Cavity Wall Insulation: How It Works and What to Check

Many UK homes built from around the 1920s onwards have walls made of two layers, an outer and an inner leaf, with a gap between them called the cavity. If the cavity is empty, it can be filled with insulation to reduce heat loss. Done well, it is one of the most effective ways to cut heating costs. Done badly, it can cause problems, so it pays to do it carefully.

Is your home likely to have a cavity?

Look at the brickwork on an outside wall. If the bricks are laid in a regular pattern with all the long sides showing (stretcher bond), the wall is probably a cavity wall. If the bricks alternate between long and short faces (Flemish bond), it is usually a solid wall, which needs a different approach. A survey or an installer can confirm this, and can check whether the cavity is already filled.

How the work is done

An installer drills small holes in the outer wall at regular intervals, then blows insulation material, such as mineral wool beads or foam, into the cavity. The holes are then filled and made good. It usually takes a few hours for an average house and causes little disruption.

Is your home suitable?

Not every cavity is suitable. A reputable installer will carry out a survey and consider:

The width and condition of the cavity and whether it is clear of rubble.
The state of the brickwork and pointing, which should be sound.
Whether the property is in an area exposed to driving rain, where wet walls can allow moisture to cross the cavity.
Existing damp problems, which should be treated first.
The height of the building and the type of walls.

Risks and how to reduce them

The main risk is moisture crossing the cavity and causing damp patches on inner walls. This is more likely with unsuitable properties or poor installation. In recent years there have been reports of problems with some insulation work carried out under government schemes, and the National Audit Office has raised concerns about quality. To protect yourself:

Use an installer who is registered with a recognised competent person scheme and offers an independent guarantee, such as one backed by a guarantee body for cavity work.
Ask for a written survey and a clear statement that your property is suitable.
Check that work follows the relevant standards and that you receive paperwork when it is done.
Keep the guarantee safe, and photograph the property before and after.
Be cautious about unsolicited calls or door-to-door offers.

If you have problems later

If damp appears after cavity wall insulation, contact the installer and the guarantee provider promptly, and get an independent survey. Removing insulation is possible but specialist work.

Solid walls

Solid walls lose more heat but are harder and more expensive to insulate, using internal or external wall insulation. These need careful design and should always be done by qualified specialists. Start with the cheaper measures, such as loft insulation and draught-proofing, and take expert advice on walls.