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Last edited 19 Jan 2021
Thermal comfort in buildings
When people are dissatisfied with their thermal environment, not only is it a potential health hazard, it also impacts on their ability to function effectively, their satisfaction at work, the likelihood they will remain a customer, and so on.
BS EN ISO 7730 defines thermal comfort as '…that condition of mind which expresses satisfaction with the thermal environment.', i.e. the condition when someone is not feeling either too hot or too cold.
The human thermal environment is not straight forward and cannot be expressed in degrees. Nor can it be satisfactorily defined by acceptable temperature ranges. It is a personal experience dependent on a great number of criteria and can be different from one person to another within the same space.
The Health and Safety Executive (HSE) suggest that an environment can be said to achieve 'reasonable comfort' when at least 80% of its occupants are thermally comfortable. This means that thermal comfort can be assessed by surveying occupants to find out whether they are dissatisfied with their thermal environment.
 Factors influencing thermal comfort
 Air velocity
The velocity of the air that a person is in contact with (measured in m/s). The faster the air is moving, the greater the exchange of heat between the person and the air (for example, draughts generally make us feel colder).
The temperature of a persons surroundings (including surfaces, heat generating equipment, the sun and the sky). This is generally expressed as mean radiant temperature (MRT, a weighted average of the temperature of the surfaces surrounding a person, which can be approximated by globe thermometer) and any strong mono-directional radiation such as radiation from the sun.
 Relative humidity (RH)
The ratio between the actual amount of water vapour in the air and the maximum amount of water vapour that the air can hold at that air temperature, expressed as a percentage. The higher the relative humidity, the more difficult it is to lose heat through the evaporation of sweat.
Clothes insulate a person from exchanging heat with the surrounding air and surfaces as well as affecting the loss of heat through the evaporation of sweat. Clothing can be directly controlled by a person (i.e. they can take off or put on a jacket) whereas environmental factors may be beyond their control.
In addition, thermal comfort will be affected by whether a thermal environment is uniform or not. For example, draughts and heaters can create a scorched face / frozen back effect and hot feet/cold head and hands effect.
'Thermal alliesthesia' goes beyond this, proposing that the hedonic qualities of the thermal environment (qualities of pleasantness or unpleasantness, or 'the pleasure principle') are determined as much by the general thermal state of the subject as by the environment itself.
In its simplest form, cold stimuli will be perceived as pleasant by someone who is warm, whilst warm stimuli will be experienced as pleasant by someone who is cold. Introducing a spatial component to this, it can for example be pleasurable to wrap cool hands around a warm mug. See Thermal pleasure in the built environment for more information.
 Controlling thermal comfort
- Environmental monitoring and control (automated or user-controlled systems, active systems such as heating and cooling and passive systems such as shading). NB: User-controlled systems require that users are properly trained.
- Adapting or changing clothing. Businesses can allow people to wear different clothing depending on conditions. They can also provide things like cloak rooms or lockers so that people can change clothes or take off and put down coats. The golden rule is layering, generally 3 layers, and use zips and buttons to regulate temperature.
- Allowing flexible working hours or changing start and finish times.
- Adjusting tasks. For example, allowing breaks or reducing the length of time people are exposed to particular conditions.
- Providing information telling people what sort of conditions to expect so that they can dress and behave appropriately.
- Providing or allowing personal equipment such as desk fans.
- Separating people from sources of discomfort. For example, putting heat generating equipment such as ICT equipment in separate rooms, insulating pipes, preventing draughts and so on. NB: Draughts can be caused by high local surface temperature differences even in a space where there is no air infiltration – for example, a cold down-draught near a window.
- Providing protective clothing (PPE Personal Protective Equipment). This should be a last resort option.
 Predicting thermal comfort
There are a great number of techniques for estimating likely thermal comfort, including; effective temperature, equivalent temperature, Wet Bulb Globe Temperature (WBGT), resultant temperature and so on, and charts exist showing predicted comfort zones within ranges of conditions.
PMV and PPD were developed by Professor Ole Fanger based on research undertaken at Kansas State University and the Technical University of Denmark. Research was carried out to find out if people felt comfortable in different conditions and this was used to develop equations that would predict comfort. The equations take into account; air temperature, mean radiant temperature, air movement, humidity, clothing and activity level.
Where non-uniform conditions exist, multiple assessments may be necessary, and in complex environments, Computational Fluid Dynamics (CFD) analysis may be necessary to accurately assess thermal comfort.
NB BREEAM UK New Construction, Non-domestic Buildings (United Kingdom), Technical Manual, SD5078: BREEAM UK New Construction 2018 3.0, published by BRE Global Limited suggests that: 'In BS EN ISO 7730:2005: Ergonomics of the thermal environment. Analytical determination and interpretation of thermal comfort, thermal comfort is defined using the calculation of PMV and PPD indices and local thermal comfort criteria. It is also defined as ‘that condition of mind which expresses satisfaction with the thermal environment.’ The term ‘thermal comfort’ describes a person’s psychological state of mind and is usually referred to in terms of whether someone is feeling too hot or too cold. Thermal comfort is difficult to define because it needs to account for a range of environmental and personal factors in order to establish what makes people feel comfortable. HSE considers 80% of occupants as a reasonable limit for the minimum number of people who should be thermally comfortable in an environment. The purpose of this issue is to encourage appropriate and robust consideration of thermal comfort issues, and specification of appropriate occupant controls to ensure both maximum flexibility of the space and thermal comfort for the majority of building occupants.'
It also suggests that: 'Thermal comfort analysis tools can be subdivided into a number of methods of increasing complexity. The most complex of these and the one that provides greatest confidence in results is the full dynamic model. This type of model enables annual heating or cooling loads, overheating risks and control strategies to be assessed.'
The Approved Code of Practice (Workplace health, safety and welfare. Workplace (Health, Safety and Welfare) Regulations 1992. Approved Code of Practice ) suggests a minimum temperature of 16 degrees Celsius, or 13 degrees Celsius if work involves severe physical effort. However, these are only guidelines.
The Health and Safety Executive (HSE) previously defined thermal comfort in the workplace, as: '…roughly between 13°C (56°F) and 30°C (86°F), with acceptable temperatures for more strenuous work activities concentrated towards the bottom end of the range, and more sedentary activities towards the higher end.'
 Related articles on Designing Buildings Wiki
- Boiler markets and the green recovery.
- Building engineering physics.
- BREEAM Thermal comfort.
- BREEAM Visual comfort View out.
- Cold stress.
- Comfort in low energy buildings.
- Domestic boiler market 2019.
- Evolving opportunities for providing thermal comfort.
- Healthy excursions outside the thermal comfort zone.
- Heat metering.
- Heat stress.
- Heating degree days.
- Home Quality Mark high temperature reporting tool.
- Indoor air velocity.
- Industrial gas boilers market 2020.
- Maximum and minimum workplace temperatures.
- Excess cold.
- Mean radiant temperature.
- Operative temperature.
- Predicted percentage dissatisfied.
- Predicted mean vote.
- Preventing overheating.
- Psychometric charts.
- Radiant heating.
- Retrofit and traditional approaches to comfort.
- Sling psychrometer.
- Tempering heating.
- Thermal comfort and wellbeing.
- Thermal environment.
- Thermal pleasure in the built environment.
- Thermal indices.
- Underfloor air conditioning at London Grade II listed landmark.
- UV disinfection of building air to remove harmful bacteria and viruses.
- When hospital buildings aren’t healthy.
 External references
- BS EN ISO 9888:2001 Evaluation of thermal strain by physiological measurements
- BS EN ISO 7730 Ergonomics of the thermal environment. Analytical determination and interpretation of thermal comfort using calculation of the PMV and PPD indices and local thermal comfort criteria)
- BS EN ISO 10551 Ergonomics of the thermal environment. Assessment of the influence of the thermal environment using subjective judgement scales
- Thermal comfort, Andris Auliciems and Steven V. Szokolay, 2007.
- Types of building services.
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