Introduction
There are so many things to consider when designing and constructing a building. Buildings are often graded by how well they meet regulations, codes, standards, and performance metrics. On paper, they may be flawless, structurally sound, energy-efficient, and fully compliant. But step inside, and the story is rarely so neat. Occupants navigate spaces in ways designers never imagined: rearranging furniture, improvising shading, or bending rules just to make the environment work for them.
People are the ultimate co-designers, constantly flipping the script on designs outlined on paper. The buildings that truly succeed are the ones that anticipate this unpredictability, spaces that are intuitive, flexible, and designed with the human factor at their core.
The intuition gap
When designing a space, it is essential to consider both its intended function and the people who will occupy it. For thermal comfort, residential spaces are usually planned per apartment or unit, while commercial or office spaces are often designed per floor plate. However, design parameters often rely on fixed values that do not reflect real world variability.
How industry design parameters are set
To manage these complexities, designers rely on established psychrometric values – minimum, maximum or average conditions – that provide a standardised basis for design decisions.
These values include:
In naturally ventilated buildings, internal humidity is uncontrolled, so fixed values are not provided and industry standard assumptions apply.
Why these parameters don’t always match reality
Although these psychrometric values are essential for guiding design, they cannot fully capture the natural variability of environmental conditions or the unpredictability of how occupants use a space. As a result, real world performance can differ from what was predicted on paper.
This divergence is particularly important in areas such as condensation risk assessment. The design values selected at the outset determine how internal comfort is modelled and how risks such as internal or interstitial condensation are evaluated. Condensation – whether on internal surfaces or within the wall build up – can lead to corrosion, rot and mould growth, all of which can have significant impacts on material durability and occupant health.
In practice, this means that the values established at the design stage must be understood as a starting point rather than a fixed representation of reality. Conditions fluctuate over time and once a building is occupied, everyday habits, movement patterns and practical routines inevitably reshape how the space operates. These shifts – both environmental and behavioural – are what ultimately drive the gap between design assumptions and real world performance.
Humidity: another design assumption that fails in real world use
In the same way that thermal comfort and ventilation rarely play out as predicted, humidity levels inside a home or workplace are also highly dependent on human behaviour, yet designers often treat them as fixed values.
The level of humidity in a space is the result of a dynamic relationship between temperature, ventilation, and the introduction of water vapours within that space. Occupancy, how the space is used, and its contents—including houseplants—all influence moisture levels. Activities such as cooking, bathing, indoor clothes drying and the use of humidifiers can significantly increase humidity, making effective ventilation critical.
Where this becomes important is when humidity control depends on ventilation.
In naturally ventilated spaces, the design relies on windows being opened often enough and for long enough to achieve the air changes needed to remove humid air and bring in drier, fresher air. This requirement, however, is wholly dependent on user behaviour.
As a result, ten identically designed spaces with ten different occupants can perform very differently, simply because of how often windows are opened or left closed.
Temperature and humidity assumptions vs The lived experience
During winter, night time external temperatures may be assumed to be -5oC, based on minimum historic weather data values, whilst the internal temperature may be assumed to be 21oC for human comfort. External temperature values are often based on an average minimum, but in reality, the actual minimum can be lower, and this usually occurs at night.
In contrast, the average internal temperature of 21oC is typically a day time average, as most people do not typically sleep with the heating on to maintain a steady 21oC.
Where humidity is uncontrolled, a value of between 45% and 55% is typically used. However, this does not consider if there are, for example, four people in a room filled with plants, or the use of a humidifier within the space, or perhaps wet clothes spread out to dry in a space that has not had the windows opened the entirety of winter.
Where the space is perhaps mechanically vented with air conditioning, this may be less exaggerated. However, it is mindful to consider the tussle between trying to get more humidity into a space for health reasons whilst the mechanical systems keep activating the removal of any moisture it deems excessive due to its design settings.
Design parameters and occupant demographics
Design parameters also do not consider that a home may have young children or neurodiverse individuals who have a different perception of risk and may climb out of upper floor windows. In homes where this has occurred, preventative measures are often taken, including permanently locking windows or blocking access with furniture. For occupant safety, these spaces are therefore not ventilated as designed.
Space variations in offices
Offices are often designed as a single floor plate for MEP (Mechanical, Electrical and Plumbing), structural and façade design considerations. This, however, may not be in alignment with the market requirements at the time the project comes into use and commercial / lease preferences may require smaller compartmentations or partitioning.
In situations where the potential compartmentation has not been considered in the initial design, the following issues may arise:
Overheating – when energy efficiency meets reality
With the drive to create buildings that perform exceptionally well in winter to reduce heating costs, many buildings are now heavily insulated and airtight to limit heat loss. However, in the warmer summer months, these same characteristics can create spaces that become heat traps. Typical UK residential dwellings are not designed with active climate control for summer and rely solely on natural ventilation through user operated windows.
Designing for overheating in residential dwellings relies heavily on window operability to provide natural ventilation. Design assumptions often include windows being open during the day for ventilation and fully open at night to mitigate overheating. What these assumptions do not account for how people actually use their homes both day and night.
Principles of intuitive building design in façade engineering
Façade engineering plays a central role in how a building actually performs, because the façade is the main pathway for heat and moisture to move between inside and outside. The thermal values set by the Mechanical, Electrical and Plumbing (MEP) engineer only become meaningful once filtered through the façade design, which must account for product availability, durability, installation quality and, crucially, how occupants will use the building.
Designing every space to suit every individual behaviour is impossible. But the assumptions used in design must be questioned regularly to check they still reflect how people live. Meeting a performance target on paper is not enough; designers must consider whether the space will work for occupants and whether small design adjustments could improve comfort and usability.
In use feedback is vital yet often limited. To understand real performance, designers need insights not just on comfort but on occupant demographics, health conditions, family changes, heating routines, day night behaviour, and how façade elements interact with these patterns. Without this, it's hard to know whether “average values” genuinely represent the people who will live or work in the building.
In the absence of detailed feedback, the responsibility falls to proactive designers to go beyond textbook values and incorporate an understanding of real human behaviour. Ultimately, even the best designed façade must contend with human free will: people will always use spaces in ways the design never anticipated.
Conclusion
As this article shows, building performance is shaped as much by human behaviour as by technical design. Façades sit at the centre of this relationship: they translate theoretical MEP values into the lived experience of heat, ventilation, moisture and comfort. But when assumptions about temperature, humidity or occupant behaviour fail to match real life, even well designed buildings can perform unpredictably.
For designers, the task ahead is clear, to question static assumptions, seek better in use feedback, and create façades and building systems that respond more intuitively to the people who rely on them. Buildings must do more than comply; they must work with the realities of everyday living.
