Thermal comfort and ASHRAE Standard 55

Thermal comfort is the condition of mind that expresses satisfaction with the thermal environment. That definition, from ASHRAE Standard 55, captures an important truth: comfort is subjective. Two people in the same office can disagree about whether it is too warm. The goal of HVAC design is not to please everyone, which is impossible, but to create conditions that a large majority find acceptable.

ASHRAE Standard 55, Thermal Environmental Conditions for Human Occupancy, is the main North American reference for defining those conditions. It is widely referenced in Canadian design specifications, green building programs and owner's project requirements, even though it is not generally mandated by the National Building Code. This guide explains its key concepts and how to apply them.

The six factors of thermal comfort

Standard 55 recognizes that air temperature alone does not determine comfort. Six factors interact:

A space can have the "right" air temperature and still be uncomfortable because of cold windows, drafts or direct sun. Understanding all six factors is the key to solving comfort complaints; see HVAC troubleshooting for hot and cold complaints.

Metabolic rate and clothing

One met is the metabolic rate of a seated, quiet person. Typical office work is around 1.0 to 1.3 met. Clothing insulation for typical indoor clothing is often taken as around 0.5 clo in summer and around 1.0 clo in winter. These personal factors explain why winter and summer comfort ranges differ, and why dress codes affect comfort as much as thermostat settings.

Operative temperature

Operative temperature combines air temperature and mean radiant temperature into a single value. In typical spaces with low air speed, it is roughly the average of the two. Standard 55 expresses comfort zones in terms of operative temperature, not air temperature.

This matters in buildings with large glazed areas, radiant systems or poorly insulated walls. A room with 22 °C air but cold windows can have a noticeably lower operative temperature. Conversely, radiant heating can provide comfort at a lower air temperature.

The PMV and PPD model

For mechanically conditioned spaces, Standard 55 uses the predicted mean vote (PMV) model, based on the work of P. O. Fanger.

• PMV predicts the average thermal sensation of a large group on a seven-point scale from -3 (cold) to +3 (hot), with 0 being neutral.
• PPD (predicted percentage dissatisfied) estimates the percentage of people who would be dissatisfied at a given PMV.

Standard 55 defines acceptable conditions as PMV between -0.5 and +0.5, which corresponds to a PPD below about 10%. Even at a perfect PMV of 0, the model predicts about 5% of people will be dissatisfied, which reflects individual differences.

The standard provides a graphical method for simple cases and a computer model for the general case. Free online tools based on the standard's calculations are widely used for design checks.

The adaptive comfort model

For naturally ventilated spaces where occupants control operable windows and there is no mechanical cooling, Standard 55 provides an adaptive model. It recognizes that people in such buildings adapt their expectations and clothing to the outdoor climate, accepting a wider and seasonally shifting range of indoor temperatures.

The adaptive model applies only when specific conditions are met, including occupant control of openings and sedentary activity. It can support designs that use natural ventilation in shoulder seasons, which is relevant in mild Canadian climates such as coastal BC.

Local thermal discomfort

A person can be comfortable overall yet uncomfortable in one part of the body. Standard 55 sets limits for:

• Draft: unwanted local cooling from air movement, especially at the neck and ankles.
• Vertical air temperature difference: warmer air at head height than at ankle level. Stratification from overhead heating is a common cause.
• Radiant temperature asymmetry: for example a cold window on one side or a warm ceiling overhead.
• Floor surface temperature: floors that are too cold or too warm for comfort, relevant to radiant floors and slab-on-grade spaces.

These limits are particularly relevant to air distribution design and perimeter heating. Proper diffuser selection and throw prevent drafts; see diffuser and grille selection. Perimeter heating or high-performance glazing addresses cold window downdrafts and radiant asymmetry.

Humidity

Standard 55 places an upper humidity limit on the comfort zone in its graphical method but does not set a lower humidity limit for thermal comfort. Low humidity can cause dry skin, eye irritation and static electricity, which are addressed by other guidance. In Canadian winters, the decision to humidify involves trade-offs with energy use and condensation risk on the building envelope; see humidification in cold climates.

Elevated air speed

Increased air speed, from ceiling fans or personal fans, can offset higher temperatures. Standard 55 includes a method for accounting for this cooling effect. Where occupants control the air speed, wider temperature ranges are acceptable. This is a low-energy strategy that can reduce cooling loads.

Applying Standard 55 in practice

In design

1. Define the design conditions, including the expected clothing and activity levels, in the owner's project requirements.
2. Select indoor design temperatures and humidity limits that satisfy the PMV range.
3. Check perimeter zones for radiant asymmetry and window downdrafts, especially with large glazing in cold climates.
4. Select air distribution to avoid drafts and stratification.
5. Document the comfort basis in the design documents.

In commissioning and verification

Standard 55 describes methods for evaluating comfort in existing buildings, including physical measurements and occupant surveys. Measurements of air temperature, mean radiant temperature, air speed and humidity at the occupant's location, at several heights, show whether conditions comply. Occupant satisfaction surveys provide a direct check on whether the space works for the people using it.

Green building programs such as LEED include thermal comfort credits that reference Standard 55, which may require design documentation, controls or post-occupancy surveys.

Frequently asked questions

Is ASHRAE Standard 55 required by Canadian codes?

It is not generally mandated by the NBC or provincial building codes. It is commonly referenced in project specifications, owner standards and green building programs. Requirements depend on the project and jurisdiction.

What temperature should an office be?

There is no single answer. It depends on clothing, activity, humidity, air speed and radiant conditions. Standard 55 defines the acceptable range for given conditions, which typically differs between summer and winter.

Why do people near windows complain more?

Cold glass in winter lowers the mean radiant temperature and can cause downdrafts. Direct sun in summer raises it. Both affect comfort even when the thermostat reads correctly.

Key takeaways

• Comfort depends on six factors, not just air temperature.
• Operative temperature combines air and surface temperatures.
• The PMV range of -0.5 to +0.5 corresponds to about 10% or fewer dissatisfied.
• The adaptive model applies to naturally ventilated spaces with occupant control.
• Local discomfort from drafts, stratification and cold surfaces must be designed out.