Reductions in energy use and provision of comfortable indoor environment to occupants are both key objectives of the building sector all around the world. However, establishing the appropriate balance between these often competing issues is challenging. Is it possible to achieve thermal comfort in buildings without increasing energy use?

The key point is to understand the occupants’ real thermal demand. To maintain the indoor environment variables within narrow range is known to consume copious energy, but is the steady iso-thermal environment with minimal variations really necessary for thermal comfort? Previous studies have shown that staying in a steady thermal environment for long time periods may actually be harmful to human body, since it weakens the physiological thermoregulatory resilience and acclimation when people are finally exposed to heat stress. We now have enough evidence to show that tight control of indoor temperatures drives high energy costs and greenhouse gas emissions, and may not always provide benefit for occupant comfort and health. The current indoor environment standards for mechanically heated and cooled buildings are based on the PMV-method for specifying an acceptable comfort temperature range. The same standards also include an adaptive approach for office buildings relying on operable windows instead of mechanical cooling systems (e.g. ISO 7730, EN 15251, ASHRAE 55).

The concept of adaptive thermal comfort is regarded as a great contribution which may play an important role in low energy building design and operation. It was firstly proposed by Charles Webb (UK Building Research Establishment) in the 1960s to explain the phenomenon that the comfortable temperature inside naturally ventilated buildings tracked outdoor climatic trends. That work was continued in the UK by Humphreys and Nicol in the 1970s, and in the early 1980s. Richard de Dear (Australia) established that the climatic dependency of indoor comfort temperatures exceeded what could simply explained by clothing, air speed or other “heat-balance” parameters in the PMV comfort model. In the late 1990s, Richard de Dear (Australia) and Gail Brager (USA) pointed out that people was not just passively accepted the environment but also interacted with the environment through multiple feedback cycle and the "human - environment" system. They established the adaptive thermal comfort theory and classified thermal adaptation process into three approaches, which are physiological acclimatization, psychological adaptation and behavioral regulation. The adaptive thermal comfort model for natural ventilated buildings was also developed based on the large global thermal comfort field study database assembled for ASHRAE RP-884 project. The model was considered suitable for free-running buildings (without cooling/heating) and adopted by ASHRAE Standard 55 (from the 2004 version) and then repeated again on a smaller European database for EN 15251 in 2007.

The research literature on adaptive thermal comfort reveals that the acceptable indoor temperature range may relate with the outdoor climate changing through annual seasonal cycles. As long as indoor temperature is maintained within the acceptable range appropriate to the season, it is possible for people to achieve thermal comfort through the three adaptive approaches (physiological, psychological, and behavioral). That means there is no need to always maintain a steady indoor thermal environment with mechanical cooling/heating, and some natural bioclimatic design resources such as natural ventilation could play a role in achieving thermal comfort for occupants while simultaneously reducing energy consumption as well. Furthermore, it has been discovered that degrees of freedom for individual personal environmental control have positive impacts through both psychological and behavioral adaptive approaches, which could further enhance occupant’s satisfaction with their indoor thermal environment. If the building’s services systems could be running in a “part-time & part-space” mode depending on its occupants’ individual demand instead of the “whole-time & whole-space” mode prevalent in many buildings today, energy use could also be reduced.

Currently, there are still existing problems to be solved in this field of research:

1)      Although the adaptive effect has been recognized by many researchers, the mechanism of the adaptive process is still unclear. How do the physiological, psychological, and behavioral factors work alongside conventional thermal heat balance explanations (clothing, airspeed, metabolic rate) cannot be readily explained by the current adaptive model, which is partially due to the inherent limitations of the last generation of thermal comfort field study database.

2)      The specific thermal adaptive mechanisms of people in diverse climate regions could be quite different, which may suggest different building design and operational strategies, and indoor environment standards. The understanding for occupants’ adaptive thermal demand in different climate regions remains limited at this point.

3)      Apart from the pure free-running buildings and pure mechanically heated/cooled buildings, mixed-mode buildings (cooling/heating together with natural ventilation) are actually the most general cases. However, there are no specific evaluation criteria for this kind of building to date. Only mechanical cooling/heating standards are used for the mixed-mode buildings now, which may represent an unnecessarily conservative barrier to their more widespread adoption. Most clients refuse to recognize the potential effect of energy saving by adaptive modes in mixed-mode buildings with indoor thermal conditions departing from the conventional HVAC comfort range in current standards.

To solve these problems, more focused, quantitative studies are required. Currently, there are over a dozen research groups around the world working in adaptive thermal comfort and building energy performance separately. To form an international cooperation is extremely important to accelerate growth of knowledge in this area and its application in the building sector. That’s the reason Prof. Yingxin Zhu and Dr. Bin Cao at Tsinghua University (China) submitted a proposal to IEA-EBC (International Energy Agency – Energy in Buildings and Communities Programme) in 2014, in order to establish an international network on adaptive thermal comfort within the scope of IEA-EBC. The proposal was approved by the EBC Executive Committee by the end of 2014 and the project was assigned “Annex 69: Strategy and Practice of Adaptive Thermal Comfort in Low Energy Buildings”, which officially started in 2015 and is expected to finish at the end of 2018.


The planned deliverables from this Annex are:

-              Database with user interface including information of human thermal reaction together with their behavior and energy consumption,

-              Model and criteria for the application of adaptive thermal comfort in built environment,

-              Guidelines for low energy building design based on adaptive thermal comfort concept, and

-              Guidelines for personal thermal comfort systems in low energy buildings.


The project beneficiaries will be:

-              the building research community and associated specialists,

-              policy and decision makers involved in developing standards and building performance evaluation,

-              architects and design companies, engineering and consulting offices in building physics, HVAC systems, and energy consumption, 

-              developers and manufacturers for HVAC devices with an interest in high performance as well as energy conservation, and

-              educational institutions.



Participating Countries (in the alphabetic order):










    The Netherlands



    United Kingdom

    United States


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