Thermal comfort models¶
The models used to judge comfort, and the factors they take account of: clothing, activity, 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 (shown in the image above) 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 of HVAC engineers to size their heating and cooling systems. However, field research has shown that people are comfortable in much larger ranges than the mathematical model predicted. Thus, the adaptive comfort model was born, it’s based on the idea that in addition to the factors above, people actively (and subconsciously) change their behaviour and environment to aid the thermal regulation process. So instead of viewing comfort as a simple balancing scale, it’s more a continuous feedback loop (shown below).
Heat balance
VS
Feedback loop
What the feedback loop diagram is saying is that your body changes the situation at all 4 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. For example, when it’s very cold, you huddle and curl up into a ball, to preserve your heat, but when its hot you might spread out while lying down on the bed. Your clothing can be added or removed to adapt to the surrounding temperatures. Finally, you can change your environment by opening windows, finding a better space, or closing the curtains for example.
Clothing¶
According to the ASHRAE 55-2010 and the BS ISO 9920. The clothing level can be estimated in measurements of ‘clo’. The table below shows the clo equivalent to typical ensembles. 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 less clothes than they would in an office with a suit and tie policy. You can use the clo values in the Climate-Consultant software to help you assess the strategies available to achieve comfort.
Activities¶
How much heat you emit from the surface of your body depends on what you’re 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 (W/m2). 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. On the other hand, moving can help keep you warm in colder weather.
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 sweating by making evaporation happen faster. This is especially important in humid areas. However, after if the air temperatures exceed a certain point, the fan can no longer provide a cooling effect. This cut off point may be around 32oC-35oC for hot humid climates and up to 40oC for hot arid climates. See ‘Outstanding Research Questions’ section on this specific topic.
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 that is derived from the air speeds in the area. So, for example, 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 temperatures. That’s why data loggers, which only log air temperature, need to be placed somewhere away from heat/coolth sources to be able to use the air temperatures as substitutes for operative temperature. Although ideally you have a black globe thermometer to separately measure radiant temperatures. However, in a project with floor to wall windows for example, the radiant temperatures would be really high, therefore measuring just the air temperatures wouldn’t accurately reflect what the occupants are experiencing.
Source: ARC Technical Criteria - 635
Reference · confidence medium · Imported 2026-09-12, not yet reviewed · next review 2026-12-12
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