U Values

The heat loss from a house iscalculated using U – values. The energy lost per second P through asurface

is proportional to the area ofthe surface,

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

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

We can writethenthe U – value of a surface equals rate of heat loss per unit areaper Kelvin temperature difference.

Example: An uninsulated roofmeasures 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 / ˚Cthe rate of heat loss through the roof is

Some typical U – values aregiven 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