Thermal Comfort in Buildings: PMV, Adaptive Comfort

Two people in the same office can disagree completely about whether it is too cold. Thermal comfort is the condition of mind that expresses satisfaction with the thermal environment, and it depends on far more than the number on the thermostat. Understanding what drives it is the key to buildings that feel pleasant while using less energy — a particularly live question in Singapore, where air-conditioning dominates building energy use.

This article explains the physics of how the body exchanges heat, the two main models designers use to predict comfort, how humidity and air movement change the picture in the tropics, and the practical design and operational choices that follow.

The body as a heat engine

The human body continuously produces heat through metabolism and must lose it at the same rate to keep its core temperature stable. It does this by four routes: convection to the surrounding air, radiation to surrounding surfaces, evaporation of sweat and moisture from the skin and lungs, and a small amount of conduction through contact. Comfort occurs when this balance is achieved with little effort — no shivering, no visible sweating.

From that balance come the six factors that comfort standards use. Four are environmental: air temperature, mean radiant temperature (the average temperature of the surfaces you exchange radiation with), air speed and humidity. Two are personal: metabolic rate, which depends on activity, and clothing insulation. Change any one and the others must shift to compensate. A sunlit glass wall raises radiant temperature; a ceiling fan increases convective and evaporative loss; a suit and tie add insulation that a T-shirt does not.

The PMV model: predicting the average vote

The most widely used model was developed by P. O. Fanger from climate-chamber experiments and underpins ISO 7730 and ASHRAE Standard 55. It calculates the Predicted Mean Vote (PMV) — the average response a large group would give on a seven-point scale from −3 (cold) through 0 (neutral) to +3 (hot) — from the six factors above. A companion index, the Predicted Percentage Dissatisfied (PPD), estimates how many people would be unhappy. Even at a PMV of exactly zero the model predicts some dissatisfied occupants, which is an honest acknowledgement that no single condition satisfies everyone.

PMV works well in steady, mechanically conditioned spaces where people’s clothing and activity are predictable. Its weakness is that it treats occupants as passive. Field studies have repeatedly found that people in naturally ventilated buildings accept a wider range of temperatures than PMV predicts, and that occupants in some tropical air-conditioned offices report feeling cool, suggesting buildings are often cooled more than comfort requires.

Adaptive comfort: people are not passive

The adaptive model, developed from large field datasets by researchers including Richard de Dear and Gail Brager and incorporated into ASHRAE 55 and EN 16798-1, starts from observation rather than the chamber. It links the comfortable indoor temperature to the recent outdoor temperature. People adapt behaviourally (changing clothing, opening windows, switching on fans), physiologically (acclimatisation) and psychologically (expectations shaped by climate and building type).

The practical outcome is that in occupant-controlled, naturally ventilated spaces, the acceptable temperature band is wider and rises with outdoor conditions. For a warm climate this legitimises comfortable conditions well above typical air-conditioned set points, provided occupants have control over openings and air movement. The adaptive model is the scientific basis for passive cooling and well-designed natural ventilation; it is not a licence to ignore heat.

Thermal comfort in a hot, humid climate

In Singapore, two factors deserve more weight than they get in temperate guidance. The first is humidity. When the air is already moist, sweat evaporates slowly, so the body’s most powerful cooling route is throttled. Air-conditioning therefore has to remove moisture as well as heat, and a space at a moderate temperature but high humidity can feel clammy.

The second is air movement. Elevated air speed increases both convective and evaporative heat loss, so a gentle breeze makes a warmer room feel cooler. ASHRAE 55 explicitly allows higher operative temperatures when occupants have control of air speed. This is the reasoning behind the Go 25 movement, co-organised by the Singapore Green Building Council and the Ministry of Sustainability and the Environment, which encourages setting air-conditioning at 25°C or higher and pairing it with fans. Raising set points reduces cooling energy, and with adequate air movement and humidity control, most people remain comfortable.

Radiant temperature also matters more than many realise. Large areas of glazing exposed to the tropical sun raise the mean radiant temperature near the façade, so occupants by the window may feel warm even when the air is cool. Shading, low-e glass and a controlled solar heat gain coefficient are comfort measures as much as energy measures.

Designing for thermal comfort

Good thermal comfort starts with the building, not the plant. The design sequence that tends to work is:

Reduce heat gain first — orientation, external shading, a well-insulated roof, appropriate glazing ratios and light-coloured surfaces.
Use thermal mass with care — in the tropics the night is not much cooler than the day, so heavy mass offers less benefit than in climates with large daily swings, and can hold heat in poorly ventilated rooms.
Provide air movement — cross ventilation, ceiling fans and personal fans extend the comfortable range.
Control humidity — in air-conditioned spaces, design for dehumidification, not just sensible cooling.
Give occupants control — operable windows, local fans and adjustable set points improve satisfaction, consistent with the adaptive model.

These measures sit naturally within tropical architecture, where deep overhangs, verandahs and raised floors evolved precisely to manage sun, rain and breeze.

Measuring and managing comfort in operation

Comfort can be measured with instruments that record air temperature, globe temperature (for radiant effects), air speed and humidity, combined with occupant surveys. In practice, complaint patterns are informative: cold complaints in air-conditioned offices often point to over-cooling, poor air distribution or draughts from diffusers, while warm complaints near façades usually point to solar gain. Fixing the cause is more effective than adjusting the central set point to suit the loudest voice.

Frequently asked questions

What is a comfortable indoor temperature in Singapore?

There is no single figure, because comfort depends on humidity, air movement, radiant temperature, clothing and activity. In air-conditioned spaces, national initiatives encourage set points of 25°C or higher combined with fans. In naturally ventilated spaces, the adaptive comfort model accepts higher temperatures where occupants have control and good air movement.

What does PMV mean in thermal comfort?

PMV stands for Predicted Mean Vote. It estimates the average thermal sensation of a large group on a scale from −3 (cold) to +3 (hot), with 0 as neutral, based on air temperature, radiant temperature, air speed, humidity, activity and clothing. It is defined in ISO 7730 and used in ASHRAE Standard 55.

What is the adaptive comfort model?

The adaptive model links comfortable indoor temperatures to recent outdoor temperatures, based on field studies showing that people in naturally ventilated buildings adapt through clothing, window opening, fans and expectation. It allows a wider and warmer comfort band than PMV in occupant-controlled, naturally ventilated spaces.

Why do fans make a room feel cooler without lowering the temperature?

Moving air increases heat loss from the skin by convection and by speeding up the evaporation of sweat. The air temperature does not change, but the body sheds heat faster, so the same room feels cooler. This is why fans allow higher air-conditioning set points without loss of comfort.

Does humidity affect thermal comfort?

Yes, significantly in hot climates. High humidity slows the evaporation of sweat, which is the body’s main cooling route when it is warm. A room at a moderate temperature but high humidity can feel stuffy and clammy, so air-conditioning in the tropics must manage moisture as well as heat.

Working on a building where comfort, glazing and cooling loads all need to be balanced with the structure? Talk to our consultancy or get in touch.