Understanding comfort: thermal neutrality explained

The human body has adapted to survive across an enormous range of climates. It does so through subconscious responses — changing blood flow to the skin, shivering, sweating — and through conscious ones: what we wear, when we are active, and the kind of buildings we put ourselves in.

All of it serves one purpose. The body is trying to reach balance. When it gets there, we call the feeling comfort. The technical term is thermal neutrality: the condition in which heat production is neither raised nor lowered to cope with heat or cold stress.

We are homeotherms

Humans hold their internal environment at a stable temperature through metabolic activity: a core temperature around 37 to 38 °C, and a skin temperature around 31 to 34 °C.

Holding it there means shedding excess heat in a warm environment and conserving it in a cold one. This is not a preference. A core temperature above 39 °C or below 34 °C has serious consequences for health, and the body will spend whatever it has to in order to prevent either.

Four ways the body loses heat

At rest, in ordinary indoor conditions, heat leaves the body by four routes in roughly these proportions:

• Radiation — about 60%. By a wide margin the largest.
• Evaporation — about 22%. Breath and insensible perspiration.
• Convection — about 15%. Air moving across the skin.
• Conduction — about 3%. Direct contact with objects.

Read that list again, because it is the reason this company exists. Radiation is the dominant route by which a resting person loses heat, and it is the one almost every heating system ignores. A radiator, a heat pump or a warm-air unit works on the convective 15%, and leaves the 60% to look after itself.

If the surfaces around you are cold, you radiate heat to them and you feel cold — regardless of what the air thermometer says. Anyone who has been chilly in a room at 22 °C with a cold external wall has felt this directly.

Six variables govern all of it

How fast and how much heat you lose by any of those four routes depends on six things:

Humidity. High humidity limits evaporation; low humidity dries out nose, throat and skin. The comfortable band is 40 to 60% relative humidity.

Air speed. Faster air across the skin means more convective and conductive loss, and a colder-feeling environment. This is what wind chill is.

Air temperature. Above roughly 27 °C the body has to work to shed heat; below roughly 18 °C it has to work to keep it.

Radiant temperature. The same relationship, but driven by the temperature of the surfaces around you rather than the air — and arguably more important than air temperature, given radiation carries 60% of the loss.

Metabolic rate. More activity means more heat generated internally, and therefore a different balance point.

Clothing insulation. Measured in clo values; more insulation slows every route of heat loss at once.

It is never one variable

Because these six interact, and because people differ in age, metabolism and clothing habits, thermal neutrality is better understood as a zone than as a set point. There is no single correct temperature for a room, which is why arguments about the thermostat never resolve.

The ASHRAE 55 comfort chart maps this out as a range of acceptable operative temperature and humidity — one zone for winter clothing at around 1.0 clo, another for summer clothing at around 0.5 clo. Operative temperature combines air temperature, radiant temperature and air velocity into the single figure that actually predicts how a room will feel.

That combination is the useful insight for anyone designing a heating system. You can raise the operative temperature by heating the air hard, or you can raise it by warming the surfaces. The second route gets you to the same comfort at a lower air temperature, which is where the energy saving comes from and why a room heated by its own surfaces feels different from a room heated by a radiator.

More on each individual variable — and the myths attached to it — in later articles. Why infrared sets out how we apply it.