THERMAL COMFORT · ENGINEERING REFERENCE
Indoor thermal comfort basics
Understand indoor comfort through occupant experience, environmental factors and field observations.
What affects thermal comfort?
Thermal comfort describes a person’s subjective satisfaction with the surrounding thermal environment; people can experience the same space differently. An investigation typically considers four environmental factors together with two personal factors, rather than judging comfort from the thermostat setting alone.
Air temperature
The temperature of air around the occupant; a wall thermostat reading may not represent every occupied location.
Mean radiant temperature
Surrounding surfaces and radiation affect heat exchange, particularly near windows, hot equipment or cold surfaces.
Humidity
Air moisture influences evaporative heat loss. Relative humidity changes with temperature and needs to be considered with the air state.
Air speed
Air movement affects convective and evaporative heat exchange; depending on conditions it may feel cooling or cause an unwanted draft.
Clothing
Clothing layers and coverage influence thermal resistance between the body and the surroundings; occupants may dress differently.
Activity
Metabolic heat production varies with activity; a seated person and someone doing physical work need not have the same conditions.
These factors interact, and individual responses vary. The UK HSE guide to thermal comfort discusses these factors in workplaces.
Overall comfort and local discomfort
Similar room averages do not guarantee the same experience at every position. Supply air can create a local draft; cold or warm windows and solar radiation may affect people nearby; temperatures can differ between head and feet, and floor temperature can affect local sensation. Consider location, direction, timing and occupant feedback.
How to investigate a space
- Record occupied locations, activities and clothing, together with when and where complaints arise and how the equipment is operating.
- Measure air temperature, humidity, air speed and the radiant environment at representative occupied locations and times; consider instrument calibration and measurement conditions.
- Compare locations and times, and investigate spatial differences from supply air, sun and surface temperatures; add local measurements when needed.
- Use the applicable project standard to select the assessment method, measuring points, duration and instruments. These investigation ideas do not replace its measurement procedure.
Example: near a window and at room centre
Suppose the air-temperature readings are similar but an occupant near a window feels cold. Alongside checking air temperature, investigate window radiation, sun, local air movement, clothing and activity. Equal air temperatures alone cannot establish equal comfort; this example makes no compliance or personal comfort determination.
The moist-air state tool and psychrometric chart help describe the air state at a given temperature, humidity and pressure; they do not calculate human comfort.
Assessment and safety limits
Thermal comfort and occupational heat stress address different questions; subjective satisfaction does not establish that a workplace is safe. This site does not provide PMV, PPD, compliance decisions or occupational exposure limits from this guide.
The appropriate comfort method depends on the space, its operation, occupants and the applicable standard. Check the version adopted for the project and seek professional assessment. For occupational heat exposure, see heat stress, wet bulb and WBGT and assess the site under applicable local requirements.
Public references
- UK HSE: thermal comfort at work: workplace factors and investigation guidance.
- ASHRAE: Standard 55: verify the standard version applicable to the project.
- OSHA: indoor air quality overview: indoor temperature, humidity, ventilation and air-quality concerns.
- NIOSH: occupational heat stress: a separate source on heat-exposure risks.
This guide is independently written using public HSE, OSHA and NIOSH references; it does not reproduce standard tables or replace standards or safety assessments.