A perfectly sealed and insulated building would hold heat for ever and
thus would need no heating. The two dominant reasons why buildings
lose heat are:
In the standard model for heat loss, both these heat flows are proportional
to the temperature difference between the air inside and outside. For a
typical British house, conduction is the bigger of the two losses, as we’ll
The rate of conduction of heat through a wall, ceiling, floor, or window is
the product of three things: the area of the wall, a measure of conductivity
of the wall known in the trade as the “U-value” or thermal transmittance,
and the temperature difference –
power loss = area × U × temperature difference.
The U-value is usually measured in W/m2/K. (One kelvin (1 K) is the
same as one degree Celsius (1 °C).) Bigger U-values mean bigger losses of
power. The thicker a wall is, the smaller its U-value. Double-glazing is
about as good as a solid brick wall. (See table E.2.)
The U-values of objects that are “in series,” such as a wall and its inner
lining, can be combined in the same way that electrical conductances
There’s a worked example using this rule on page 296.
To work out the heat required to warm up incoming cold air, we need the
heat capacity of air: 1.2 kJ/m3/K.
In the building trade, it’s conventional to describe the power-losses
caused by ventilation of a space as the product of the number of changes
N of the air per hour, the volume V of the space in cubic metres, the heat
capacity C, and the temperature difference ΔT between the inside and