U Values

The heat loss from a house is calculated using U – values. The energy lost per second P through a surface

is proportional to the area of the surface,

is proportional to the temperature difference on either side of the surface,

depends on the nature of the surface, expressed by the U – value.

We can writethen the U – value of a surface equals rate of heat loss per unit area per Kelvin temperature difference.

Example: An uninsulated roof measures 10 m by 10 m. The temperature inside the house is 25 ˚Cand the temperature outside is -5 ˚C.If the U – value of the roof is 2.5 W/m 2 / ˚The rate of heat loss through the roof is

Some typical U – values are given in the table below.


U - value

Roof, tiled, uninsulated


Roof, tiled, insulated


Wall with air cavity


With with insulated cavity


Single glazed window


Double glazed window