Thermal comfort

Factors affecting occupant comfort. (Source: IBPSA-USA BEMP Training Workshop)

Thermal comfort for building occupants is defined by ASHRAE Standard 55 as the "condition of mind that expresses satisfaction with the thermal environment." It goes beyond simply meeting a single temperature setpoint, depending instead on a complex balance of several factors related to the occupants and the surrounding environment.

ASHRAE Standard 55 thermal comfort criteria

ASHRAE Standard 55 is the benchmark for determining thermal comfort in buildings, defining it as the condition of mind that expresses satisfaction with the thermal environment. The standard provides criteria for designing and evaluating indoor spaces to ensure a substantial majority of occupants (at least 80%) feel comfortable. It achieves this by using six key inputs to calculate a space's Predicted Mean Vote (PMV) and Predicted Percentage of Dissatisfied (PPD).

The six inputs used to calculate thermal comfort are split into two categories:

1. Personal Inputs

These factors relate to the human occupants of the space.

• Metabolic Rate (Met): This is the rate at which the human body converts chemical energy into heat and mechanical work. It is measured in "met" units, where 1 met is the metabolic rate of an average person at rest. The value is adjusted for different activities, such as light office work (1.2 met) or walking (2.0 met).
• Clothing Insulation (Clo): This measures the thermal insulation provided by the clothing an occupant is wearing. It is measured in "clo" units, where 1 clo is the insulation of a typical winter business suit. A person in light summer clothes might have a clo value of 0.5.

2. Environmental Inputs

These factors relate to the physical conditions of the space.

• Air Temperature (Ta​): The temperature of the air surrounding the occupants.
• Mean Radiant Temperature (Tmr​): The average temperature of all surfaces surrounding an occupant (e.g., walls, windows, and ceilings). This is a crucial factor as people can feel uncomfortable if one surface is significantly hotter or colder than others, even if the air temperature is fine.
• Relative Humidity (RH): The amount of water vapor in the air, expressed as a percentage. It affects the body's ability to cool itself through sweat evaporation.
• Air Speed (v): The velocity of the air moving across the occupant. Higher air speed can increase discomfort (draft) in cooler conditions but can improve comfort in warmer conditions.