Human thermal comfort is defined by ASHRAE as the state of mind that expresses satisfaction with the surrounding environment (ASHRAE Standard 55). Maintaining thermal comfort for occupants of buildings or other enclosures is one of the important goals of HVAC design engineers.

Thermal comfort is affected by heat conduction, convection, radiation, and evaporative heat loss. Thermal comfort is maintained when the heat generated by human metabolism is allowed to dissipate, thus maintaining thermal equilibrium with the surroundings. Any heat gain or loss beyond this generates a sensation of discomfort.[1] It has been long recognised that the sensation of feeling hot or cold is not just dependent on air temperature alone.

Importance of thermal comfortEdit

Thermal comfort is very important to many work-related factors. It can affect the distraction levels of the workers, and in turn affect their performance and productivity of their work. Also, thermal discomfort has been known to lead to Sick Building Syndrome symptoms. [2][3] The US EPA BASE study found that higher indoor temperatures, even within the recommended thermal comfort range, increased worker symptoms[4]. The occurrence of symptoms increased much more with raised indoor temperatures in the winter than in the summer due to the larger difference created between indoor and outdoor temperatures.

Factors determining thermal comfortEdit

Factors determining thermal comfort include:


When measuring metabolism rates, many factors have to be taken into account. Each person has a different metabolism rate, and these rates can fluctuate when a person is performing certain activities, or under certain environmental conditions. Even people who are in the same room can feel significant temperature differences due to their metabolic rates, which makes it very hard to find an optimal temperature for everyone in a given location. [5] [6]

Clothing insulationEdit

During cold weather, layers of insulating clothing can help keep a person warm. At the same time, if the person is doing a large amount of physical activity, lots of clothing layers can prevent heat loss and possibly lead to overheating. Generally, the thicker the garment is the greater insulating abilities it has. Depending on the type of material the clothing is made out of, air movement and relative humidity can decrease the insulating ability of the material. [7] [8]

The amount of clothing is measured against a standard amount that is roughly equivalent to a typical business suit, shirt, and undergarments. Activity level is compared to being seated quietly, such as in a classroom. This standard amount of insulation required to keep a resting person warm in a windless room at 70 °F (21.1 °C) is equal to one clo. Clo units can be converted to R-value in SI units (m²·K/W) or RSI) by multiplying clo by 0.155 (1 clo = 0.155 RSI). (In English units 1 clo corresponds to an R-value of 0.88 °F·ft²·h/Btu.)

Relative humidityEdit

The human body has sensors that are fairly efficient in sensing heat and cold, but they are not very effective in detecting relative humidity. Relative humidity creates the perception of an extremely dry or extremely damp indoor environment. This can then play a part in the perceived temperature and their thermal comfort. The recommended level of indoor humidity is in the range of 30-60%. [9][10]

A way to measure the amount of relative humidity in the air is to use a system of dry-bulb and wet-bulb thermometers. A dry-bulb thermometer measures the temperature not relative to moisture. This is generally the temperature reading that is used in weather reports. In contrast, a wet-bulb thermometer has a small wet cloth wrapped around the bulb at its base, so the reading on that thermometer takes into account water evaporation in the air. The wet-bulb reading will thus always be at least slightly lower than the dry bulb reading. The difference between these two temperatures can be used to calculate the relative humidity. The larger the temperature difference between the two thermometers, the lower the level of relative humidity. [11] [12]

The wettedness of skin in different areas also affects perceived thermal comfort. Humidity can increase wetness on different areas of the body, leading to a perception of discomfort. This is usually localized in different parts of the body and local thermal comfort limits for local skin wettedness differ between different skin locations of the body.[13] The extremities are much more sensitive to thermal discomfort from wetness than the trunk of the body. Although local thermal discomfort can be caused from wetness, the thermal comfort of the whole body will not be affected by the wetness of certain parts[12].

Recently, the effects of low relative humidity and high air velocity were tested on humans after bathing. Researchers found that low relative humidity engendered thermal discomfort as well as the sensation of dryness and itching. It is recommended to keep relative humidity levels higher in a bathroom than other rooms in the house for optimal conditions.[14]

Thermal stressEdit

The concept of thermal comfort is closely related to thermal stress. This attempts to predict the impact of solar radiation, air movement, and humidity for military personnel undergoing training exercises or athletes during competitive events. Values are expressed as the Wet Bulb Globe Temperature or Discomfort Index.[15][16] Generally, humans do not perform well under thermal stress. People’s performances under thermal stress is about 11% lower than their performance at normal thermal conditions. Also, human performance in relation to thermal stress varies greatly by the type of task you are completing. Some of the physiological effects of thermal heat stress include increased blood flow to the skin, sweating, and increased ventilation. [17] [18]

Effects of natural ventilation of thermal comfortEdit

Many buildings use a HVAC (Heating Ventilation Air Conditioning) unit to control their thermal environment. Recently, with the current energy and financial situation, new methods for indoor temperature control are being used. One of these is natural ventilation. This process can make the controlled indoor air temperature more susceptible to the outdoor weather, and during the seasonal months the temperatures inside can become too extreme. During the summer months, the temperature inside can rise too high and cause the need for open windows and fans to be used. In contrast, the winter months could call for more insulation and layered clothing to deal with the less than ideal temperatures. [19][20]

Operative temperatureEdit

The ideal standard for thermal comfort can be defined by the operative temperature. This is the average of the air dry-bulb temperature and of the mean radiant temperature at the given place in a room. In addition, there should be low air velocities and no 'drafts,' little variation in the radiant temperatures from different directions in the room, and humidity within a comfortable range.

The operative temperature intervals varied by the type of indoor location. They also vary by the time of year. [21] ASHRAE has listings for suggested temperatures and air flow rates in different types of buildings and different environmental circumstances. For example, a single office in a building has an occupancy ration per square meter of 0.1. In the summer the suggested temperature is between 23.5 and 25.5 degrees Celsius, and airflow velocity of 0.18 m/s. In the winter, the recommended temperature is between 21.0 and 23.0 degrees Celsius with an airflow velocity of 0.15 m/s. [22]

Thermal sensitivity of individualsEdit

The thermal sensitivity of an individual is quantified by the descriptor FS, which takes on higher values for individuals with lower tolerance to non-ideal thermal conditions[23]. This group includes pregnant women, the disabled, as well as individuals whose age is below 14 or above 60, which is considered the adult range. Existing literature provides consistent evidence that sensitivity to hot and cold surfaces declines with age and that there is also a gradual reduction in the effectiveness of the body in thermoregulation after the age of 60. This is mainly due to a more sluggish response of the counteraction mechanisms in the body that are used to maintain the core temperature of the body at ideal values[24].

Situational factors include the health, psychological, sociological and vocational activities of the persons.

Gender differencesEdit

While thermal comfort preferences between genders seems to be small, there are some differences. Females are much more likely to be sensitive to thermal conditions. Females are also more likely to be uncomfortable with the room temperature, and will find the temperature too hot or too cold before many men would. Many times, females will prefer higher temperatures. But while females were more sensitive to temperatures, males tend to be more sensitive to relative humidity levels. [25] [26]

Models of thermal comfortEdit

When discussing thermal comfort, there are two different models that can be used. These are the static model and the adaptive model.

The adaptive model states that there is an optimal temperature for a given indoor environment depending on the outdoor air temperature. It takes into account that humans can adapt and tolerate different temperatures during different times of the year. The optimal temperature for a given time is determined by looking at the mean outdoor temperatures of each month of the year. Also, field studies are performed in these areas to see what the majority of people would prefer as their set-point temperature indoors at different times of the year.[27]

On the other side, the static model states that the indoor temperature should not change as the seasons do. Rather, there should be one set temperature year-round. This is taking a more passive stand that humans do not have to adapt to different temperatures since it will always be constant. [28]

More advanced research on thermal comfort considers the heat balance of the human body and calculates sensation and comfort for local body parts. [29] [30] [31]

Thermal comfort in different regionsEdit

In different areas of the world, thermal comfort needs may vary based on climate. In China there are hot humid summers and cold winters causing a need for efficient thermal comfort. Energy conservation in relation to thermal comfort has become a large issue in China in the last several decades due to rapid economic and population growth[32]. Researchers are now looking into ways to heat and cool buildings in China for lower costs and also with less harm to the environment.

In tropical areas of Brazil, urbanization is causing a phenomenon called urban heat islands (UHI). These are urban areas, which have risen over the thermal comfort limits due to a large influx of people and only drop within the comfortable range during the rainy season. UHI's are caused by surface and atmospheric modifications from the overcrowding of people in an already hot climate[33].

In the hot humid region of Saudi Arabia, the issue of thermal comfort has been important in mosques where people go to pray. They are very large open buildings which are used only intermittently making it hard to ventilate them properly. The large size requires a large amount of ventilation but this requires a lot of energy since the buildings are used only for short periods of time. Some mosques have the issue of being too cold from their HVAC systems running for too long and others remain too hot. The stack effect also comes into play due to their large size and creates a large layer of hot air above the people in the mosque. New designs have placed the ventilation systems lower in the buildings to provide more temperature control at ground level[34]. Also new monitoring steps are being taken to improve the efficiency.

Thermal comfort of livestockEdit

Although thermal comfort of humans is the main focus of thermal comfort studies, the needs of livestock must be met as well for better living and production. The Department of Animal Production in Italy produced a study on ewes, which tested rumen function and diet digestibility of ewes chronically exposed to a hot environment[35]. These two bodily functions were reduced by the hot temperatures offering insight that thermal comfort levels are important to livestock productivity.


These factors were explored experimentally in the 1970s. Many of these studies led to the development and refinement of ASHRAE Standard 55 and were performed at Kansas State University by Ole Fanger and others. Perceived comfort was found to be a complex interaction of these variables. It was found that the majority of individuals would be satisfied by an ideal set of values. As the range of values deviated progressively from the ideal, fewer and fewer people were satisfied. This observation could be expressed statistically as the % of individual who expressed satisfaction by comfort conditions and the predicted mean vote (PMV)

This research is applied to create Building Energy Simulation (BES) programs for residential buildings. Residential buildings can vary much more in thermal comfort than public and commercial buildings. This is due to their smaller size, the variations in clothing worn, and different uses of each room. The main rooms of concern are bathrooms and bedrooms. Bathrooms need to be at a temperature comfortable for a human with or without clothing. Bedrooms are of importance because they need to accommodate different levels of clothing and also different metabolic rates of people asleep or awake[36].

Thermal comfort research in clothing is currently being done by the military. New air-ventilated garments are being researched to improve evaporative cooling in military settings. Some models are being created and tested based on the amount of cooling they provide[37].

See alsoEdit


  1. Welcome To Home Energy Magazine Online
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  16. About the WBGT and Apparent Temperature Indices
  17. Hancock, P. A., Ross, J. M., & Szalma, J. L. (2007). A meta-analysis of performance response under thermal stressors. Human Factors, 49(5), 851.
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  29. Zhang, Hui, Arens, Edward, Huizenga, Charlie, Han, Taeyoung. Thermal sensation and comfort models for non-uniform and transient environments: Part I: local sensation of individual body parts, 2009. 
  30. Zhang, Hui, Arens, Edward, Huizenga, Charlie, Han, Taeyoung. Thermal sensation and comfort models for non-uniform and transient environments: Part II: local comfort of individual body parts, 2009. 
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