ASHRAE 55

The standard that defines what indoor conditions people find thermally comfortable.

Quick Answer

ASHRAE Standard 55, Thermal Environmental Conditions for Human Occupancy, specifies combinations of indoor environmental and personal factors that produce thermal conditions acceptable to a majority of occupants. It considers air temperature, radiant temperature, humidity, air speed, clothing, and activity level, and is widely used in HVAC design and green building programs.

The Full Picture

Thermal comfort is not just a temperature setpoint. ASHRAE 55 treats comfort as the result of six factors: air temperature, mean radiant temperature, air speed, humidity, the insulation value of clothing, and the metabolic rate of occupants. Changing any one of these shifts the conditions people experience.

The standard defines a comfort zone, commonly shown on a psychrometric chart, where a target share of occupants, typically at least 80 percent, would be expected to find conditions acceptable. It uses the Predicted Mean Vote model for mechanically conditioned spaces and includes an adaptive method for naturally conditioned spaces where occupants control windows and dress for the weather.

Beyond averages, the standard also addresses local discomfort, such as drafts, vertical temperature differences, radiant asymmetry from large cold windows, and floor temperature. These matter in glassy buildings and spaces with radiant systems, where an acceptable average can still hide uncomfortable zones.

ASHRAE 55 is a design and evaluation reference rather than a building code in most places, although it is referenced by green building and health programs and sometimes by owner requirements. It informs decisions about diffuser selection, zoning, glazing, shading, and controls.

Real Examples

→Open office zoning: A mechanical engineer adds more thermostat zones along a glass perimeter because radiant asymmetry would otherwise leave desks near the window uncomfortable in winter.

→Naturally ventilated atrium: A design team uses the adaptive comfort method to justify wider acceptable temperature ranges in a naturally conditioned space.

→Owner requirement: An owner's design criteria cite ASHRAE 55 compliance, so the engineer includes thermal comfort analysis in the submitted documentation.

Common Misconceptions

People assume: ASHRAE 55 prescribes a single indoor temperature.

Actually: It defines a range of combined conditions that vary with humidity, air speed, clothing, and activity rather than one setpoint.

People assume: Meeting the standard means every occupant will be comfortable.

Actually: It targets acceptability for a majority of occupants. Individual preferences vary, so some people will still be dissatisfied.

Frequently Asked Questions

What factors does ASHRAE 55 consider?

Air temperature, mean radiant temperature, air speed, humidity, clothing insulation, and metabolic rate.

What is PMV?

Predicted Mean Vote is a model that estimates the average thermal sensation of a group of people on a scale from cold to hot based on the six comfort factors.

Is ASHRAE 55 required by code?

Generally not by itself. It is used as a design reference and is cited by programs such as green building certifications or by owner requirements.

What is the adaptive comfort model?

A method in the standard for naturally conditioned spaces that relates acceptable indoor temperatures to outdoor conditions, recognizing that occupants adapt through clothing and window use.