Existing thermal comfort standards and methods cover mainly thermal comfort conditions under steady state conditions. Most of the thermal comfort studies have been carried out in laboratories and are based on evaluations of the heat transfer between the human beings and their environment and of the required physiological conditions for thermal comfort. Given the thermal interaction between the building envelope, the occupants and the heating and cooling system, it is very rare to encounter steady state conditions in real buildings, and it is evident that the temperature in free running buildings is far less likely to be steady.
Field comfort studies carried out around the world have shown that the so called adaptive approach describes comfort conditions in non air conditioned buildings better. The fundamental assumption of the adaptive approach is expressed by the ‘adaptive principle’ that stipulates:
“If a change occurs such as to produce discomfort, people react in ways which tend to restore their comfort”
This principle codifies the behaviour of building occupants which takes two basic forms:
- Adjustments to the optimal comfort temperature by changes in clothing, activity, posture, etc. so that the occupants are comfortable in prevailing conditions
- Adjustment of indoor conditions by the use of controls such as windows, blinds, fans and in certain conditions mechanical heating or cooling. Occupants may also migrate around the room to find improved conditions
It is as a result of these adaptive behaviours that field surveys have verified that the comfort temperature is very closely related to the mean indoor temperature. It is suggested that such an effect could be seen as the result of feedback between the thermal sensation of subjects and their behaviour as part of the processes by which the thermal situation is preserved. Very important research has been carried out in order to develop an international adaptive comfort standard. As a result ASHRAE has proposed an adaptive comfort standard (ACS ASHRAE Standard 55), applicable for naturally ventilated buildings. In parallel, the new proposed thermal comfort standard of CEN involves an adaptive thermal comfort part. Analysis from various researchers has shown that when variable indoor temperature comfort standards based on adaptive theory are used in air conditioned buildings remarkable energy savings may occur. The expected energy saving in European buildings is more than 18% over that from using a constant indoor temperature as reported, while the corresponding energy savings for UK conditions have been estimated close to 10 %.
The VIP on adaptive comfort offers the basic scientific and practical information on adaptive comfort and discusses the impact of air speed on thermal comfort.