What is thermal comfort?

Thermal comfort is defined in British Standard BS EN ISO 7730 as:

‘that condition of mind which expresses satisfaction with the thermal environment.’

So 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 very difficult to define because you need to take into account a range of environmental and personal factors when deciding what will make people feel comfortable. These factors make up what is known as the ‘human thermal environment’.

Thermal comfort is not measured by air temperature, but by the number of employees complaining of thermal discomfort.

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

Environmental factors,

Air temperature,

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

                       

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).

                       

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.

                       

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.

Personal factors:

                       

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 (ie they can take off or put on a jacket) whereas environmental factors may be beyond their control.

                       

Metabolic heat.

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

                       

Well being generally and sickness, such as the common cold or flu which affect our ability to maintain body temperature, 37C 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.

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.

Sources:

Thermal comfort:

http://www.designingbuildings.co.uk/wiki/Thermal_comfort_in_buildings

http://www.hse.gov.uk/temperature/thermal/explained.htm