Last edited 15 Sep 2020

Temperature in buildings


[edit] Introduction

Temperature is a measure of the average kinetic energy of molecules. This is important as the average temperature relates to the movement of molecules in a substance or material. A cup of boiling water will be hotter than the Atlantic Ocean but the latter will contain much more heat than the former.

Temperature will tend to vary throughout a body depending on its heat exchange with its surroundings. Temperature can be expressed in degrees Celsius (°C), kelvin (°k) or Fahrenheit (°F), with Kelvin the scientific measure, Celsius being used as it relates to the properties of water, zero is freezing and 100 is steam..

Measuring temperature actually records the temperature of the sensor that is being used to carry out the measurement (usually a thermometer). As a consequence, different types of measurement and calculation have been developed to represent the temperature of different types of body. Some of these methods are described below.

[edit] Thermal comfort

Temperature can be a particularly important measure in the built environment when considering the thermal comfort of occupants and in the design of building services systems. As a consequence, many of the measures of temperature are intended to represent the thermal comfort of people, or some component of their thermal comfort. Thermal comfort is a complex science in its own right.

Making a building comfortable is not as simple as delivering an average internal air temperature of 21 °C throughout the year. Thermal comfort is dependent on a range of environmental factors in addition to air temperature, such as; air velocity, radiant temperature, relative humidity and the uniformity of conditions. It also depends on personal factors such as; clothing, metabolic heat, state of health, acclimatisation, expectations, and even access to food and drink. See thermal comfort for more information.

NB: Heat stress is a form of overheating that the occupants of a building may experience when the measures that their body uses to regulate internal temperature begin to fail. This can occur for example in buildings where an industrial process is being carried out, such as; smelting, brick-firing, cooking, and so on. See Heat stress for more information.

[edit] Measures of temperature

[edit] Dry-bulb temperature

Dry-bulb temperature (Tdb, DBT or Td), is a measure of air temperature. It is referred to as dry-bulb temperature because the thermometer bulb is dry and so the temperature recorded does not vary with the moisture content of the air. See dry-bulb temperature for more information.

[edit] Wet-bulb temperature

Wet-bulb temperature (Twb or Tw) is the temperature recorded by a thermometer that has its bulb wrapped in cloth and moistened with distilled water. See wet-bulb temperature for more information.

[edit] Wet-bulb globe temperature

Wet-bulb globe temperature (WBGT) is an index that is widely used for the assessment of heat stress. It combines wet-bulb temperature, dry-bulb temperature and globe temperature. See wet-bulb globe temperature and heat stress for more information.

[edit] Mean radiant temperature

Mean radiant temperature (MRT) is a measure of the average temperature of the surfaces that surround a particular point. If the point is exposed to the outside, this may include the sky temperature and solar radiation. See mean radiant temperature for more information.

[edit] Globe temperature

Globe temperature (or black-globe temperature) was introduced as a means of assessing the combined effects of radiation, air temperature and air velocity as they influence human comfort. It is measured using a globe thermometer, a hollow copper sphere painted matt back to absorb radiant heat with a temperature sensor at its centre. This can be used in conjunction with air temperature and air velocity to calculate mean radiant temperature. See globe temperature for more information.

[edit] Sling psychrometer

Sling psychrometers hold a wet-bulb thermometer and a dry-bulb thermometer. They can be used to determine the physical and thermal properties of moist air by using standard tables. See sling psychrometer for more information.

[edit] Thermal comfort

BS EN ISO 7730 defines thermal comfort as '…that condition of mind which expresses satisfaction with the thermal environment.', ie the condition when someone is not feeling either too hot or too cold. The Health and Safety Executive suggest that an environment can be said to achieve 'reasonable comfort' when at least 80% of its occupants are thermally comfortable. See Thermal comfort for more information.

[edit] Operative temperature

Operative temperature (previously known as resultant temperature or dry resultant temperature, but renamed to align with ASHRAE and ISO standards) is a simplified measure of human thermal comfort derived from air temperature, mean radiant temperature and air speed. See operative temperature for more information.

[edit] Resultant temperature

A measure of thermal comfort now re-named 'operative temperature'. See operative temperature for more information.

[edit] Predicted mean vote

The predicted mean vote (PMV) is an empirical fit to the sensation of thermal comfort. It predicts the average vote of a large group of people on the a seven-point thermal sensation scale where +3 is hot and -3 is cold. See predicted mean vote for more information.

[edit] Running mean temperature

An exponentially-weighted outside running mean temperature can be used to account for the time-dependency of thermal comfort. Adaptive comfort theory suggests that the occupants of a building will adapt to their environment over time, adjusting clothing, modifying behaviour and so on and so might accept conditions that would otherwise have been predicted to be unsatisfactory. See running mean temperature for more information.

[edit] Accumulated temperature

Median accumulated temperature above 0°C from January to June which gives a measure of heat energy input and soil drying potential and correlates with crop growth and yield.

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