You may have heard reference to U-Values when discussing thermal insulation in buildings. In short it is a measure of heat loss through a particular element in the building. This could be a wall, a floor, roof or window.

The building regulations require new elements of a building to achieve certain minimum U-Values or heat loss in order to reduce carbon emissions.

Here is a table from the building regulations illustrating the required u-values for new thermal elements (in new or existing buildings).

Extract from Approved Document L1
Conservation of fuel and power in dwellings

U-Value is measured in W/m2K.

This is a measure of energy in the form or watts being lost through the area of the thermal element. Watts per square meter per kelvin. So you can effectively calculate how many watts are being lost at any given temperature difference.

For example if the desired internal temperature is 20 degrees celsius and it is 0 degrees outside then the heat loss through a 15 sqm wall with a U-Value of 0.26 would be (15 x 20 x 0.26) = 78 Watts

Electricity costs about 25p per kWh. So the above example heat loss would be 0.078kW so the cost per hour is 1.9pence/h.

Gas or power from a heat pump is considerably cheaper at the moment in the UK, generally about 1/3rd, so the heat loss would cost more like 0.6p/h for gas or ASHP.

If the wall only achieved 0.3W/m2K then the heat loss through that wall would cost 2.25pence/h with electricity or 0.75p/h with gas.