Thermal
Thermal Comfort
The condition in which a person is satisfied with the thermal environment. It is a perceptual state rather than a temperature, governed by six factors of which air temperature is only one.
In more detail
ASHRAE Standard 55 models comfort from six variables: air temperature, mean radiant temperature, air speed, humidity, clothing insulation, and metabolic rate. Four are properties of the room and two belong to the person. A thermostat reads one of them.
Mean radiant temperature is frequently missing from residential comfort discussions, and it can explain complaints that air temperature alone cannot. Bodies exchange heat with surrounding surfaces by radiation, so a room at 21°C air temperature with a large expanse of cold glass feels cold, and the same room with a sunlit floor feels warm. Turning up the thermostat answers a radiant problem with an air-temperature solution, which is expensive and only partly works. At low air speeds, operative temperature can often be approximated from air temperature and mean radiant temperature, and it tracks perception better than air temperature alone.
Air speed cuts both ways. A draft at 20°C is a complaint and the same air movement at 27°C is relief, which is the entire argument for ceiling fans in a cooling climate. Humidity matters most at the warm end, where it limits evaporative cooling from skin.
Comfort is also statistical. Standard 55 targets a proportion of occupants satisfied, not all of them, because individual variation in clothing, activity, and physiology means a room that satisfies everyone simultaneously is not a design target that exists.
Why it matters
A house that fails at comfort gets operated badly for the rest of its life — overheated in winter, overcooled in summer, with rooms that go unused because they never feel right. Most of the causes are enclosure and glazing decisions rather than equipment. Window performance, insulation continuity, and which way the glass faces set the radiant environment that the mechanical system then spends decades compensating for. Comfort complaints in a finished house are usually surface temperature complaints wearing a thermostat's clothing.
The common misunderstanding
That comfort is a thermostat setpoint, and that a room which fails to feel right needs a bigger system. Air temperature is one of six inputs. Cold glass, an uninsulated slab, a draft from a leaky rim joist, or a sunlit west wall will each defeat a correctly sized system, and none of them are solved by moving the setpoint. They are solved at the surfaces.
Related terms
On Compound
Where this term does real work — and where the decisions get made.
Sources
Last reviewed July 2026 by Compound Editorial. Definitions are educational and not project-specific engineering or medical advice.