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.

Surface |
U - value |

Roof, tiled, uninsulated |
2 |

Roof, tiled, insulated |
0.4 |

Wall with air cavity |
1.5 |

With with insulated cavity |
0.5 |

Single glazed window |
5.6 |

Double glazed window |
3.0 |