Requirements for thermal comfort study material

2.1. Clothing and metabolic rate

Fanger (1970) showed that the overall thermal sensation can be predicted by “the comfort equation” (PMV). Equation connects six variables that have a large influence on comfort: activity level (met) thermal resistance of the clothing (clo), air temperature, mean radiant temperature, relative air velocity, and water vapor pressure in the ambient air. Fanger’s work constitutes the basis for comfort standard EN ISO 7730 and ASHRAE 55.

Clothing, through its insulative properties, is an important modifier of body heat loss and comfort. Clothing unit represents the thermal resistance of the clothing (1 clothing unit = 1 clo = 0,155 m2K/W) and the activity level (metabolic rate) means the released heat from human body per unit skin area (1 metabolic unit = 1 met = 58,2 W/m2, an average person is 1,7 m2) (ISO 7730).

Figure 2.21 shows the predicted percentage of dissatisfied, based on this comfort equation, for sedentary office work and different types of clothing. A total acceptance (0% of dissatisfied) for a given room climate cannot be reached. EN ISO 7730 thus specifies three comfort categories A, B and C with a maximum permissible rate of dissatisfied of 6, 10 and 15%, respectively. For office spaces, the categories A and B are appropriate.

Figure 1 Predicted percentage of dissatisfied (PPD) for a range of room temperatures (operative temperatures) and different clothing (sedentary activity). Comfort ranges for winter/transitional period and summer. (EN ISO 7730)

Clothing or garment insulation is quantified in clo units. Value Icl is used to describe the insulation provided by clothing ensembles. The contribution of an individual garment to overall insulation is expressed in terms of its effective insulations – Iclu. Clothing worn by people indoors is modified to a great extent by the season and outdoor weather conditions. During the summer months, typical clothing in commercial establishments consist of lightweight dresses, lightweight trousers, short- or long-sleeved shirts and blouses, and occasionally a suit jacket or sweater. These ensembles have clothing insulation values (Icl) rating from 0,35 to 0,6 clo. During the winter season, people wear garments constructed of thicker, heavier (i.e., warmer) fabrics and often add more garment layers to an ensemble. A typical indoor winter ensemble would have an Icl value rating from 0,8 to 1,2 clo (Ashrae, 2010).