E   Heating II

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:

  1. Conduction – heat flowing directly through walls, windows and
  2. Ventilation – hot air trickling out through cracks, gaps, or deliberate
    ventilation ducts.

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

Conduction loss

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.

Ventilation loss

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

kitchen 2
bathroom 2
lounge 1
bedroom 0.5
Table E.1. Air changes per hour: typical values of N for draught-proofed rooms. The worst draughty rooms might have N = 3 air changes per hour. The recommended minimum rate of air exchange is between 0.5 and 1.0 air changes per hour, providing adequate fresh air for human health, for safe combustion of fuels and to prevent damage to the building fabric from excess moisture in the air (EST 2003).