How does cavity wall insulation work?

Heat will always flow from a warm area to a cold one. In winter, the colder it is outside, the faster heat from your home will escape into the surrounding air.

Cavity wall insulation slows down the rate at which it escapes, keeping as much of it as possible inside your home for as long as possible. How? Insulation makes it much more difficult for heat to pass through your walls by filling up the cavity with a material with lots of air pockets in it. These pockets greatly reduce what is known as your walls' U value - which is a measure of how quickly they lose heat - from around 1.5 to 0.5 W/m²K . The lower the U value, the slower heat is lost - and the less energy you need to keep your home warm.

You may on occasions also see references to an R-value. This is a measure of thermal resistance and is the inverse of a U-value - the higher a U-value is the lower the R-value is.

More about cavity walls

For professional installers, cavity walls are simple to insulate. The external walls of a house are actually made of two walls, an inner wall and an outer wall, with a gap of at least 50mm between the two.

In the UK, houses have been built with cavity walls since around 1920. The cavity was originally designed to stop homes getting damp.