heatlossYou can undertake Heat Loss Calculations if provided with the necessary information. This is not a substitute for SAP calculations required to accompany submissions under Part L for Building Regulations Approval.

However, small extensions not requiring SAP calculations benefit from such analysis to ensure sufficient heating when primary heating is required.

Design Heat Loss Calculations

The design heat loss of a dwelling is calculated using details of the building construction –
•Individual room sizes
•Temperature to be maintained
•Rates of air change

The design heat loss has two main component parts –
•Fabric heat losses
•Infiltration heat losses

Fabric heat losses are due to the transmittance of he by conduction through various elements of the building structure, such as, windows, walls, roof and the floor.

Infiltration heat losses are due to warm air from within the home escaping to the outside.

Design Temperature

The design external temperature is traditionally taken as -1C although some designers are tending to use -3C. The value applies throughout England and Wales and is simply a standard figure for design purposes, not a heating limit.

Heating systems sized in accordance with this guide can cope on days colder than -1C, but on very cold days the proportion of direct acting heating would be greater.

Fabric Heat Loss

The rate at which heat is lost through the fabric depends upon its construction and the temperature difference between inside and outside air. For each element of a building there is an overall coefficient of heat transmission. This is called the U-value and indicates the steady state heat transmission through a unit area of building fabric for unit temperature rise. The units for U-value are W/m2K.

To determine the heat loss through a surface the U-value is multiplied by the surface area and by the design temperature difference, the answer being in Watts.

The total fabric heat loss for a room is the sum of the individual losses for each surface (wall, window, floor and roof). Within the dwelling there are losses or gains through the fabric where adjoining rooms are at different temperatures.


The heat loss due to natural infiltration is the product of the room volume, number of air changes per hour, temperature difference and the specific heat of air (usually taken as 0.33 W/m3K). The design air change rates should always be used when calculating the design heat loss.

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