The definition of comfort

The main parameters

The main parameters that affect
thermal comfort are:

> air temperature;
> mean radiant temperature;
> air speed;
> relative humidity;
> activity (metabolism);
> clothing;
> subjective factors.

Air temperature (°C)

Air temperature is understood as the dry-bulb temperature. It is the most important factor in determining thermal comfort.

Mean radiant temperature (MRT, °C)

The MRT is the weighted mean temperature of the temperatures of the surfaces that form the boundary of the room, including the effect of incident solar radiation. It affects transfer by radiation. Together with air temperature, MRT is the factor that most affects the sensation of warmth because the radiation that falls on the skin stimulates the skin‘s sensory organs. If the body is exposed to cold surfaces, a significant amount of heat is emitted in the form of radiation towards these surfaces, producing a sensation of cold. A variation of 1°C in the air temperature can be compensated by a counter variation between 0.5 and 0.8°C in the MRT. The most comfortable condition was considered to be that corresponding to an MRT 2°C higher than the air temperature. An MRT up to 2°C higher is also tolerable if the radiation emitted by the body is almost the same in all directions. This happens only if the surface temperatures of the surrounding environment are practically uniform. We can also define the operating temperature as the average of the air temperature and the mean radiant temperature, to evaluate heat transfer by convection and by radiation with a single value.

Air speed (m/s)

Air movement produces thermal effects even without a variation in air temperature and can facilitate the dissipation of heat, through the surface of the skin, in the following ways:

1) Increase in heat dissipation by convection, until the air temperature remains below that of the skin;

2) Acceleration in evaporation and therefore the activation of physiological cooling. At low humidities (< 30 %) questo effetto è irrilevante in quanto si ha già una intensa evaporazione anche con aria ferma; alle alte umidità (> 80 %) evaporation is somewhat limited and air movement has no major cooling effects. However, evaporation can be significantly accelerated in average humidity levels (40-50 %): if the air is still, the layer closest to the skin quickly becomes saturated, preventing further evaporation whereas moving air allows continuous evaporation.

However, the use of moving air to cool down an environment can be limited by its annoying effects. Average subjective reactions to different speeds are as follows:

up to 0.25 m/s:

0.25-0.50 m/s:

0.50-1.00 m/s:
sensation of moving air

1.00-1.50 m/s:
airflow from light to annoying

above 1.50 m/s:

Relative humidity (RU, %)

Relative humidity is the ratio of the amount of water contained in one kilo of dry air at a certain temperature and the maximum amount of water that may be contained at the same temperature in the same kilo of air.

Atmospheric humidity, if not extremely high or low, has a minor effect on comfort. At comfortable temperatures there is no need for evaporative cooling while at higher temperatures this becomes the most important means of heat dissipation. Saturated air (100% RU) prevents any kind of evaporative cooling. When RU is below 20%, the mucous membranes dry out, which increases the chances of infection.

At low temperatures, very dry air enhances the sensation of cold because moisture reaching the surface of the skin evaporates, giving an unpleasant sensation of cold. For air temperatures above 32°C and RU over 70%, any sensation of heat is made worse since sweat cannot evaporate. In stable conditions, an increase in RU of 10% has the same effect as a temperature increase of 0.3°C.

The influence of RU increases if you move between environments with different conditions of temperature and humidity (i.e. dynamic conditions), increasing its influence on the sensation of comfort by up to 2 or 3 times.






Relative humidity

50-60 %

40-50 %

Air speed

0.1-0.2 m/s

0.05-0.1 m/s

Recommended values for temperature RU and air speed according to season

Activity (rate of metabolism)

The body constantly produces heat in varying quantities: "metabolism" is the term that describes these biological processes. The rate of metabolism is the energy released per unit of time from the processing food. The amount required by the body depends on the level of activity. It is expressed in Watt/square metre of body surface area (approximately 1.8 m2) or in "Met" (1 Met = metabolic rate of a person at rest = 58 W/m2).

Example: sleep (0.7 Met - 40 W/m2), sedentary office work (1.4 Met - 80 W/m2)

Clothing affects heat loss by evaporation and heat transfer by conduction and radiation. Clothing is our thermal insulation and changing our clothes is the most effective and conscious way we can control heat loss.

The thermal insulation provided by clothing is expressed in "Clo" (1 Clo = typical indoor winter clothing = 0.155 m2 K/W).

Example: light summer clothes (0.08 m2 K/W - 0.5 Clo), winter clothes (0.23 m2 K/W - 1.5 Clo)

Evaluating thermal comfort

All the factors listed above interact to create a sensation of comfort or discomfort. It is impossible to judge environmental comfort on the basis of only one of these parameters. We can numerically evaluate the environmental conditions that correspond to sensations of thermal comfort using statistical tests that evaluate the degree of satisfaction of groups of people inside variously conditioned environments.