Thermal Comfort Is Not Simply a Matter of Temperature - It Depends on Six Factors
A building that maintains a thermostat setting of 72°F can still produce thermally uncomfortable conditions depending on surface temperatures, air movement, humidity levels, clothing worn by occupants, and the physical activity of those occupants. ASHRAE Standard 55, Thermal Environmental Conditions for Human Occupancy, establishes the conditions under which 80% of occupants would be expected to find the thermal environment acceptable. This standard is referenced in building codes, LEED credits, and design guidelines, and it is tested on the ARE PPD exam because thermal comfort connects the mechanical systems that condition air with the envelope design decisions that control radiant heat exchange between occupants and surfaces.
The Six Factors Affecting Thermal Comfort
ASHRAE Standard 55 identifies six factors that together determine thermal comfort. Two are environmental (controlled by the building systems and envelope); two are personal (characteristics of the occupants themselves); and two are additional environmental factors:
- Air temperature: The temperature of the air surrounding the occupant. The most commonly monitored factor, but not the only important one.
- Radiant temperature: The temperature of surrounding surfaces (walls, ceiling, floor, windows). Cold windows in winter create discomfort through radiant heat loss even when air temperature is adequate. The Mean Radiant Temperature (MRT) averages the radiation exchange with all surrounding surfaces. A cool exterior wall in winter can make occupants feel cold even with 72°F air temperature because of radiant heat loss to the cold surface.
- Air velocity: Air movement increases convective heat loss from the body. At low temperatures, air movement increases discomfort (wind chill effect). At high temperatures, air movement can increase comfort by increasing evaporative cooling.
- Relative humidity: High humidity slows evaporation of perspiration, making warm conditions feel warmer. Low humidity accelerates evaporation. ASHRAE 55 establishes acceptable humidity ranges; very low humidity (below 20% RH) also causes dry skin and respiratory discomfort. Most building systems target 30–60% RH for occupied spaces.
- Clothing insulation (Clo value): The insulating value of clothing worn by occupants. Standard office clothing is approximately 1.0 Clo; summer clothing is approximately 0.5 Clo. Seasonal clothing variation is part of why acceptable temperature ranges shift between winter (with warmer clothing) and summer (with lighter clothing).
- Metabolic rate (Met value): The rate of heat production by the body based on activity level. Seated at rest is 1.0 Met; light office work is 1.1–1.2 Met; walking is 2.0–3.0 Met. ASHRAE 55's standard conditions assume sedentary activity; spaces with high activity levels (gyms, manufacturing) require different comfort targets.
The PMV/PPD Model and the Adaptive Model
ASHRAE 55 includes two methods for evaluating thermal comfort. The Predicted Mean Vote (PMV) model uses all six factors to predict, on a scale from -3 (cold) to +3 (hot), the average thermal sensation vote of a large group of occupants. PMV = 0 is neutral; the acceptable range is approximately -0.5 to +0.5, within which the Predicted Percentage Dissatisfied (PPD) is 10% or less. The Adaptive Comfort Model is applicable to naturally ventilated spaces without mechanical cooling; it recognizes that in spaces where occupants can control their environment (open windows, adjust clothing), acceptable temperature ranges are wider than in fully conditioned spaces, because occupants adapt both physiologically and behaviorally to the prevailing outdoor conditions.
Key Exam Points
- Six comfort factors: air temperature, mean radiant temperature, air velocity, relative humidity, clothing (Clo), metabolic rate (Met).
- MRT: average surface temperature surrounding the occupant; cold surfaces cause radiant discomfort even with adequate air temperature.
- PMV scale: -3 (cold) to +3 (hot); acceptable range ±0.5; PPD ≤ 10%.
- Adaptive comfort model: wider acceptable range in naturally ventilated spaces where occupants can adapt.
- Humidity: ASHRAE 55 range typically 30–60% RH for occupied spaces.
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