Heat guide
Thermal Comfort: More Than Air Temperature
Thermal comfort depends on the balance between the body’s heat production and heat exchange with the environment, not on air temperature alone.
Environmental variables
Air temperature, mean radiant temperature, air speed and humidity all affect body heat exchange. A room can therefore feel different at the same thermostat setting when surface temperatures or airflow change.
Personal variables
Clothing insulation and metabolic rate strongly influence comfort. Someone doing physical work produces more metabolic heat than someone seated quietly.
PMV and PPD
Predicted Mean Vote estimates the average thermal sensation of a large group under steady conditions, while Predicted Percentage Dissatisfied estimates the proportion likely to feel thermally dissatisfied.
Model limitations
PMV is not a diagnosis and does not predict every individual. Adaptation, expectations, local discomfort, asymmetry, drafts, vertical temperature differences and non-steady conditions can matter.
Comfort and heat safety are separate questions
A condition can be uncomfortable without being medically dangerous, and a person can face heat strain even when a simple comfort index appears moderate. Occupational heat safety requires a separate assessment.
Adaptive comfort recognises that expectations change with context
People in naturally ventilated buildings often tolerate and prefer a wider range of indoor temperatures when outdoor conditions change gradually and occupants can use windows, fans or clothing adjustments. Adaptive comfort models therefore relate acceptable indoor conditions partly to recent outdoor temperature rather than assuming one fixed neutral temperature all year.
This approach is distinct from steady PMV-based assessment and applies only within the conditions defined by the relevant standard. Building type, occupant control and mechanical conditioning strategy determine which framework is appropriate.
Comfort depends on more than air temperature
Human thermal comfort reflects the balance between metabolic heat production, clothing insulation and heat exchange with the environment. Air temperature, mean radiant temperature, air speed and humidity all influence that exchange.
A room with cool air but hot surrounding surfaces can feel warmer than the air temperature suggests because radiative heat loss from the body is reduced. Strong air movement can alter both convective and evaporative heat transfer.
PMV and PPD describe group response under defined conditions
The Fanger PMV model combines environmental and personal variables to estimate the average thermal sensation of a large group on a seven-point scale. PPD estimates the percentage expected to remain dissatisfied under those model assumptions.
These indices do not predict exactly how one individual will feel. Adaptation, expectation, local discomfort, asymmetric radiation, drafts and personal preference can produce responses outside the model average.
Local discomfort can occur even when average conditions look acceptable
A person can experience cold feet, a warm head, drafts, radiant asymmetry or temperature stratification even when room-average measurements fall within a nominal comfort range. Floor temperature and proximity to windows or hot surfaces can therefore matter.
Useful comfort assessment combines whole-body indices with local measurements and occupant feedback rather than relying on a single air-temperature target.
Operative temperature combines air and radiant effects
People exchange sensible heat with both surrounding air and surrounding surfaces. In many indoor conditions, operative temperature provides a useful combined measure of air temperature and mean radiant temperature. A room can therefore feel cooler near a cold window even when the thermostat reports a comfortable air temperature.
This explains why improving surface temperatures through better glazing or insulation can improve comfort without changing the air-temperature setpoint by the same amount.
Local discomfort can exist even when whole-body conditions look acceptable
Drafts, vertical temperature differences, hot or cold floors and radiant asymmetry can create local discomfort even when an overall thermal-comfort index falls within an acceptable range. A person seated beneath a cold window or directly under a supply diffuser may experience conditions very different from the room average.
Comfort assessment therefore combines whole-body indices with checks for local environmental asymmetry and occupant activity.
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Sources and further reading
Use the linked primary or authoritative resources for additional detail, standards and source-specific conditions.