The definition of comfort

Invisible comfort

Comfort is the achievement of equilibrium with our environment in the absence of any discomfort. It is already clear from the definition that comfort cannot be measured analytically but only statistically, since it depends on too many variables, some of which are purely subjective and psychological in nature.

The variables are:

  • thermal and hygrometric comfort;
  • olfactory comfort (related to air quality);
  • visual comfort (related to lighting);
  • psychological comfort.

The analysis that follows will focus primarily on thermal and hygrometric comfort.

Thermal and hygrometric comfort

Thermal comfort depends on:

  • physical parameters: air temperature, mean radiant temperature, relative humidity, air speed, atmospheric pressure;
  • external parameters: activity affecting metabolism, clothing;
  • organic factors: age, sex, individual physical characteristics;
  • cultural and psychological factors.

In addition, there are varying degrees of acceptance of so-called uncomfortable situations according to social and environmental conditions. The experience of a prolonged period of discomfort can lead to some environmental situations being considered "normal" when in a different context they would be judged uncomfortable. In a highly developed civilisation like our own we have come to expect a high degree of comfort.

Il comfort termico e igrometrico



Air temperature

26 °C

20 °C

Relative humidity

30 % < U < 60 %

30 % < U < 50 %

Air speed

0.1-0.2 m/s

0.05-0.1 m/s

Actual temperature

20-22 °C

16-18 °C

Tab. 1. Average limits for what are considered optimal hygrothermal conditions (Melino C. 1992).

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.

The importance of a radiant system for comfort

Comfort in the home is affected by many factors. It is dependent on our needs as individuals and our personal perception of comfort as the seasons change. UNI EN ISO 7730 sets out the parameters for indoor comfortand identifies ideal conditions as a perceived room temperature of approximately 20°C and a percentage humidity of 50-55%.

A radiant system operating at a surface temperature of 26-27°C triggers transfer by radiation, bringing the entire structure to a temperature in the region of 22-24°C. This temperature is not just found in certain points of the house but is evenly distributed across the entire floor area. This ideal distribution of temperature in each room ensures a high degree of comfort.

The body is enveloped in a warm "cocoon" and doesn‘t suffer the sensation of cold experienced in rooms fitted with traditional systems, such as radiators, or from turning one‘s back to a window or poorly insulated wall. The "warm cocoon" allows the body to tolerate a lower air temperature without experiencing discomfort.

Colder air is also less dry which is better for the respiratory system.

The cause of inflammation of the nasal mucous membranes, laryngitis and bronchitis is excessive heating of the air as a certain degree of moisture is necessary for the mucous membranes of the respiratory system - the first natural filter against external bodies - to work properly.

Air at an even temperature in a room prevents the generation of annoying air currents, the cause of airborne dust in radiator-heated environments.

These air currents are caused by expanding air molecules. When air molecules come into the vicinity of a radiator they heat up, increasing their volume. Since they are lighter they tend to rise, before cooling and falling back down again.