THERMAL COMFORT

Thermal comfort: from a subjective perception to a shared standard

When an individual is inside an environment, he or she may perceive a sensation of heat or cold, a very subjective perception since it depends on the type of activity carried out, the position occupied by the person, or more simply on how he or she is dressed.
The sensation of comfort is strongly conditioned by the thermoregulation system of the human body, i.e. by the whole set of heat exchange mechanisms that allow a constant temperature of 37°C to be maintained.
These mechanisms can be: breathing, sweating, heat exchange between the individual and the environment. Thus, a person undergoing average physical activity, such as standing, might feel warmer than a person sitting for hours in front of a computer. Or, in an office where it is compulsory to wear a jacket and tie, people might feel warmer during the summer period, which is why the temperature needs to be reduced.
The sensation of comfort is linked either to an overall condition consisting of air and wall temperature, a slight air movement, an excessive air temperature difference between head and ankles, or even an inadequate floor surface temperature, i.e. it is linked to everything that can be understood as ‘local discomfort’.
People most susceptible to local discomfort are generally those who are sedentary for many hours, or who are resting. This is why the design of offices and bedrooms rather than living areas, theatres or cinemas must necessarily take these aspects into account.

THERMAL WELL-BEING: THE PARAMETERS OF GLOBAL COMFORT

But how can you tell if a room is more or less thermally comfortable?

Precisely because thermal comfort is due to a condition of well-being and is subjective, it is not enough to measure temperature and humidity. In fact, it can happen that two individuals within the same environment can perceive two different and opposite sensations. But if a representative sample of people were to be received in the same room at successive times, it is likely that most of them would express a uniform opinion on comfort in that room. It is also possible to find a certain uniformity of assessment of the environment by people dressed alike or of the same age.
This is where it becomes useful to assess comfort with an actual rating on thermal sensation, expressed by a substantial number of people. Specifically, thermal sensation is expressed on a 7-point scale, where 0 expresses thermal neutrality, +3 indicates a very hot sensation and -3 a very cold sensation. The individual ratings will be scattered to an average value: this is the so-called PMV index (Predicted Average Rating), which is the basis for all thermal comfort considerations.

il comfort termico grafico parametri del comfort globale

Next to the PMV there is the PPD (Expected Percentage of Dissatisfied), which expresses the number of thermally dissatisfied people. This value can never be equal to 0 because several studies involving thousands of people have shown that even in the best situations of thermal neutrality there is always at least 5% of dissatisfied people.

Since PMV expresses the heat balance between the human body and its environment, the factors involved in its determination are related to:

Metabolic activity of the person, expressed in metric or W/m²
Effective mechanical power, expressed in W/m² borne by the person
Air temperature conditions, generally at 1.1 m from the ground (corresponding to the height of a seated person's neck)
Average radiant temperature, typically 1.1 m above ground (corresponding to the height of a seated person's neck)
Air velocity
Type of clothing, expressed with the clo index.

The PMV can be assessed by:

Calculation, by means of the formulas given in standard UNI EN ISO 7730
From a table, according to EN ISO 7730, knowing information on type of clothing, activity, operating temperature and air speed
By direct measurement using special sensors (using a microclimatic control unit)
Comfort parameters such as air temperature and mean radiant temperature can be
Estimated by calculation
Measured using a microclimatic control unit according to the instructions in ISO 7726.

THE PARAMETERS OF LOCALISED DISCOMFORT

Local or localised discomfort expresses that sensation of heat or cold perceived in a specific area of the body and which inevitably contributes to the perception of overall comfort. The main parameters of local discomfort are as follows:

Floor temperature
Head-to-toe air temperature difference
Radiant asymmetry
Air currents

The values given in the tables below are derived from UNI EN ISO 7730

FLOOR TEMPERATURE

Temperatura del pavimento

Description
For people standing or sitting, wearing light shoes, the floor temperature affects the thermal sensation.

Correct value
Min 19°C in cold
Max 29°C in warm

Elements affected
Radiant water floors
Electrically heated floors but for short durations

Affected quantities
Surface temperature of the floor

AIR TEMPERATURE DIFFERENCE BETWEEN HEAD AND ANKLES

Differenza di temperatura dell'aria tra testa e caviglie

Description
When there is a high vertical air temperature difference between head and ankles, the person may experience discomfort.

Correct value
Max. 2°C for seated persons (between 1.1 m and 0.1 m above the ground).

Affected elements
Radiant water floors, displacement bottom ventilation systems, glazing.

Affected quantities
- Heat flow exchanged by walls and floor
- Mechanical ventilation
- Infiltration

RADIANT ASYMMETRY

Asimmetria radiante

Description
When a person stands between two walls with a high temperature difference, he or she may feel thermal discomfort; the same applies when there is a high temperature difference between the floor and ceiling.

Correct value
Horizontal asymmetry
warm wall <23°C
cold wall <10°C

Vertical asymmetry
warm ceiling < 5°C
cold ceiling < 14°C

Affected elements
Radiant ceilings, radiant walls, windows, poorly insulated walls or floors.

Affected quantities
Average surface temperature around the person.

DRAUGHT RATING

Correnti d'aria fredda

Description
Cold draughts, especially in the vicinity of the neck, can annoy people; the risk of discomfort (DR%) is higher for people who perform sedentary activities and feel hot. The DR% parameter ranges from 0 to 100%.

Correct value
<10%

Affected elements
Radiant ceilings, radiant walls, windows, poorly insulated walls or ceilings.

Affected quantities
- Air temperature
- Average air velocity
- Turbulence intensity

Air temperature (Ta): Room air temperature is important to define the convective exchange between a person's surface or skin and the environment. For comfort purposes it is important to assess it at ankle level, at the head height of a seated person (1.1 m) and at the height of a standing person (1.7 m).

Mean radiant temperature (Tmr): The mean radiant temperature is defined as the uniform temperature of a cavity in which the heat loss due to radiation is equal to that of a person under examination inside a real room; it therefore expresses the temperature of all surfaces surrounding a person inside a room. Calculating this temperature is quite complex and requires knowledge of the temperatures of all surfaces and the view factors (i.e. the geometric relationship expressing the radiative heat transfer between the various surfaces). Alternatively, the mean radiant temperature can be measured with a globoprobe, according to ISO 7726.

Operating temperature (To): The operating temperature expresses the overall heat exchange of a person with his or her surroundings. It is conceptually equal to the sum of the sensible exchange exchanged by convection with air (Ta) and the radiant exchange exchanged with surfaces (Tmr). For moderate thermal environments, if the difference between mean radiant temperature and air temperature is less than 4°C and the air velocity less than 0.2 m/s, the operating temperature can be easily calculated with the following formula: