Prof. Dr. Wouter van Marken Lichtenbelt is the former head of the research group Thermophysiology & Metabolism at Maastricht University, whose work has significantly advanced the understanding of how the human body interacts with indoor environments. Through pioneering studies on cold exposure, brown fat activation and adaptive thermal responses, he has shown that mild deviations from constant indoor temperatures can have positive effects on both metabolic health and energy use.

In the context of climate change, his work provides a crucial perspective: rather than designing buildings to maintain static ‘neutral’ conditions, indoor environments can support human adaptation, resilience and reduced energy consumption through controlled thermal variability.

BUILD UP (BUP): Having significantly advanced our understanding of thermal comfort, how do you see this evolving knowledge challenging the way ‘optimal’ indoor conditions are currently defined in buildings?

Wouter van Marken Lichtenbelt (WML): I think there is a gradual improvement from a focus on average comfort, a constant indoor environment and combined energy-costly indoor climate management, towards a healthier indoor environment with attention to individual differences in physiology and expectations. The adaptive comfort model together and with recent results from physiological and behavioural studies, provides a sound step forward in this respect. This model is increasingly used as input for built environment guidelines, e.g., the Dutch ISSO publication 74 and the ANSI/ASHRAE Standard 55.

BUP: Today’s smart buildings often aim to maintain tightly controlled indoor conditions. From your perspective, what are the limitations of this approach when it comes to human health, comfort and adaptation?

WML: This indeed is still often the case; in fact, it is a relic from the comfort models used in the 1970s. Even building and installation directives gradually use more modern insights, it is the rather conservative attitude by installers and building managers to still focus on a constant indoor temperature, virtually independent of the outdoor conditions. 

Research has clearly shown that occupants adapt to more variable indoor environments, although individual responses may be different. Many studies have also shown that both heat and cold exposure may improve metabolic health.

BUP: Thermal comfort and health are not always aligned. How should this distinction be better reflected in building design and policy frameworks, such as the Energy Performance of Buildings Directive (EPBD)?

WML: I think the effect of variable indoor temperatures on health is undervalued and not well understood. An increasing amount of evidence comes from studies examining mild temperature changes on our metabolic health. Mild low temperatures make the human body increase its energy expenditure, in a way comparable to exercise. Although the mechanism may differ from sports, we also see a positive effect of low temperatures on sugar and fat metabolism. Repeatedly, we observed an increase in insulin sensitivity. Likewise, effects are observed after a week of exposure to an increase in temperature.

On top of that, we often observe that people gradually become comfortable over the course of days with both high and low temperatures. Importantly, resilience to more extreme temperatures also increases, which is nowadays especially important during heat waves.

It is, however, important to realise that during the first days of relatively cool or relatively warm environments, people must get used to these conditions. The sound provision of information is very important. Also, personal control systems may be very useful. For instance, occupants are all in the same office, but individuals can make use of local temperature control systems.

BUP: Moving away from constant indoor temperatures implies a shift not only in design, but also in expectations from occupants. What are the implications of this transition for both building performance and user experience?

WML: I think a slight change in thinking and expectations is needed. Therefore, more information is needed. Since more than a century, we have been adapted to so-called stable indoor environments, relatively high in winter and low in summer. We gradually think this is normal, although this is not the case. More normal and natural is a more variable temperature. It is not the intention to use extreme temperatures as may have occurred in nature in the past, but more changing environmental conditions to which humans can easily adapt. More information is needed not only for the occupants, but also for building owners, managers and installation designers. 

BUP: Looking ahead, what key changes would you recommend to better integrate human physiology and adaptive comfort into building standards and performance frameworks?

WML: Together with sound information and well-designed installations, it is to be expected that there will be good experiences. It is crucial that people really realise what is happening, and from the start will be encouraged to behave in a healthier way. We can think of designing instruments similar to those we nowadays use to use for exercise promotion (e.g. a step counter stimulates many people to walk).  In such a way, people can become more aware and be more motivated to actually use more variable indoor temperatures.