Andy Walker explains U Values - a measure of how good different materials are at conducting heat.

Every surface in a house, indeed every material, has a different ability to allow heat to pass through it. Some materials are very good conductors of heat, metals for example, whilst other materials are very good at blocking the flow of heat eg. insulation.

When making a Whole House Energy Review its important to look for the thermal weak spots. The first part of this work is to identify the actual boundary between heated space and unheated space. This is not always where you think it is.

Then assumptions must be made about the materials that have been used to build up the surface. Forexample the external wall of a house will typically have an outer leaf made of stone or brick, a cavity of some kind, and then an inner leaf made of stone, bricks or composite blocks. Then there's likely to be some kind of surface finish like a plaster skim.

Each element of the wall will have a different thermal conductivity associated with it and that coupled with the thickness of each layer gives us a measure of how much heat energy will flow through the wall for a fixed temperature difference between inside and outside.

This quantity is called the 'U Value' and is a crucial way of describing the performance of each surface in your house. It is then possible to quickly see what the improvement would be by upgrading the insulation on the wall, window or door - and directly calculate the reduction in heat energy lost.

Knowing the U Value of typical elements in a house is a fundamental starting point to making a thermal assessment of a home.