Last edited 19 Aug 2019

Thermal comfort in buildings


[edit] Introduction

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.

For example, a person walking up stairs in a cold environment whilst wearing a coat might feel too hot, whilst someone sat still in a shirt in the same environment might feel too cold.

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.

Thermal comfort.jpg

[edit] Factors influencing thermal comfort

Thermal comfort results from a combination of environmental factors and personal factors:

[edit] Environmental factors

[edit] Air temperature

The temperature of the air that a person is in contact with, measured by the dry bulb temperature (DBT).

[edit] 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).

[edit] Radiant temperature

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.

[edit] 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.

[edit] Personal factors

[edit] Clothing

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.

[edit] Metabolic heat

The heat we produce through physical activity. A stationary person will tend to feel cooler than a person who is exercising.

[edit] Wellbeing and sicknesses

Such as the common cold or flu which affect our ability to maintain a body temperature of 37°C at the core.

Other contributing factors can include; access to food and drink, acclimatisation (this can be more difficult where there is a high outdoor-indoor temperature gradient) and state of health.

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.

[edit] Controlling thermal comfort

Thermal comfort can be controlled or adjusted by a number of different measures:

  • 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.

[edit] 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.

However, BS EN ISO 7730 and BS EN ISO 10551 suggest thermal comfort can be expressed in terms of Predicted Mean Vote (PMV) and Percentage People Dissatisfied (PPD).

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.

PMV is an index that predicts the mean vote of a group of people voting on how comfortable they are in an environment. PPD is a function of PMV.

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.

[edit] Regulation

Temperatures in the workplace are governed by the Workplace (Health, Safety and Welfare) Regulations 1992, which oblige employers to provide a reasonable temperature in the workplace.

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.'

However, the complexity of thermal comfort means that there is no meaningful maximum guideline temperature, particularly at higher temperatures.

[edit] Related articles on Designing Buildings Wiki

[edit] External references


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