u-values

A U-value measures the heat loss through a material - in the case of construction - heat loss through the roof, external walls and ground floor of a building.  So a windo for example would have a U-value of its own which would contribute to the overall U-value of a wall. 

The lower the U-value, the lower the level of heat loss.  U-values are indicated in units of "watts per metre squared per degree kelvin" or W/m2K.

Maximum U'values permissible under the latest Building Regulations are 0.35 for walls and floors and 0.25 for roofs.  Windows have a permitted U-value of 1.8.  Changes in the Building Regulations in 2006 specified that where a house is being renovated, if 25% or more of the total surface area is being renovated, then the whole house needs to comply with U-value requirements.