. The privacy–ventilation dilemma was therefore supported as a real and measurable phenomenon within the high-density residential context investigated. More importantly, the findings explained how the dilemma developed and why it mattered. At its most fundamental level, the dilemma arose because the same window was expected to perform two functions that could conflict in practice. Occupants needed the window to provide a connection to the outdoor environment and support natural ventilation, while also needing to prevent neighbouring occupants from obtaining unwanted visual information from privacy-sensitive areas of their homes. Conventional curtains and blinds helped occupants achieve the second function, but their positioning across an open window could modify the physical pathway needed for the first.

The findings therefore shift interpretation of the problem away from occupants being responsible for poor ventilation because they close windows or use curtains or blinds. Their behaviour was generally a rational adaptation to the conditions provided by the building and its surrounding urban environment. When occupants perceived that neighbours could see into areas or activities they considered private, they responded using the controls available to them. Leaving the window open while partially or substantially closing a curtain or blind represented an attempt to preserve ventilation without surrendering privacy. This has an important implication for architectural and urban design. Providing an operable window does not by itself ensure that its intended natural-ventilation function will be fully available during actual occupation. The realised performance of the window depends partly on whether occupants can use it without compromising other legitimate requirements. A naturally ventilated dwelling may therefore be technically designed with adequate openings but perform differently in everyday life because the surrounding architecture creates visual relationships that occupants subsequently manage through curtains or blinds.

The findings similarly broaden the meaning of visual privacy in high-density housing. Privacy cannot be understood adequately from building separation alone. What matters to occupants is whether people outside can obtain recognisable visual information about them, their activities and privacy-sensitive areas of their homes. Consequently, the three-dimensional relationship between neighbouring buildings, windows and interior spaces becomes part of the privacy problem. Urban density itself is therefore not necessarily the problem; the way density creates lines of sight into occupied spaces is critical. The findings also demonstrate why privacy requirements should not be treated as fixed characteristics of occupants. The need for privacy changed according to where occupants were within the home and what they were doing. A window that presented little difficulty during an ordinary activity could become problematic during a privacy-sensitive activity without any change to the surrounding architecture. Building performance was therefore dynamically influenced by the interaction between relatively fixed architectural conditions and changing human activities.

This interaction became especially important when activities inside the dwelling generated pollutants. Such activities increased the functional importance of ventilation for dilution and removal, yet occupants could simultaneously experience a strong need for visual privacy. The need for cleaner indoor air did not make the need for privacy disappear, and the need for privacy did not make the need for ventilation disappear. This is the essence of the dilemma. Occupants could be placed in a situation where satisfying one legitimate requirement made it more difficult to satisfy another. Scientifically, the findings demonstrate the importance of treating building performance as a human–building interaction rather than solely as a physical property of the building. Architectural geometry influenced potential visual exposure; occupants interpreted that exposure; their privacy requirements shaped their behaviour; and their behaviour changed the physical configuration through which natural ventilation occurred. Human perception and decision-making therefore formed part of the pathway through which architectural design translated into realised environmental performance.

This interpretation also has methodological implications for natural-ventilation research. Classifying windows simply as open or closed can overlook an important part of how ventilation openings actually exist during occupation. The window and the curtain or blind should be considered together as an operational configuration when privacy devices occupy or interact with the airflow pathway. Similarly, indoor pollutant concentrations should be interpreted alongside information about indoor pollutant-generating activities so that changes caused by pollutant generation are not mistakenly attributed to ventilation behaviour.

For building occupants, the findings validate the practical reality of trying to achieve privacy and ventilation simultaneously. The appropriate response is therefore not simply to advise occupants to open their windows more widely or remove privacy screening. Such advice transfers responsibility for resolving an architectural conflict to the occupant and may require them to accept an outcome they reasonably regard as unacceptable. A more appropriate design objective is to create residential environments in which occupants do not have to sacrifice one fundamental requirement to obtain another. For architects, engineers, planners and developers, the findings consequently suggest that visual privacy should be considered as part of natural-ventilation design from the beginning of the design process, particularly in high-density housing. Window orientation, neighbouring sightlines, internal spatial arrangement and privacy-sensitive activities should be considered alongside conventional ventilation parameters. Designing these matters independently risks producing a technically ventilatable dwelling that occupants cannot comfortably operate as intended.

Research conducted to answer RQ1 therefore reframed the privacy–ventilation dilemma from an apparent problem of occupant behaviour into a design problem arising from the interaction between architecture and legitimate human needs. Occupants were attempting to make the available building system work for them. The deeper problem was that the system could require them to negotiate visual privacy and natural ventilation through the same opening. This interpretation establishes the logical transition to the subsequent research questions without prejudging their findings. Research conducted to answer RQ1 established the dilemma, its context and the mechanism through which it operated. Research conducted to answer RQ2 therefore examined the magnitude and significance of its environmental and human consequences, while research conducted to answer RQ3 investigated whether the underlying conflict could be resolved through design. The broader scientific challenge emerging from research conducted to answer RQ1 was consequently not how to persuade occupants to tolerate less privacy in order to obtain more ventilation, but how buildings could enable visual privacy and natural ventilation to coexist without requiring occupants to trade one against the other.

Research Findings for Research Question 2

Overview

The findings from Research Question 2 built directly on the findings from Research Question 1 by quantifying the environmental and human consequences of the privacy–ventilation dilemma established there. The findings from Research Question 1 showed how visual intrusion and privacy requirements influenced occupants’ window and curtain or horizontal louvred-blind configurations and how these configurations modified the natural-ventilation pathway. The findings from Research Question 2 extended this finding by determining what the resulting differences in ventilation meant for indoor pollutant concentrations, thermal comfort, healthy indoor air, and conditions supporting healthy living. Evidence from field investigation, controlled experiments, Computational Fluid Dynamics (CFD) simulations, and occupant-centred assessments showed that the ventilation changes associated with different privacy-related configurations had measurable consequences for the indoor environment. When curtains or horizontal louvred blinds restricted or redirected airflow through an open window, the resulting ventilation condition influenced how effectively pollutants generated indoors were diluted and removed and how heat was exchanged between the indoor and outdoor environments.

The consequences for healthy indoor air depended on both pollutant generation and pollutant removal. Under comparable pollutant-generating conditions, configurations associated with more effective ventilation generally produced lower pollutant accumulation and faster pollutant removal, whereas configurations associated with weaker ventilation allowed pollutants to reach higher concentrations or remain indoors for longer. The same ventilation differences also affected air movement and thermal conditions experienced by occupants. These environmental consequences subsequently influenced conditions relevant to healthy living. Occupant-centred evaluation showed that the issue could not be understood from pollutant concentrations or thermal measurements alone. Visual privacy, thermal comfort, perceived indoor air conditions, environmental satisfaction, and occupants’ ability and willingness to use natural ventilation all contributed to whether a residential environment supported healthy everyday living. The findings from Research Question 2 therefore demonstrated the consequences of the mechanism established by the findings from Research Question 1. Privacy-related configurations changed ventilation; the resulting ventilation conditions affected indoor pollutant concentrations and thermal comfort; these effects influenced healthy indoor air and occupants’ environmental experience; and, together with visual privacy, they influenced the conditions supporting healthy living. Thus, the findings from Research Question 1 established the privacy–ventilation dilemma and its mechanism, while the findings from Research Question 2 quantified what that dilemma ultimately meant for the indoor environment and the people living within it.

Field Evidence Confirmed That Privacy-Related Configurations Had Environmental Consequences

The field data collected concurrently for Research Questions 1 and 2 showed that the ventilation changes associated with privacy-related configurations translated into measurable differences in the indoor environment and occupants’ experiences. The findings from Research Question 1 established why these configurations occurred; the findings from Research Question 2 quantified what they meant for indoor pollutant concentrations, thermal comfort, healthy indoor air, and conditions supporting healthy living. Curtains and horizontal louvred blinds were analysed separately because they interacted differently with airflow. With an unobstructed open window, mean occupied-zone air velocity was 0.31 m/s. Under substantial privacy screening, this decreased to 0.13 m/s with curtains and 0.17 m/s with horizontal louvred blinds, corresponding to reductions of approximately 58% and 45%, respectively.

These ventilation differences were reflected across the pollutants measured. During comparable occupied periods after adjustment for indoor source activity, mean CO₂ increased from 720 ppm under the unobstructed condition to 1,090 ppm with substantial curtain screening and 940 ppm with substantial horizontal louvred-blind screening. Median PM₂.₅ increased from 18 to 31 and 26 μg/m³, while PM₁₀ increased from 32 to 49 and 43 μg/m³, respectively. TVOCs increased from 210 to 365 and 310 μg/m³, formaldehyde from 21 to 34 and 29 μg/m³, and NO₂ from 14 to 22 and 19 μg/m³. During relevant source events, CO increased from 0.7 to 1.1 and 0.9 ppm, while ozone decreased slightly from 18 to 15 and 16 ppb, consistent with reduced outdoor-air exchange. The overall pattern therefore showed that reduced ventilation generally increased the persistence or accumulation of pollutants generated indoors, although ozone behaved differently because part of its indoor presence originated from outdoor air. Thermal comfort was also affected. Mean operative temperature increased from 27.4°C under the unobstructed condition to 29.1°C with substantial curtain screening and 28.5°C with substantial horizontal louvred-blind screening, while the proportion reporting thermally acceptable conditions decreased from 78% to 49% and 61%, respectively. When pollutant and ventilation indicators were integrated, the proportion of occupied periods satisfying the study’s healthy-indoor-air criteria decreased from 76% to 43% and 55%, respectively. The proportion of occupants rating the combined conditions of privacy, thermal comfort, perceived indoor air, environmental satisfaction, and normal residential functioning as satisfactory was 81% under the best-performing balanced condition, compared with 54% for substantial curtain screening and 66% for substantial horizontal louvred-blind screening.

Controlled Experiments Established the Physical Effect of Configuration

The controlled experiments extended the field findings by reproducing representative privacy-related configurations while holding relevant environmental and pollutant-source conditions constant. The purpose was to determine whether the differences observed in occupied apartments persisted when competing influences were controlled and the effects of curtains and horizontal louvred blinds were examined separately. With the window fully open and unobstructed, mean volumetric airflow was 485 m³/h and occupied-zone air velocity was 0.32 m/s. Substantial curtain screening reduced these values to 203 m³/h and 0.13 m/s, representing reductions of approximately 58% and 59%, respectively. Substantial horizontal louvred-blind screening produced 267 m³/h and 0.18 m/s, corresponding to reductions of approximately 45% and 44%. For horizontal louvred blinds, increasing the slat angle towards a more privacy-protective position further reduced airflow. Under standardised pollutant-generation conditions, these ventilation differences produced corresponding changes in indoor pollutant concentrations. Mean CO₂ increased from 710 ppm under the unobstructed condition to 1,120 ppm with curtains and 960 ppm with horizontal louvred blinds. PM₂.₅ increased from 17 to 33 and 27 μg/m³, PM₁₀ from 30 to 52 and 44 μg/m³, TVOCs from 205 to 382 and 318 μg/m³, formaldehyde from 20 to 36 and 30 μg/m³, NO₂ from 13 to 23 and 19 μg/m³, and CO from 0.6 to 1.1 and 0.9 ppm, respectively. Ozone showed the opposite tendency, decreasing from 19 to 15 and 17 ppb, consistent with reduced outdoor-air exchange. Thermal consequences followed the same physical pattern. Mean operative temperature increased from 27.3°C to 29.2°C with curtains and 28.6°C with horizontal louvred blinds, while thermal acceptability decreased from 80% to 47% and 60%, respectively.

Consequently, the proportion of experimental periods satisfying the predefined healthy-indoor-air criteria decreased from 78% under unobstructed conditions to 41% with substantial curtain screening and 54% with substantial horizontal louvred-blind screening. Corresponding satisfaction with the combined conditions supporting healthy living decreased from 82% to 52% and 65%, respectively. Importantly, the controlled experimental results were consistent with the field findings in both the direction and approximate magnitude of the observed effects. For example, substantial curtain screening reduced occupied-zone air velocity by approximately 59% experimentally, compared with approximately 58% in the field, while substantial horizontal louvred-blind screening produced reductions of approximately 44% and 45%, respectively. Similar agreement was observed across the measured pollutant concentrations, thermal conditions, healthy-indoor-air assessment, and conditions supporting healthy living. The close correspondence strengthened confidence that the field associations were not merely consequences of uncontrolled differences among apartments, occupants, weather conditions, or pollutant-generating activities. Instead, reproducing the configurations under controlled conditions produced the same overall pattern of environmental and human consequences. The controlled experiments therefore confirmed the field findings under standardised conditions: privacy-related reductions and redirection of ventilation produced measurable consequences for pollutant concentrations and thermal comfort, which subsequently affected healthy indoor air and the conditions supporting healthy living.

Pollutant Removal Was Governed by Both Source Strength and Ventilation

The controlled experiments provided further insight into how the ventilation conditions created by different privacy configurations affected the accumulation and removal of pollutants generated indoors. Unlike the field investigation, pollutant source strength, location, duration, and timing were standardised, allowing differences in pollutant concentration over time to be attributed more confidently to the ventilation configuration. For a standardised particulate-generating event, peak PM_2.5 concentration reached approximately 86 µg/m³ under the unobstructed open-window condition, 112 µg/m³ with partial curtain screening, and 157 µg/m³ with substantial curtain screening. Following termination of the source, the corresponding PM₂.₅ half-life increased from approximately 18 minutes under unobstructed conditions to 27 minutes with partial curtain screening and 43 minutes with substantial curtain screening. Substantial curtain screening therefore increased the time required for the PM_2.5 concentration to decrease by half by approximately 139% relative to the unobstructed condition. The type of privacy device also mattered. Under comparable substantial-screening conditions, the PM_2.5 half-life was approximately 43 minutes with curtains compared with 35 minutes with horizontal louvred blinds, consistent with the greater airflow restriction produced by curtains. For horizontal louvred blinds, pollutant removal also varied with slat angle because the remaining openings between the horizontal slats provided different pathways for air and pollutant transport.

The same underlying pattern occurred for gaseous pollutants, although the magnitude differed according to pollutant characteristics. TVOCs, formaldehyde, NO₂ and CO generated indoors generally persisted longer as ventilation became more restricted, while CO₂ accumulation and subsequent removal similarly reflected the available ventilation. Ozone behaved differently where outdoor air represented an important source, reinforcing the need to interpret each pollutant according to both its source and removal mechanisms. The important finding was therefore not merely that different privacy configurations produced different pollutant concentrations, as established in the preceding findings. The pollutant-decay analysis explained why these differences occurred: privacy-related changes to ventilation altered the rate at which pollutants accumulated and, critically, the rate at which the indoor environment could remove them after generation ceased.

CFD Explained the Spatial Mechanism

The validated CFD models extended the physical measurements by revealing how and where air, heat, and pollutants moved throughout the residential space, including locations where physical sensors were not positioned. Validation showed close agreement between simulated and measured air velocities, with a normalised root-mean-square error of 8.7%, coefficient of determination R² = 0.91, and mean absolute error of 0.024 m/s across validation locations. Temperature predictions had a mean absolute error of 0.34°C, while simulated contaminant concentrations differed from corresponding measurements by an average of 7.9%. This agreement supported subsequent parametric analyses, subject to the stated modelling assumptions and uncertainty. The simulations showed that curtains and horizontal louvred blinds changed both the amount and direction of airflow, creating recirculation and low-air-movement regions that affected pollutant removal and thermal conditions. Under the unobstructed configuration, approximately 82% of the occupied-zone volume remained above the selected minimum local air-speed criterion. This decreased to 61% with partial screening and 37% with substantial screening, while low-air-movement or stagnation regions increased from 11% to 34%. These changes explained why greater privacy screening was associated with higher concentrations and longer persistence of pollutants generated indoors, reduced air movement around occupants, and less favourable thermal conditions.

The CFD results also demonstrated why whole-room ventilation rate alone could be misleading. Two representative configurations differed in whole-room air-change rate by only 7%, yet breathing-zone contaminant removal differed by approximately 24% because privacy screening redirected incoming air above and around the main occupied region. Thus, similar overall ventilation did not necessarily produce equally healthy indoor air or thermal conditions where occupants actually lived and breathed. The simulations therefore provided the physical explanation connecting privacy configuration to the broader findings: curtain characteristics and positioning or horizontal louvred-blind slat angle altered airflow pathways; these changes affected pollutant dilution and removal and thermal conditions; and the resulting occupied-zone environment influenced healthy indoor air, thermal comfort, and conditions supporting healthy living.

Environmental Conditions Modified the Magnitude of the Privacy Effect

Multivariable modelling showed that the environmental consequences of privacy screening were not fixed but depended on the combined effects of the privacy device, window opening, and prevailing ventilation-driving conditions. Curtains and horizontal louvred blinds were analysed separately because their different physical characteristics produced different levels of airflow resistance and redirection. For substantial curtain screening, volumetric airflow decreased by approximately 46% under low-wind conditions, 58% under moderate-wind conditions, and 64% when wind direction produced less favourable pressure conditions across the façade, relative to the corresponding unobstructed open-window conditions. Substantial horizontal louvred-blind screening produced smaller reductions of approximately 34%, 45%, and 51%, respectively. The configuration-by-wind-condition interaction was statistically significant (p = 0.008), confirming that the effect of privacy screening changed with the environmental conditions driving natural ventilation.

Window opening produced a similar interaction. With the window fully open, 50% curtain closure reduced airflow by approximately 28%, compared with approximately 20% for a horizontal louvred blind at a comparable privacy-screening position. When the window opening was reduced to 50%, the corresponding reductions increased to approximately 39% and 30%, respectively. At approximately 25% window opening, the combined restrictions reduced airflow by approximately 57% with the curtain and 45% with the horizontal louvred blind, relative to their corresponding unobstructed conditions. These differences had consequences beyond airflow itself. Conditions producing greater ventilation reductions were also associated with higher accumulation and slower removal of pollutants generated indoors, lower occupied-zone air movement, and less favourable thermal conditions. The magnitude of the resulting effects on healthy indoor air and conditions supporting healthy living therefore varied with the complete combination of window opening, privacy-device type and position, and prevailing environmental conditions. The findings consequently showed why assigning a single ventilation penalty to privacy screening would be scientifically misleading. The environmental consequence of a curtain or horizontal louvred blind depended on how it was configured, how far the window was open, and the environmental forces available to drive natural ventilation.

Human Consequences, Healthy Indoor Air and Integrated Findings

This section examines the human and healthy indoor air consequences of the privacy–ventilation dilemma and integrates the findings to establish how the observed effects were connected. The subsections progressively examine how restricted effective ventilation affected healthy indoor air, how improved privacy influenced ventilation-related experience, how these combined experiences shaped support for healthy living, and how the complete pathway was quantified through an integrated structural model.

Healthy Indoor Air Deteriorated as Effective Ventilation Was Restricted: Healthy indoor air was evaluated as a multidimensional condition rather than through one pollutant or ventilation measure. Using a composite healthy-indoor-air assessment based on prespecified ventilation, pollutant-removal and occupied-zone criteria, 81% of monitoring intervals under the unobstructed open-window reference condition satisfied all applicable criteria simultaneously, decreasing to 63% under partial privacy screening, 38% under substantial screening and 19% with the window closed. Substantial screening was associated with approximately 70% lower odds of satisfying the combined criteria (adjusted OR = 0.30, 95% CI 0.21 to 0.43, p < 0.001). The effect became stronger during active indoor pollutant generation. The pollutant-removal criterion was satisfied during 74% of unobstructed, 49% of partially screened and 27% of substantially screened open-window periods (interaction p = 0.012). Thus, the dilemma became most environmentally consequential when pollutant generation increased the need for ventilation.

Privacy Improved While Ventilation-Related Experience Deteriorated: Occupants experienced the opposing consequences directly. Mean perceived privacy increased from 2.3/7 under the fully open unobstructed condition to 4.8/7 under partial screening and 6.1/7 under substantial screening, with an unobstructed-to-substantial difference of +3.8 points (95% CI +3.4 to +4.2, p < 0.001). Conversely, perceived air freshness decreased from 5.8/7 to 4.9/7 and 3.7/7, while perceived air movement decreased from 5.6/7 to 4.5/7 and 3.3/7, respectively. Acceptance for normal daily use was 54% for the fully unobstructed condition, 72% for partial screening and 48% for substantial screening. This inverted-U pattern showed why partial screening was frequently adopted: it did not maximise privacy or ventilation but provided the least unacceptable compromise between them.

Support for Healthy Living Reflected the Combined Experience: Healthy living was not interpreted as a directly measured medical outcome. The composite measure of support for healthy daily living, incorporating environmental satisfaction, thermal comfort, perceived privacy, perceived freshness, perceived control and ability to undertake normal residential activities, averaged 5.3/7 under partial screening, 4.7/7 under the fully unobstructed condition and 4.4/7 under substantial screening. Occupants therefore evaluated the overall living condition rather than any single engineering outcome. Mediation analysis demonstrated two opposing pathways. Greater screening supported healthy living through improved perceived privacy (standardised indirect effect +0.31, 95% CI +0.22 to +0.41) but reduced it through poorer ventilation-related environmental satisfaction and healthy indoor air (−0.27, 95% CI −0.36 to −0.18). Conventional privacy management therefore produced benefits and costs simultaneously.

Integrated Structural Model Quantified the Complete Pathway: SEM connected the behavioural pathway established by Research Question 1 with the environmental and human consequences quantified by Research Question 2. Perceived visual intrusion predicted privacy requirement (β = 0.67, 95% CI 0.56 to 0.76), which predicted greater window and curtain or horizontal louvred-blind obstruction (β = 0.52, 95% CI 0.39 to 0.63). Greater obstruction predicted poorer ventilation performance (β = −0.58, 95% CI −0.68 to −0.46), ventilation predicted healthy indoor air (β = 0.64, 95% CI 0.52 to 0.74), and healthy indoor air predicted conditions supporting healthy living (β = 0.41, 95% CI 0.28 to 0.53); all pathways were p < 0.001. The indirect effect from privacy requirement to healthy indoor air was −0.19 (95% CI −0.28 to −0.12), while increased screening simultaneously improved privacy satisfaction. Model fit was strong (CFI = 0.95, TLI = 0.94, RMSEA = 0.046, 90% CI 0.039 to 0.054; SRMR = 0.041). Together, these opposing pathways quantitatively demonstrated the central dilemma: conventional privacy protection improved visual privacy while simultaneously creating environmental conditions that could undermine healthy indoor air and healthy living.

Integrated Interpretation of the Findings

Taken together, the findings from Research Question 2 provided sufficient evidence to reject H₀₂ in favour of H₁₂. Privacy-related window and curtain or horizontal louvred-blind configurations produced meaningful differences in the environmental and human outcomes investigated. The findings therefore confirmed that the privacy–ventilation dilemma established by Research Question 1 had consequences extending beyond occupants’ window and privacy-management behaviour to indoor pollutant concentrations, thermal comfort, healthy indoor air, and conditions supporting healthy living. The central scientific interpretation is that an open window should not automatically be interpreted as an effectively ventilating window. Natural ventilation depended on the complete physical pathway through which outdoor air entered the dwelling and moved towards occupied areas. When a curtain or horizontal louvred blind was positioned across an open window for privacy, it became functionally part of that pathway. Its material, degree of closure, geometry and, for horizontal louvred blinds, slat angle influenced how much air could pass through and where that air subsequently travelled. This means that assessments of naturally ventilated residential buildings that consider window position without considering privacy devices may substantially misrepresent the ventilation conditions actually experienced by occupants.

The findings also clarify why whole-room ventilation alone is insufficient for understanding healthy indoor air. Outdoor air entering a dwelling is useful only if it reaches the places where it is needed and contributes effectively to diluting and removing pollutants. Privacy screening could redirect airflow away from occupied and breathing zones, creating areas with relatively weak air movement even when the overall room still received outdoor air. Consequently, building-performance assessment should consider not only how much air enters a dwelling but also where it travels and how effectively it supports pollutant dilution and removal where occupants actually live and breathe. This interpretation becomes particularly important when pollutants are being generated indoors. Cooking, cleaning, human occupancy and other everyday activities can create periods during which pollutant removal becomes more important. The findings showed that privacy-related restrictions to ventilation could allow pollutants generated during such activities to accumulate to higher concentrations or remain indoors for longer. The dilemma is therefore not simply about occupants choosing between privacy and a subjective preference for fresh air. It can involve choosing between two legitimate requirements for healthy residential living: protecting oneself from unwanted visual exposure and maintaining indoor environmental conditions capable of effectively managing pollutants generated through everyday life.

Thermal comfort adds another dimension. The same airflow that supports pollutant dilution and removal can contribute to heat exchange and air movement around occupants. Restricting or redirecting that airflow therefore affected not only healthy indoor air but also how occupants experienced the thermal environment. Privacy management consequently influenced several aspects of residential experience simultaneously rather than producing a single isolated ventilation effect. Importantly, the findings should not be interpreted as showing that occupants behaved incorrectly by closing curtains or adjusting horizontal louvred blinds. Their behaviour was a rational response to the architectural conditions established in Research Question 1. Stronger screening improved visual privacy, while weaker screening generally supported better ventilation-related environmental performance. Occupants therefore faced a genuine conflict created partly by the physical environment: improving one legitimate function could diminish another. This explains why partial screening emerged as an attractive everyday compromise. It could provide more privacy than an unobstructed window while retaining more ventilation than substantial screening. However, its acceptability should not be mistaken for resolution of the underlying problem. A compromise means that occupants continue to surrender some performance on both sides. They receive neither the strongest privacy protection nor the environmental performance available from an unobstructed opening. The distinction between curtains and horizontal louvred blinds further demonstrates that privacy devices should not be treated as interchangeable. Their different physical forms produced different airflow behaviour. Horizontal louvred blinds could preserve airflow pathways between their slats and redirect airflow according to slat angle, whereas curtains generally formed a more continuous obstruction. This has practical implications for architects, façade designers and building engineers because privacy devices should be considered not merely as interior accessories but as components capable of influencing the realised environmental performance of naturally ventilated dwellings.

The agreement among field observations, controlled experiments and CFD modelling strengthened this interpretation. The field investigation demonstrated that the phenomenon occurred during normal residential life. The controlled experiments showed that comparable environmental consequences remained when important competing influences were controlled. CFD then explained the physical mechanisms by showing how privacy devices changed airflow pathways, stagnation regions, pollutant transport and occupied-zone conditions. Occupant-centred evidence completed the explanation by demonstrating how these physical consequences were experienced alongside privacy requirements. Together, these methods connected what occupants did, why they did it, what happened physically, and what those changes meant for their everyday living environment. The findings therefore have an important implication for building design. Simply encouraging occupants to open windows cannot fully resolve the problem because occupants may already have opened them while simultaneously using privacy devices that restrict or redirect airflow. Likewise, maximising ventilation by removing privacy screening would disregard a legitimate human requirement. The problem is therefore partly architectural rather than merely behavioural. This interpretation established the design challenge for Research Question 3. The objective should not be to identify another compromise between privacy and ventilation, but to investigate whether the underlying functions can be decoupled. The required solution should preserve the environmental benefits associated with a substantially unobstructed open window while providing the visual protection occupants seek from conventional privacy screening. Research Question 2 therefore transformed the dilemma established by Research Question 1 into a clear performance requirement for subsequent design investigation: Clear outward view × Visual privacy × Full ventilation. Research Question 3 consequently addresses whether these three functions can coexist rather than requiring building occupants to continue trading one against another.

Findings for Research Question 3

Overview

The findings from Research Question 3 demonstrated that the privacy–ventilation dilemma established and explained by Research Question 1, and its environmental and human consequences quantified by Research Question 2, could potentially be resolved without requiring occupants to compromise between visual privacy and natural ventilation. The findings showed that the three functions previously in conflict could be achieved simultaneously under the investigated conditions: Clear outward view × Visual privacy × Full ventilation. The final Directional Optical Air Curtain Window maintained a physically unobstructed window opening while substantially preventing external observers from obtaining recognisable visual information from the privacy-sensitive interior. At the same time, occupants retained a clear outward view. The findings therefore demonstrated that visual-information transmission and airflow could be functionally separated within the same architectural opening, rather than privacy being achieved by physically obstructing the pathway required for natural ventilation.

Environmental performance was correspondingly maintained close to the unobstructed open-window reference established by Research Question 2. Unlike substantial curtain or horizontal louvred-blind screening, the directional optical system did not produce the same reductions in occupied-zone airflow or associated deterioration in pollutant dilution and removal, healthy indoor air, thermal comfort, and conditions supporting healthy living. The findings also showed that achieving privacy alone was insufficient. Prototype configurations that compromised outward-view clarity or interfered materially with airflow failed the integrated performance requirement, even when they improved visual privacy. The final design therefore represented a shift from managing the privacy–ventilation trade-off to potentially removing its underlying physical cause. The findings establish functional feasibility, the required performance envelope, governing design principles and an experimentally supported design framework under the investigated conditions. They do not establish commercial readiness or universal performance across all buildings, climates and viewing conditions.

Prototype Development and Validation of Integrated Functional Performance

The following findings establish how iterative prototype development progressed towards the simultaneous achievement of Clear outward view × Visual privacy × Full ventilation, and whether the final prototype satisfied each of these non-compensatory requirements. They also provide the experimental and computational evidence explaining how visual privacy could be achieved while preserving outward view and the natural-ventilation functionality of the physically unobstructed fully open window.

Iterative Development Revealed Why Functional Separation Was Necessary: The prototype findings showed why the privacy–ventilation dilemma could not be satisfactorily resolved by simply improving conventional physical screening. As established by Research Questions 1 and 2, increasing curtain or horizontal louvred-blind screening improved privacy but could restrict or redirect airflow. The prototype results showed that overcoming this trade-off required visual-information transmission to be controlled without physically obstructing the open-window airflow pathway. Consequently, Clear outward view × Visual privacy × Full ventilation remained non-compensatory: strong performance in one function could not compensate for failure in another. The first integrated optical prototype prevented external recognition of protected interior targets in 71.4% of predefined viewing trials while retaining 91.2% of reference outward-view quality. Although outward visibility for occupants remained relatively high, external recognition remained possible in almost three out of every ten trials; the prototype therefore failed the visual-privacy requirement. The second generation increased privacy protection to 89.6%, but outward-view fidelity decreased to 88.7%. This showed that increasing optical privacy alone could create another trade-off by degrading occupants’ ability to see outside clearly.

The final prototype substantially reduced this conflict. External recognition was prevented in 98.6% of predefined viewing trials while outward-view fidelity reached 96.8% of the unobstructed reference. Outward-view performance was therefore only 3.2 percentage points below the reference, compared with 11.3 percentage points for the second-generation prototype. A similar progression occurred for ventilation. An early frame configuration physically projected into the window opening and retained 91.5% of reference volumetric airflow. Recessing the field-generating components into the head, sill and side frames increased retained airflow to 96.9%, while the final configuration retained 98.7% of the unobstructed-window reference airflow. The important finding was therefore not that any single performance measure had been maximised. Rather, successive failures revealed where privacy, outward view or ventilation remained compromised. The final prototype brought all three functions close to their predefined performance requirements simultaneously within the same operating condition, demonstrating the functional separation required to potentially remove, rather than merely manage, the privacy–ventilation trade-off.

Visual Privacy Was Maintained Across the Defined Viewing Envelope: The findings showed that the Directional Optical Air Curtain Window maintained visual privacy across the defined external viewing envelope, including the most demanding perpendicular viewing position. This was important because a privacy solution that worked only when observers viewed the window from favourable side angles would leave occupants exposed when someone looked directly through the open window. With the window fully open and the photonic field inactive, external observers correctly recognised predefined interior targets in 96.4% of trials. When the final photonic field operated across the same physically open aperture, recognition decreased to 1.4%, representing an absolute reduction of 95.0 percentage points and a relative reduction of approximately 98.5%. Thus, the window remained physically open while recognisable visual information reaching external observers was almost completely suppressed.

Importantly, performance remained consistent across viewing directions. Recognition with the photonic field active was 1.8% from the perpendicular position, 1.2% from oblique positions, 1.5% from elevated positions and 1.1% from lower positions. The photonic field strongly reduced recognisability (odds ratio = 0.0019, 95% CI 0.0012 to 0.0031, p < 0.001), while its effectiveness did not differ significantly among the tested viewing-position categories (interaction p = 0.31). The significance of this finding extends beyond making the interior appear blurred or less visible. Privacy was demonstrated by whether external observers could recognise predefined occupants, objects, surfaces or activities within the protected privacy-sensitive area. The findings therefore demonstrated that visual privacy could be maintained even through the direct perpendicular line of sight while leaving the window physically open, supporting the functional separation of visual privacy from the airflow obstruction associated with conventional privacy screening.

Clear Outward View Was Preserved Simultaneously: The findings showed that the visual privacy achieved by the Directional Optical Air Curtain Window did not require occupants to sacrifice their ability to see clearly outside. This was essential to the integrated requirement of Clear outward view × Visual privacy × Full ventilation. A system that prevented people outside from seeing in by making the window effectively opaque in both directions would simply replace the privacy–ventilation dilemma with a privacy–outward-view dilemma. With the conventional window fully open and the photonic field inactive, outward-view performance was defined as the 100% reference condition. The final prototype retained 96.8% of reference visual-detail performance, improving substantially from 88.7% in the preceding prototype generation. Participants correctly recognised predefined features in the external residential scene in 97.1% of trials without the field and 95.6% with the field, a difference of only 1.5 percentage points.

Participants’ perception of outward-view clarity was consistent with these objective measurements. On a seven-point scale, mean clarity decreased only from 6.63 ± 0.41 under the unobstructed reference condition to 6.39 ± 0.48 with the final prototype, a mean difference of −0.24 points (95% CI −0.34 to −0.14). Colour fidelity and contrast also remained within the predefined acceptable optical-performance range, without substantial deterioration towards the edges of the protected viewing envelope. Most importantly, the directional difference was pronounced: external recognition of the protected interior decreased from 96.4% to 1.4%, while occupants retained 95.6% recognition of external visual features. The findings therefore demonstrated that strong visual privacy and a clear outward view occurred simultaneously rather than one being achieved at the expense of the other, supporting the directional separation of inward and outward visual-information transmission required by the proposed solution.

Full Ventilation Was Preserved Relative to the Fully Open Window: The findings showed that the visual privacy achieved by the Directional Optical Air Curtain Window did not require a meaningful sacrifice in natural ventilation. The decisive comparison was between the same physically unobstructed, fully open window with the photonic field inactive and active. This provided the most demanding and scientifically appropriate reference because the prototype was required to preserve full ventilation, rather than merely provide more ventilation than a closed, partially open, or physically screened window. With the unobstructed fully open window defined as 100%, operation of the final photonic system retained 98.7% of reference volumetric airflow, a reduction of only 1.3%. The estimated airflow ratio was 0.987 (95% CI 0.978 to 0.996).

Air-change effectiveness was similarly preserved at 0.96 (95% CI 0.92 to 1.00) for the reference and 0.95 (95% CI 0.91 to 0.99) with the photonic field operating, an absolute difference of only −0.01. Breathing-zone air velocity was also essentially maintained, changing from 0.31 ± 0.07 m/s to 0.30 ± 0.07 m/s, while the spatial coefficient of variation changed only from 0.28 to 0.29. Thus, the system did not retain total airflow simply by redirecting it away from where occupants lived and breathed. Equivalence testing confirmed the practical significance of these findings. Using the prespecified ±5% equivalence margin, the 95% confidence interval for normalised airflow remained entirely within the equivalence bounds, supporting aerodynamic equivalence to the unobstructed fully open window within the defined experimental tolerance. The implication is important: privacy was not obtained by accepting a reduced but supposedly adequate level of ventilation. The prototype retained 98.7% of the ventilation available from the physically unobstructed fully open window while simultaneously providing visual privacy and preserving a clear outward view.

CFD Explained Why Ventilation Was Preserved: The CFD findings explained why the Directional Optical Air Curtain Window could provide visual privacy while retaining the ventilation performance of the fully open window. Validation against measured air velocities showed close agreement, with a normalised root-mean-square error of 6.8% and R² = 0.93 across validation locations. Mesh refinement changed the principal airflow outcome by less than 1.5%, supporting numerical independence at the selected resolution. The simulations showed that the privacy system did not place a physical barrier across the open-window airflow pathway. Only small, localised airflow disturbances occurred near the field-generating components integrated into the window frame. The effective high-velocity inflow region remained 97.9% of that produced by the unobstructed reference window, while the principal circulation of air through the occupied zone remained substantially unchanged.

Similarly, pressure differences immediately upstream and downstream of the window aperture differed by less than 2% between the photonic-field-active and reference conditions across the principal wind scenarios investigated. This showed that operating the privacy function did not materially change the pressure-driven mechanism responsible for natural ventilation. The CFD findings therefore provided the physical explanation for the experimentally observed 98.7% retention of reference airflow. Unlike curtains or horizontal louvred blinds, which physically restricted or redirected airflow in Research Question 2, the photonic approach acted on visual-information transmission rather than the physical movement of air. Consequently, the simulations showed why visual privacy could operate while the principal airflow pathways, occupied-zone air circulation, and ventilation mechanism of the fully open window remained substantially intact, supporting the simultaneous achievement of Clear outward view × Visual privacy × Full ventilation.

Healthy Indoor Air, Human Evaluation, Robustness and Contribution to Knowledge

The following findings examined whether the integrated performance of the final prototype extended beyond the simultaneous achievement of visual privacy, clear outward view and full ventilation. They established what this performance meant for healthy indoor air and occupants’ experiences, whether it remained robust under representative operating conditions, and its broader contribution to knowledge.

Preservation of Ventilation Preserved Healthy Indoor Air Performance: Healthy indoor air was evaluated by determining whether preserving ventilation also preserved performance for the individual pollutants investigated in Research Questions 1 and 2. For each pollutant comparison, the source type, emission rate, source location, generation duration and timing were maintained identical between the unobstructed fully open-window reference and photonic-field conditions, allowing differences in pollutant concentration and removal to be attributed to the resulting ventilation performance. Under these standardised source conditions, pollutant concentrations with the photonic field operating remained close to those measured with the unobstructed fully open-window reference. Mean CO₂ concentration was 718 ppm under the reference condition and 726 ppm with the photonic field operating. Corresponding concentrations were 17.2 and 17.6 μg/m³ for PM₂.₅, 30.4 and 31.0 μg/m³ for PM₁₀, 204 and 209 μg/m³ for TVOCs, 20.1 and 20.5 μg/m³ for formaldehyde, 13.2 and 13.4 μg/m³ for NO₂, and 0.60 and 0.61 ppm for CO, respectively. Ozone remained similarly comparable at 19.0 and 18.9 ppb, indicating that preservation of outdoor-air exchange did not materially change ozone entry relative to the fully open reference.

Pollutant-removal performance showed the same pattern. Following standardised generation events, the median time required for source-related concentration to decrease by 50% was 18.4 minutes under the reference and 18.8 minutes with the photonic field, a difference of approximately 2.2%. Contaminant-removal effectiveness was 1.03 ± 0.09 and 1.01 ± 0.09, respectively, while cumulative excess pollutant concentration was only 2.6% higher with the field and remained within the predefined environmental-equivalence margin. CFD analysis showed that the airflow pathways responsible for these outcomes were also substantially preserved. The findings therefore demonstrated that CO₂ removal, particulate dilution and removal, VOC removal, removal of internally generated NO₂ and CO, and outdoor-air-related ozone behaviour remained close to the unobstructed fully open-window reference. The photonic field therefore did not clean indoor air; it preserved the natural-ventilation functionality responsible for maintaining healthy indoor air while simultaneously providing visual privacy.

Full-Scale Human Evaluation Confirmed Resolution of the Experienced Dilemma: Full-scale human evaluation showed that the physical performance translated into an experience occupants could perceive. Participants experienced an unobstructed fully open window, conventional physical privacy screening, and the fully open Directional Optical Air Curtain Window. The conventional fully open window provided strong perceived ventilation but weak privacy. Mean perceived privacy was 2.1 ± 0.9 on a seven-point scale, while outward-view clarity was 6.6 ± 0.4. Conventional screening increased privacy to 6.0 ± 0.6, but perceived ventilation decreased to 4.2 ± 0.9, reproducing the trade-off established by Research Question 2. The prototype changed this relationship. Mean perceived privacy reached 6.4 ± 0.5 while perceived ventilation remained 6.2 ± 0.6 and outward-view clarity was 6.4 ± 0.5. The result was therefore not merely greater overall satisfaction; participants simultaneously experienced high privacy, ventilation and outward-view clarity.

Perceived control increased from 4.3 ± 1.1 under conventional privacy management to 6.1 ± 0.7 with the prototype, while willingness to use the system during normal residential operation averaged 6.2 ± 0.7. Perceived indoor-air freshness was 6.1 ± 0.6 with the prototype compared with 6.2 ± 0.6 under the unobstructed reference, consistent with the small objectively measured differences in airflow and pollutant removal. Interviews reinforced this finding. Participants distinguished between merely having an open window and being able to use an open window without feeling exposed to neighbours. The prototype therefore addressed the experienced source of the dilemma rather than simply encouraging occupants to ventilate more frequently.

Healthy Living Was Supported, Not Claimed as a Clinical Health Effect: The healthy-living findings were deliberately bounded. The experimental duration could not establish reduced disease incidence, improved respiratory health or other long-term clinical outcomes. Instead, Research Question 3 evaluated whether the prototype supported environmental and experiential conditions relevant to healthy everyday living. On the seven-point composite measure, support for healthy everyday living increased from 4.7 ± 0.8 under conventional privacy screening to 6.1 ± 0.6 with the final prototype, representing an improvement of 1.4 points (95% CI 1.18 to 1.62, p < 0.001; d = 1.83). The prototype supported healthy living because occupants could simultaneously experience visual privacy, outdoor visual connection, ventilation, acceptable thermal conditions, healthy indoor-air conditions and perceived environmental control. It did not demonstrate that participants became objectively healthier during the experiment. This distinction strengthens rather than weakens the conclusion. Research Question 3 demonstrated improvement in conditions supporting healthy living; determining downstream health outcomes would require longer-term longitudinal or epidemiological investigation.

Performance Remained Robust Across Representative Conditions: Sensitivity analysis showed that the prototype’s performance was not confined to one favourable laboratory condition. Across the representative wind directions and ventilation-driving conditions investigated, normalised airflow retention ranged from 97.6% to 99.2% of the fully open reference. External target recognisability remained below 2.5% across the predefined viewing envelope, while outward-view fidelity remained above 95%. Under the most challenging tested combination of low ventilation-driving force and direct perpendicular external viewing, airflow retention remained 97.8%, external recognisability was 2.3%, and outward-view fidelity was 95.4%. Thus, all three primary functions remained within their predefined performance thresholds even under the least favourable condition investigated. These findings do not support extrapolation to every building, climate, optical environment or viewing geometry. They demonstrate, however, that the observed functional separation was not dependent on a single idealised operating condition within the tested envelope.

Operational, Energy and Safety Performance: Practical operation provided no obvious secondary performance barrier under the tested conditions. Window opening triggered the system in 100% of repeated activation trials, with a median response time of 0.42 seconds and a 95th-percentile activation time of 0.61 seconds. No unintended deactivation occurred during continuous operating trials. Mean electrical demand was 18.6 W during steady operation, with a short activation peak of 27.4 W. Frame-surface temperature remained less than 2.1°C above the corresponding inactive condition after extended operation. Measurements identified no detectable ozone increase above instrumental background attributable to system operation, while electromagnetic and optical exposure remained within the prespecified applicable safety limits used for prototype evaluation. These results do not constitute regulatory certification. Rather, no disqualifying operational, energy or safety signal was identified under the investigated conditions. Commercial deployment would require separate testing and certification against applicable regulatory standards.

Integrated Performance Demonstrated Resolution Rather Than Redistribution of the Dilemma: The strongest finding from Research Question 3 emerged when Clear outward view × Visual privacy × Full ventilation were considered simultaneously. This was important because improving only one or two functions would not resolve the privacy–ventilation dilemma; all three had to be achieved at the same time. The conventional fully open window provided 100% reference ventilation and 100% reference outward-view performance, but external observers correctly recognised protected interior targets in 96.4% of trials. It therefore provided ventilation and outward view but failed to provide visual privacy. Conversely, the conventional curtain or horizontal louvred-blind configurations investigated in Research Question 2 improved visual privacy but restricted or redirected airflow, with corresponding consequences for pollutant dilution and removal, thermal comfort, healthy indoor air, and conditions supporting healthy living. Conventional approaches therefore improved one legitimate requirement partly by compromising another.

The final Directional Optical Air Curtain Window produced a fundamentally different performance pattern. It retained 98.7% of the fully open-window reference airflow and 96.8% of reference outward-view fidelity, while reducing external recognisability of the protected interior from 96.4% to 1.4%. Thus, the same physically open architectural opening simultaneously maintained near-reference ventilation, a clear outward view, and strong visual privacy. This three-way performance represents the central contribution of Research Question 3. Within the predefined experimental tolerances and tested operating envelope, the prototype did not simply transfer the problem from privacy to ventilation or from privacy to outward view. Instead, it substantially decoupled functions that conventional privacy management had forced occupants to negotiate as a trade-off. The significance therefore lies not in maximising any single performance indicator, but in demonstrating the functional feasibility of achieving all three requirements simultaneously. This supports the governing non-compensatory design principle: Successful Solution = Clear Outward View × Visual Privacy × Full Ventilation. Because failure of any one function constitutes failure of the overall solution, the findings establish a different design approach: rather than designing a better compromise, the conflicting functions can potentially be separated so that the underlying privacy–ventilation trade-off itself is removed.

Integrated Interpretation of the Findings

Research Question 3 built on Research Questions 1 and 2, which established the privacy–ventilation dilemma and its consequences for the indoor environment and occupants. It addressed whether the underlying conflict could be removed through design rather than continually managed through behavioural compromise. The findings provided sufficient evidence to reject H₀₃ in favour of H₁₃ within the investigated experimental domain. The Directional Optical Air Curtain Window provided visual privacy while maintaining ventilation, outward-view quality and healthy-indoor-air performance sufficiently close to the unobstructed fully open-window reference. This conclusion is limited to the investigated prototype configuration, viewing geometries, environmental boundary conditions and participant population and does not establish universal performance or commercial readiness. The central scientific finding is that visual privacy and natural ventilation do not necessarily have to remain competing functions simply because visual information and air pass through the same architectural opening. Curtains and horizontal louvred blinds physically couple these functions because they protect privacy while occupying or modifying the airflow pathway. Research Question 3 demonstrated an alternative principle of functional separation, whereby visual-information transmission was controlled independently while the natural-ventilation pathway remained substantially unobstructed.

Importantly, success was not inferred simply because the window remained physically open. The prototype preserved airflow quantity, occupied-zone airflow distribution and the ventilation functionality responsible for diluting and removing pollutants generated indoors. Performance across the individual pollutants investigated remained close to the unobstructed reference, demonstrating that privacy was not achieved by accepting deterioration in healthy indoor air. The photonic field did not clean the air; it avoided substantially interfering with the natural-ventilation process responsible for pollutant dilution and removal. For building occupants, the findings mean that visual privacy need not necessarily require covering an open window and compromising natural ventilation, nor should privacy require sacrificing visual connection with the outdoors. Within the tested conditions, occupants could experience privacy, clear outward view and ventilation simultaneously. This supports Clear outward view × Visual privacy × Full ventilation as a non-compensatory design principle. Exceptional performance in one function cannot compensate for failure in another; all three functions must coexist within the same operating condition.

For architectural practice, the findings challenge the treatment of privacy and natural ventilation as separate requirements. In naturally ventilated high-density residential buildings, the window, external viewing relationships, privacy requirements, privacy-control mechanism, outward view and airflow pathway should be considered as an integrated architectural system. Privacy should not simply be left for occupants to manage afterwards through curtains or horizontal louvred blinds, because these can alter the environmental performance intended by the window design. The findings therefore shift practice from designing an opening and leaving occupants to negotiate privacy and ventilation towards designing the façade and window system so that visual privacy, clear outward view and full ventilation are deliberately resolved together. This has particular significance for increasingly high-density cities, where residential buildings, windows and everyday domestic activities can be brought into closer visual relationships. Greater building separation to achieve privacy may become increasingly difficult or spatially inefficient, while physically screening windows can compromise natural ventilation, healthy indoor air and outward visual connection. The privacy–ventilation dilemma should therefore be recognised as an urban-density design challenge rather than merely an individual occupant-management problem. Functional separation provides a potentially important direction for supporting healthy living as cities become increasingly dense without necessarily requiring occupants to trade privacy against ventilation and clear outward view.

Human evaluation further showed that engineering performance translated into occupants’ experiences, including privacy, ventilation, outward-view quality, perceived environmental control and conditions supporting healthy everyday living. The findings do not demonstrate improved clinical health; they demonstrate environmental and experiential conditions supporting healthy living. Long-term health effects would require longitudinal or epidemiological investigation. The contribution to knowledge extends beyond the prototype. Conceptually, Research Question 3 establishes functional separation as an alternative framework for the privacy–ventilation dilemma. From a design perspective, it establishes simultaneous, non-compensatory performance requirements. Technologically, it demonstrates the functional feasibility of directional visual-information control while substantially preserving fully open-window aerodynamic behaviour. Methodologically, it demonstrates how a futuristic architectural proposition can progress through modelling, proof-of-concept development, engineering and human evaluation, failure identification and iterative refinement without assuming commercial readiness. The principal contribution is therefore the experimentally supported demonstration that visual privacy, clear outward view and full ventilation need not necessarily remain competing functions of an open residential window. The findings shift the design objective from optimising a privacy–ventilation compromise to potentially removing its physical basis through functional separation, with particular relevance to healthy living in increasingly high-density cities.

………………… Chapter 5 ……………………

The morning after Corlin Brown formally completed his five-year-long PhD, he woke later than usual. For several years, his days had been organised around experiments, measurements, failed ideas, revised designs, discussions with supervisors and the persistent question of whether privacy, clear outward view and full ventilation could be achieved together. Now the thesis had been defended, the corrections had been accepted, and there was nothing left to prove to an examiner. His parents had taken him to dinner the previous evening. Professor Daniel Brown had raised his glass and told his son that he was now entitled to disagree with him professionally. Professor Eleanor Brown had laughed and reminded Daniel that Corlin had been doing that since childhood. Corlin laughed with them, but the following morning he felt strangely unsettled. He had expected completion to feel like arrival. Instead, it felt like standing at another beginning. His PhD had taught him something that extended beyond the window he had investigated. For years, he had instinctively approached dilemmas by deciding which legitimate requirement should take priority and what compromise could reasonably be accepted. His research had forced him to remain with one dilemma for long enough to discover that the apparent conflict between its requirements was not necessarily fundamental. He had learnt to look beneath the conflict for the barrier producing it. Yet Corlin did not know how far this emerging way of thinking would travel beyond the unusually concentrated intellectual environment of his PhD. His academic career would soon begin testing it against problems that were messier, less controlled and consequential to people other than himself.

Corlin’s first academic appointment as an Assistant Professor allowed him to bring his Physics background into Architecture through teaching and research in Architectural Science and Technology. The position suited him. He remained intellectually close to the physical science he loved, but the phenomena he investigated now had direct consequences for buildings and the people occupying them. Airflow was no longer interesting to him only as fluid behaviour; it mattered because people needed ventilation and healthy indoor air. Heat transfer mattered because people needed thermally comfortable spaces and buildings needed to use energy responsibly. Light mattered not only as an electromagnetic phenomenon but because people needed daylight, visual comfort, privacy and outward view. Physics increasingly became the scientific foundation through which Corlin investigated how Architecture could perform better.

A few months into his appointment, a different experience taught Corlin that knowing which questions to ask did not always mean being able to ask them. As a relatively junior Assistant Professor, he was once invited to a project meeting led by a highly respected senior professor and several experienced industry executives. A costly building-performance problem had already been discussed extensively before Corlin joined the project, and the meeting had been convened to select between two proposed interventions. The senior professor opened the discussion by explaining that the causes were already understood and that the team should concentrate on deciding which intervention offered the better balance between performance, cost and implementation time. As Corlin listened, something troubled him. One of the assumptions underlying the diagnosis did not seem sufficiently supported by the evidence being presented. He wanted to ask whether the team had established that the assumed mechanism was actually responsible for the observed performance gap. Several times he prepared to speak, but each time he hesitated. The people around the table had decades more experience than he did. Some had been investigating the problem long before he was invited. The senior professor had considerably greater academic standing, while the industry executives controlled the project and its resources. Corlin began wondering whether questioning the diagnosis would be interpreted as intellectual contribution or as a young academic failing to understand what experienced people had already established.

He remained quiet. The discussion moved downstream. The team compared the two interventions, debated their advantages and disadvantages and eventually selected one. Corlin participated in that discussion and even helped evaluate some of the technical evidence. His contribution was competent, but throughout the meeting the unanswered question remained in his mind: What if the problem we are solving is not actually the problem? Several months later, the intervention produced improvement, but considerably less than expected. Further investigation eventually revealed that another mechanism had been contributing substantially to the performance problem. Corlin felt no satisfaction in discovering that his earlier concern had been justified. Instead, he was disturbed by something more personal. He had recognised the need for further diagnosis and had still allowed the decision-making process to proceed without raising it. His old flaw had appeared in a form he had not previously recognised. This time, the problem was not that he had failed to think of the appropriate question. He had thought of it. The problem was that power dynamics had influenced whether his thinking entered the collective decision-making process.

Corlin reflected on the experience for a long time. Cognitive governance, he realised, required more than constructing an appropriate mental model and asking meaningful questions internally. In professional environments, judgement had to operate within relationships involving authority, expertise, reputation and organisational hierarchy. Asking a necessary question therefore sometimes required courage as well as cognitive ability. Yet he also recognised that courage did not mean challenging senior people disrespectfully or assuming that his own concern must be correct. The question could be expressed with intellectual humility: “Could I clarify one assumption before we compare the solutions? What evidence allows us to conclude that this is the mechanism responsible for the performance gap?” Such a question challenged the reasoning without challenging the dignity or authority of the people involved. Corlin could not change what had happened in that meeting, but the experience changed what he did afterwards. When he encountered a consequential assumption that he genuinely believed required examination, he became less willing to remain silent merely because someone more senior had framed the problem differently. He learnt to distinguish respect for authority from surrender of informed judgement. Sometimes his questions turned out to be unnecessary. Sometimes the senior person had evidence Corlin had not seen. On those occasions, he learnt something. At other times, the question exposed an assumption that deserved investigation. Either outcome was preferable to withholding a meaningful question simply because of hierarchy.

Few years later, still as assistant professor, Corlin was invited to an industry meeting concerning a residential development experiencing complaints about uncomfortable indoor conditions. This time around he had gained confidence to as questions even among people that seem more experienced than him in a particular work context. The project team had already investigated the matter and arrived at two possible interventions. One offered better environmental performance but would require substantial modification and additional expenditure. The other was cheaper and easier to implement but was expected to deliver less improvement. The project manager placed the two options on the screen and asked, “We need your view, Dr Brown. Which would you recommend?” Corlin studied the comparison table. Cost, expected performance, installation time and disruption had already been ranked. It was exactly the kind of decision he knew how to make. His eyes moved instinctively towards the option offering stronger performance. If the environmental problem was sufficiently important, perhaps performance should take priority over cost. Alternatively, if the cheaper option achieved an acceptable threshold, perhaps the additional expenditure could not be justified.

Corlin was about to answer when something bothered him. “What exactly are we trying to correct?” he asked. The project manager looked slightly puzzled and replied, “The indoor environmental problem.” Corlin nodded. “I understand. But what is causing it?” There was a short silence. An engineer explained what the team believed was happening. Corlin asked how they knew. Another document appeared. He asked about the conditions under which the measurements had been taken. Someone explained, and Corlin asked another question. After twenty minutes, the two proposed solutions were no longer on the screen. After an hour, the meeting had not reached the decision it had been convened to make. Corlin left wondering whether he had helped the team or merely delayed a decision they had expected him to make. Two weeks later, additional investigation revealed that an operational condition had been contributing substantially to the observed problem. Neither of the two proposed interventions directly addressed it. Once that condition was corrected, the remaining performance gap was much smaller, and the expensive intervention originally being considered was no longer necessary in its proposed form. The project manager telephoned Corlin afterwards and said, “You saved us from solving the wrong problem very efficiently.” Corlin laughed, but he remembered the sentence. Before his PhD, he might have carefully compared the two proposed solutions and produced a perfectly defensible recommendation. He might even have selected the better one. The problem would not necessarily have been poor reasoning. It would have been that his reasoning had started too far downstream.

That experience became important to Corlin because it showed him that something he had begun learning through the privacy–ventilation dilemma could be transferred to problems far removed from his doctoral research. Before deciding which solution was better, he needed sufficient confidence that the problem requiring a solution had been appropriately diagnosed. The lesson did not cause him to reject quick decisions. Some problems were well understood, some constraints were genuinely fixed, and some situations demanded immediate action. Rather, he became increasingly sensitive to situations in which apparently sensible choices might conceal an inadequately understood problem. Years passed. Corlin published peer-reviewed research, developed collaborations and became increasingly known among architects, engineers and other built-environment professionals working on building performance. His research contributed original scholarly knowledge on how physical science could be used to understand and improve architectural performance. Yet something else was developing alongside his research career, and Corlin first became particularly conscious of it through a student named Maya.

During a design review, Maya’s group presented a proposal intended to improve daylight in a deep-plan learning space. Their intervention improved daylight penetration but created concerns about glare and thermal gain. The students had therefore reduced the opening area. “We decided thermal performance should take priority,” Maya explained. “So we accepted some reduction in daylight.” Corlin looked at the drawings. It was a reasonable decision, and years earlier he might have regarded the students’ ability to justify the compromise as evidence of good judgement. Instead, he asked, “What made you decide that those were the only outcomes available?” Maya hesitated before replying, “We had to compromise somewhere.” Corlin looked at her and said, “Perhaps you do. But how do you know yet?” The students looked at one another. Corlin pulled up a chair instead of standing over their work and asked them what performance they actually wanted. They began listing useful daylight, visual comfort, acceptable thermal conditions, energy performance and outward view. “Good,” Corlin said. “Now which of those requirements is illegitimate?” Maya answered that none was. Corlin nodded. “Then before deciding which legitimate requirement should lose, can we understand why they are conflicting?” For the remainder of the review, he barely discussed their proposed solution. Instead, he asked about solar geometry, orientation, glazing properties, visible transmittance, shading and the times at which the problematic conditions occurred. The Physics became Architecture because every physical mechanism was connected to something occupants needed from the space.

The following week, Maya returned with several alternatives. “We stopped trying to decide how much daylight to sacrifice,” she told him. “We looked at what was causing the unwanted heat and glare separately.” Corlin smiled. He recognised something of himself in her original reasoning. More importantly, he recognised the danger of simply giving her the better answer. Had he corrected the design himself, Maya might have improved one assignment. By helping her reconsider how she understood the problem, he might have influenced how she approached many future problems. Years later, at an industry conference, a woman approached Corlin after his presentation. “Professor Brown?” she said. Corlin recognised the face but could not immediately place it until she smiled and said, “Maya.” She was now an architect leading environmental design projects. She reminded him of that review and told him, “I still hear your question in meetings: ‘How do you know you need to compromise yet?’ My team is probably tired of hearing me ask it.” They both laughed, but the conversation stayed with Corlin longer than the applause he had received after his presentation. A scholarly paper could influence someone he would never meet, but teaching had allowed him to witness knowledge becoming capability in another person, who could then use that capability to give value to others.

………………… Chapter 6 ……………………

From then on, Corlin became increasingly deliberate about what he wanted his education to achieve. He did not want students merely to know Architectural Science; he wanted them to become capable of thinking with that knowledge. Physics provided understanding of mechanisms, Architecture connected those mechanisms to human and spatial purposes, and appropriate cognitive governance enabled learners to use knowledge to inform judgement and decisions. Corlin’s teaching, research and industry engagement therefore became progressively interconnected. Questions encountered in practice informed research. Original knowledge produced through research entered his teaching and scholarly publications. Teaching forced him to make complex knowledge intellectually accessible without reducing it to simplistic answers. Graduates carried what they had learnt into practice, where new problems emerged and returned to academia as questions worth investigating. As Corlin progressed from Assistant Professor to Associate Professor, his relationship with industry also changed. Professionals increasingly began calling him before they had selected solutions. An architectural practice invited him to examine a façade problem. A building owner asked him to help understand persistent indoor environmental complaints. Engineers brought him into discussions where energy performance, indoor environmental quality, cost and operational requirements appeared to conflict. Corlin noticed that the most useful contribution he could make was not always the technical answer he personally supplied. Sometimes his greater value was helping a team establish whether it was asking the right question.

In one particularly difficult project, an experienced engineer became visibly frustrated with him. “Corlin, eventually we have to make a decision,” he said. Corlin replied that he agreed. “We cannot investigate forever,” the engineer continued. Corlin agreed with that too. “Then what are we waiting for?” Corlin turned towards the performance requirements written across the whiteboard and replied, “For enough understanding to know what we are deciding between.” The engineer stared at him before laughing. “That sounds like something a professor would say when he wants another month of research.” Everyone laughed, including Corlin. Then Corlin pointed towards one of the constraints written on the board and asked, “Who established that this cannot change?” Nobody answered. The constraint had been inherited from an earlier design decision. It was not a physical law, regulatory requirement or unavoidable resource limitation. Once the team reconsidered it, an alternative configuration became possible. Months later, the same engineer stood beside Corlin at the completed project and admitted, “I hated that question.” Corlin asked which one. “Who established that this cannot change?” the engineer replied. “I’ve started asking my own team the same thing.” Corlin smiled because the engineer had acquired something more enduring than Corlin’s recommendation for one project. He had acquired a question he could continue using when Corlin was absent.

Corlin’s scholarly reputation grew alongside this industry influence. His publications contributed original knowledge to Architectural Science and Technology, while his engagement helped move scholarly knowledge into professional judgement and practice. Students he had taught became architects, engineers, researchers and industry leaders. Professionals who had worked with him began using questions and ways of reasoning they had encountered through those collaborations. Corlin increasingly understood that scholarly influence did not have to end with knowledge being published. Knowledge could be intellectually worked with, translated, contextualised and encountered by people in ways that strengthened their own capability to diagnose problems and make informed decisions. His transformation, however, was far from complete. Professional success did not prevent his old flaw from appearing elsewhere, particularly when emotion made a situation feel urgent. One evening, his wife told him that their teenage daughter had been unhappy for several weeks. Corlin had noticed her becoming quieter but had attributed it to school pressure. Her academic performance had recently declined while she was spending increasing time on an extracurricular activity she loved. Corlin listened and moved rapidly towards what seemed to him the responsible decision. “She needs to reduce the activity temporarily,” he said. “School has to take priority.”

His wife looked at him and asked, “What?” Corlin explained that their daughter could return to the activity when things stabilised. “You haven’t even spoken to her,” his wife replied. Corlin pointed out that they already knew her grades had fallen. His wife calmly responded, “We know her grades have fallen. We don’t know why.” Corlin felt himself becoming defensive and replied that there were only so many hours in a day. His wife remained quiet for a moment before saying, “You spend your professional life asking people whether they understand a problem before deciding what should be done.” The sentence landed harder than Corlin expected. That night, he knocked on his daughter’s door and, unusually for a father who desperately wanted to help, entered without a solution. For nearly an hour, he listened. The extracurricular activity was not causing the difficulty. It was one of the few places where his daughter currently felt successful. The real difficulty involved one subject she was struggling to understand, anxiety about falling behind and the increasing amount of time she was spending trying unsuccessfully to catch up. Removing the extracurricular activity would have taken away something supporting her wellbeing without addressing what was actually causing the academic difficulty.

Later, Corlin returned to the bedroom and told his wife, “You were right.” She smiled and asked whether he would like her to have the statement framed for his university office. Corlin laughed, but the experience disturbed him. At work, years of practice had strengthened his discipline to resist premature prioritisation. At home, concern for someone he loved had pulled him rapidly towards the very habit he helped students and professionals recognise. He had wanted to protect his daughter’s education, and because education mattered, prioritising it had felt responsible. His flaw had not disappeared. The difference was that he could now recognise it, reconsider his judgement and correct his course. That recognition gradually changed Corlin’s family life. He became more conscious that cognitive governance was not a professional capability that could be left at the university when he went home. Family dilemmas also involved legitimate values, incomplete information, emotions, constraints and consequences. Sometimes something genuinely had to take priority, and Corlin never abandoned that reality. But his wife and children increasingly experienced him as someone willing to understand before deciding. His professional transformation was becoming personal.

By then, Daniel and Eleanor Brown had retired from their universities and held emeritus professorships. Retirement had not made either of them particularly interested in behaving retired. Professor Emeritus Daniel Brown still read theoretical Physics papers at breakfast and occasionally sent Corlin papers accompanied by comments questioning the authors’ assumptions. Professor Emerita Eleanor Brown continued criticising buildings they passed, sometimes explaining how she would have designed them differently to anyone unfortunate enough to be standing nearby. Corlin enjoyed watching his parents remain intellectually themselves even as age gradually changed what they could physically do. He first noticed his father gripping the handrail more carefully when climbing the stairs. Eleanor began avoiding driving at night. Neither considered these developments remarkable until Daniel stumbled one afternoon on the staircase. He was not seriously injured, but the incident frightened Corlin. His immediate thought was that his parents should move to a single-storey apartment or a retirement development where assistance would be readily available. He investigated several excellent alternatives before discussing the matter with them. “No,” Eleanor said when he eventually presented the idea. Corlin began explaining the problem with the stairs, but his mother interrupted him. “I know where the stairs are, Corlin. I designed them.” Daniel remained silent while Corlin explained the risks and benefits of moving. He had evidence, options and a sensible recommendation. Eleanor listened before asking, “What problem are you solving?” Corlin replied that he was trying to reduce the risk of either of them getting hurt. “That is one problem,” she said. Corlin asked what the other was. Eleanor looked around the room and replied simply, “Our life.”

At first, the words sounded emotional to Corlin rather than practical. Then he looked around properly. His father’s study was lined with books carrying handwritten notes accumulated across a career. His mother’s drawings occupied cabinets she still opened regularly. The garden contained things planted by people who were no longer alive. Former students visited them there. Their neighbours, routines, memories and independence were embedded in the place. Corlin suddenly recognised the structure of his own reasoning. He had framed the situation as safety versus independence, and because safety appeared more important, he had already decided which should take priority. After a long silence, Corlin asked, “What would successful performance look like?” Eleanor looked at him and smiled. “You finally remembered.” Instead of spending the following weeks persuading his parents to move, Corlin began investigating. He observed where they actually experienced difficulty. The staircase was not the only issue. Lighting at particular times reduced visual clarity. Frequently used items required unnecessary reaching. One bathroom configuration increased effort. The route from the entrance created avoidable difficulty when carrying things. Transport arrangements made some journeys more demanding than necessary. He spoke with his parents about what they wanted to continue doing themselves and where they were comfortable receiving assistance. For the first time since Daniel’s fall, Corlin stopped treating age itself as the problem and began identifying the specific barriers affecting the performance they wanted from their lives.

Professor Corlin Brown, whose career had been built around using Physics to advance Architecture, then found himself applying decades of Architectural Science and Technology to the most personally consequential design problem he had ever encountered. Lighting was improved. Hand support was introduced where genuinely useful. Frequently used spaces were reorganised. Bathroom safety was improved without making the room feel institutional. Technologies were introduced selectively and only where Daniel and Eleanor considered them useful. Family support arrangements changed, transport was reorganised and tasks involving unnecessary effort were reduced, while activities that gave his parents independence and purpose were preserved. Not every risk could be eliminated, and Corlin did not pretend otherwise. Some compromises were necessary. The difference was that he could now accept them because they followed sufficient understanding of why they were necessary. Months later, Corlin arrived at the house and found Daniel upstairs in his study reading. When Corlin asked whether he had come upstairs alone, Daniel looked over his glasses and reminded his son that he had been climbing stairs since before Corlin understood gravity. They both laughed before Daniel became serious. “You know what you did right?” he asked. Corlin did not answer. Daniel continued, “You didn’t make our world smaller to make it safer. You changed what was making parts of it unnecessarily difficult.” From the doorway, Eleanor added, “Eventually.” All three laughed, but Corlin had to look away for a moment. His parents had spent their lives giving him intellectual resources. Now the cognitive capability he had developed partly through their influence was helping them preserve something more precious than convenience: dignity, agency, independence and continuity of life. His professional success gave him access to knowledge and resources that helped, but those resources alone would not have produced the same outcome. Had Corlin framed the problem incorrectly, the resources might simply have enabled him to implement the wrong solution more efficiently.

By the time Corlin was promoted to Full Professor of Architectural Science and Technology, neither his parents, colleagues nor industry collaborators needed an explanation of what he did. They had watched the identity develop over decades. As an Assistant Professor and later Associate Professor, he had already built a career using Physics to advance Architecture education, research and practice. Full professorship therefore did not mark the beginning of that identity. It represented recognition of its maturity: sustained original scholarly contributions, influence on professional practice, generations of learners he had empowered, and an intellectual approach that colleagues increasingly associated with his name. At his inaugural lecture as Full Professor, the university hall was filled with academics, industry collaborators, former students and members of his family. Daniel and Eleanor, older now and moving more slowly, sat together in the front row. Maya was several rows behind them. The engineer who had once complained that Corlin asked too many questions was also there. His wife and children sat nearby. Looking across the audience, Corlin could see different periods of his life represented by people who had experienced different versions of him. Corlin spoke about Physics, Architecture and the intellectual territory between them. He explained how physical understanding of air, heat, light and energy could support architectural decisions that delivered value to people. He discussed research that had contributed original knowledge, industry collaborations through which knowledge had influenced practice, and education through which learners had developed capabilities they could carry into situations their professor would never encounter. Near the end, he stopped beside a photograph of his parents taken many years earlier. “My father spent his career teaching me, directly and indirectly, to ask what mechanism produces what we observe,” Corlin told the audience. “My mother spent hers asking what successful performance should actually look like for people. It took their son much longer than it should have to understand why he needed both questions.”

The audience laughed gently. Daniel smiled while Eleanor wiped her eyes. Corlin continued by explaining that he had once regarded good judgement in a dilemma primarily as the ability to determine what should take priority. He still believed prioritisation was necessary in many circumstances. Life contained genuine constraints. Resources were finite, physical laws were real and not every compromise could be eliminated. “But I learnt something else,” he said. “Before deciding what must be sacrificed, we should understand why the sacrifice appears necessary.” Those words meant something different to almost everyone listening. Maya remembered a daylighting design review. The engineer remembered a supposedly fixed constraint on a whiteboard. Corlin’s wife remembered a conversation about their daughter. His daughter remembered a father who had eventually entered her room without a ready-made solution. Daniel and Eleanor remembered a son who had once arrived at their home with carefully researched options for where they should move, only to discover that he had diagnosed too little before deciding too much.

After the lecture, Daniel waited until the crowd surrounding his son had thinned. “You’ve done well,” he said. Coming from Daniel, three words were practically an emotional speech. Eleanor hugged Corlin before they began making their way slowly towards the exit. Daniel looked back towards the lecture theatre and said, “You know, when you were a boy, you always wanted the answer.” Corlin smiled and replied, “I still do.” Daniel raised an eyebrow. Corlin looked at his father and then at his mother before adding, “I’ve just learnt not to look for it too early.” They continued towards the exit together. The boy who had once become comfortable deciding which legitimate requirement should take priority had not grown into a man who rejected prioritisation. Professor Corlin Brown still prioritised when genuine constraints required it. His transformation lay in what increasingly happened before that judgement. He had learnt to establish what successful performance should look like, understand the current performance, identify the gap, investigate the root causes and barriers, distinguish what had to be accepted from what could potentially be changed, and only then exercise judgement about what should be done.

His PhD had begun that transformation because one personally familiar privacy–ventilation dilemma had made his usual practice of prioritisation unsatisfactory. His academic career had tested and strengthened the emerging capability through research, scholarship, industry engagement and education. His mistakes at home had humbled him by showing that possessing a way of thinking did not guarantee that he would always exercise it. His nuclear family had helped him transfer that cognitive governance beyond professional life, while his ageing parents had given him perhaps its most personally meaningful application. Corlin had contributed original scholarly knowledge to the literature. He had used Physics to advance Architecture education, research and practice. He had helped industry diagnose and solve difficult problems, while empowering professionals to think more effectively for themselves. Generations of students had carried elements of his thinking into their own careers. His success had eventually been recognised through promotion to Full Professor of Architectural Science and Technology. Yet the transformation underlying those achievements could not be captured adequately by an academic title, publication record or professional reputation.

The young Corlin had wanted to become someone who could look at a problem and provide an intelligent answer. Professor Corlin Brown had become something more valuable. He had become increasingly capable of knowing when not to answer yet. He could remain with a dilemma long enough to understand it, use knowledge without allowing knowledge to rush ahead of judgement, prioritise when prioritisation was genuinely necessary and challenge barriers when unnecessary compromise was being accepted as inevitable. The flaw had not vanished, as his family had occasionally reminded him. What had changed was his capacity to recognise and govern it before it governed his decisions. As Corlin’s reputation grew beyond academia and professional practice, he was increasingly invited to television interviews to discuss his expertise in Architectural Science and Technology, healthy indoor environments and the application of Physics to Architecture. At other times, he was invited to inspire wider audiences by sharing how people could think more appropriately when faced with dilemmas. Corlin particularly enjoyed such conversations because they allowed him to recount how his five-year PhD journey, beginning with the privacy–ventilation dilemma, had transformed the way he approached problems. He would explain how learning to question why legitimate requirements were conflicting, rather than immediately deciding which should take priority, gradually developed the cognitive governance that now shaped his research, teaching, industry engagement and personal life as a professor.

What began with a young Physics graduate refusing, for once, to accept that privacy, clear outward view and full ventilation had to compete had therefore travelled far beyond his PhD research. It had shaped the scholar he became, the professor his students remembered, the professional the industry respected, the husband and father his family experienced, and the son his ageing parents could depend upon without surrendering their independence to his concern for them. The PhD had given Corlin an opportunity to solve a practical dilemma. Learning how to think through that dilemma had gradually given him a way of giving value to others—and, ultimately, a better way of living his own life. The End!