Thermal comfort models¶
The two models most standards are built on - steady-state heat balance and adaptive comfort - and the factors that feed them, from clothing and activity to humidity and air speed.
According to the ISO standard 7730 definition, thermal comfort is the "condition of mind which expresses satisfaction with the thermal environment".
There are currently two main models that most standards are built upon: the steady-state heat balance model and the adaptive comfort model.
The steady-state heat balance model theorises that a person's thermal comfort can be measured by calculating the impact of specific factors on the 'thermal load' that impacts people's own internal thermal regulation systems. By distilling it into tangible things that you can measure, this model makes it easy for HVAC engineers to size their heating and cooling systems.
ARC's approach is built on the adaptive model, extended to include humidity - see the comfort model.
The feedback loop¶
Your body changes the situation at all four levels of heat transfer: from your internal body, to skin, to the clothes you wear, and finally the environment surrounding you.
At the body level, you can shiver to produce heat, or rub your hands to create some heat from the movement. At the skin level, you can sweat, change the blood flow rates to different parts of the body, or change your posture - when it is very cold you huddle and curl up into a ball to preserve your heat, but when it is hot you might spread out while lying down on the bed. Your clothing can be added or removed to adapt to the surrounding temperatures.
Clothing¶
According to ASHRAE 55-2010 and BS ISO 9920, the clothing level can be estimated in measurements of 'clo'. Where people wear traditional clothes like turbans and saris, some local studies offer an equivalent clo.
For a typical designer the key benefit in understanding this is to check that any local surveys were based on clo levels typical of the project they are working on. For example, if a local survey was conducted in people's homes, it would mean people were likely to wear fewer clothes than they would in an office with a suit and tie policy.
Activities¶
How much heat you emit from the surface of your body depends on what you are doing. Heat will transfer from a warmer body with higher temperature to a colder body with lower temperature.
Activity level is measured in 'met' units, which is 60 watts per square metre. That specific number comes from the amount of energy you produce sitting, which is the most common activity.
During heatwaves, taking a nap is an effective way to reduce your metabolic rate and help cool down. See practical advice during a heatwave.
Humidity¶
Air that is more humid makes it harder to sweat. This is why people can tolerate higher temperatures in hot dry climates.
Air speed¶
Higher air velocities help you sweat by making evaporation happen faster. This is especially important in humid areas.
However, if the air temperature exceeds a certain point, the fan can no longer provide a cooling effect. This cut-off point may be around 32-35 °C for hot humid climates, and up to 40 °C for hot arid climates.
Still under investigation
The source records this as an open research question and points to its own
"Outstanding research questions" section for the detail. That section is
working notes rather than guidance and has not been published here - see
content/originals.yaml. Treat the figures above as ARC's current working
view, not a settled threshold.
Air and radiant temperatures¶
Air temperature is the temperature of the air around you, while radiant temperatures are impacted by solar radiation either directly or indirectly reaching you. The operative temperature, often used in comfort models, is a sort of average between the two.
The air and radiant temperatures are weighted depending on a factor derived from the air speeds in the area. So when you are inside, not near the window or any hot ceiling, and the air is still, the operative temperature is usually the same as air temperature. That is why data loggers, which only log air temperature, need to be placed away from heat and coolth sources if the air temperature is to stand in for operative temperature. See the comfort model and physiological adaptation.
Tables and diagrams not yet included
The source contains a clo table for typical ensembles, and a heat balance versus feedback loop diagram. Neither has yet been prepared for the wiki.
Source: ARC Technical Criteria - Appendix 1: Thermal comfort models
Reference · confidence medium · Imported 2026-08-29, not yet reviewed · next review 2026-11-29
Tags: reference, comfort, model, clothing, air-speed, humidity
Citation key: thermal-comfort-models