Energy used to heat a building in the heating season should be conserved to reduce heating costs. Therefore, we need to restrict the rate at which heat is lost to atmosphere. Insulation materials resist the transfer of heat between areas at different temperatures. Insulating a building will not only help to conserve heating fuel, but will also help to make the inside more comfortable by increasing the surface temperatures. Increasing the surface temperatures also reduces the risk of condensation forming, which could lead to mould growth.

How much insulation?

Insulation takes energy to manufacture and costs money to put in, so there is a point at which any additional insulation becomes pointless, i.e. it will take more energy to make than it will ever save. With existing buildings it is fair to say, the more insulation the better, but with two main considerations:

  • Heat will always find the easiest way out so insulation needs to be as equally distributed as possible. There is no point in having a a lot of insulation in the loft when all the heat is going out through the windows and walls;
  • In any existing property there are always practical limits to the thickness of insulation; the size of the cavity; the availability of head space in the loft; the space between the floor joists.


In order to determine the correct amount of insulation, we need to calculate the heat transfer through an element, e.g. a wall. This is known as the U value and is a measure of heat transfer (watts) per square metre of element (e.g. wall) for every degree of temperature change between the conditioned and unconditioned (usually outside) spaces (°C or °Kekvin K), written W.m²K. This means that the higher the U value the worse the thermal performance of the element. A low U value usually indicates high levels of insulation.