Last edited 08 Mar 2019

Heat loss

Heat is the energy that is transferred between different systems as a result of thermodynamic interactions.

Heat loss is a measure of negative heat transfer through a building’s fabric from the inside to the outside. This can be due to either convection, conduction, radiation, mass transfer, or a combination. The colder the outside temperature, the warmer the inside, and the worse the thermal insulation of the building fabric, the greater the heat loss will be.

Heat loss is typically measured in either kilowatts (kW) or British Thermal Units (BTUs).

U-values (sometimes referred to as heat transfer coefficients or thermal transmittances) are used to measure how effective elements of a building's fabric are at insulating against heat loss (or heat gain). The lower the U-value of an element of a building's fabric, the more slowly heat is able to transmit through it, and so the better it performs as an insulator. Very broadly, the better (i.e. lower) the U-value of a building's fabric, the less energy is required to maintain comfortable conditions inside the building.

The building regulations require that reasonable provision be made to limit heat gains and losses through the fabric of new buildings and works to existing buildings. The approved documents to the buildings regulations set out the limiting standards for the properties of the fabric elements of the building, described in terms of maximum U-values. For more information see: Limiting fabric standards.

Typically, the older a building is, the more it will be susceptible to heat loss. This can be due to a combination of poor (or no) insulation, thermal bridging across the building envelope, single glazing, poor airtightness and so on,

Levels of heat loss will vary depending on the type of building; for example, a terraced house will lose a higher proportion of heat through the floor and roof rather than walls, whilst nearly all the heat loss from a flat will be through the outside walls.

The following estimates indicate the proportionate heat loss from a badly insulated house:

(Ref. www.ired.co.uk)

Some of the techniques that can be used to combat heat loss in existing buildings include:

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