Imagine wrapping your house up against the cold on freezing winter days. Cavity wall insulation is like a giant scarf around your home, keeping it warmer and cosier in winter.

About a third of the heat in an uninsulated home is lost through its walls, according to the Energy Saving Trust. So cavity wall insulation could really cut your heating bills and help the environment.

If you’re not sure what kind of walls you have, read on to find out. Or you might like to visit our guide to solid wall insulation.

Diagram of 1. a solid wall with a long- and short-brick pattern, and 2. a cavity wall with a concrete-block inner wall and regular brick outer one

What’s a cavity wall?

Most houses in the UK have either solid walls or cavity walls.

  • A solid wall is what it says – a single, solid wall with no cavity. It’s usually two bricks wide or made of stone.
  • A cavity wall is in fact two walls with a gap, or cavity, between them. The outside wall is usually made of bricks. The inside one might be bricks or concrete blocks.

If you’ve got a solid wall, you can find out about insulation options in our complete guide to solid wall insulation.

How much could I save with cavity wall insulation?

The Energy Saving Trust estimates you could save between £70 and £245 a year on your heating bill, depending on what type of property you live in.

Based on fuel prices from April 2018, and usage for gas-heated homes in England, Scotland and Wales, cavity wall insulation could pay for itself in less than five years.

Detached house Semi-detached house Mid-terrace house Bungalow Flat
Savings on fuel bill (per year) £245 £145 £90 £100 £70
Average cost of unsubsidised installation £725 £475 £370 £430 £330


Source: Energy Saving Trust

Cavity wall cross-section: a layer of mineral wool insulation between a concrete-block inner wall and a brick outer wall

How does cavity wall insulation work?

Cavity wall insulation is a material that fills the cavity in your external walls, forming an extra barrier between your home and the outside world.

The insulation traps the heat inside in chilly winter weather, making your home feel warmer. It also keeps the heat out on sweltering summer days, keeping your house cooler.

The insulation material is often made of mineral wool, as in the picture. The other common types of insulation are polystyrene beads or granules, or polyurethane foam.

For more information on the pros and cons of different insulation materials, visit the National Insulation Association website.

How do I know if I’ve got cavity walls?

If your home is made of brick, there are a few ways to tell you’ve got cavity walls:

  • How old is your house? Most houses built after the 1920s have cavity walls.
  • How regular is the brickwork? If the bricks all look to be the same length, the chances are you’ve got cavity walls. A mixture of long and short bricks is a sign of a solid wall.
  • How thick are your walls? If they’re more than about 260mm thick, they’re probably cavity walls.

Some stone walls may have been built with a cavity that’s suitable for insulation. But stone walls vary in thickness and structure, so you may need to seek professional advice.

Have I already got cavity wall insulation?

Nearly all houses built from the 1990s onwards have cavity wall insulation. So if your home was built in the last 20 or 30 years, the walls probably have insulation built in.

To find out if your walls are already insulated, you can:

  • Contact your local authority’s building control department.
  • Get a registered cavity wall insulation installer to check by drilling a small hole in your wall.

Can I install cavity wall insulation myself?

Cavity wall insulation is not a DIY job, unless you’re an expert. You’ll need a professional installer with special equipment for the job.

Registered installers check that your home is suitable for cavity wall insulation, and they can advise on the best materials for your walls.

How is cavity wall insulation installed?

Your installers will drill small holes (18-25mm in diameter) in your external walls. Then they use a pipe to blow the insulating material through the holes into the cavity.

When they’ve filled the cavity with insulation, they’ll fill in the holes. They should do their best to match the filling with the colour of your walls so that the old holes aren’t noticeable.

The installation won’t take long. The whole process usually takes around two hours, depending on the size of your house.

More energy-saving tips

Cavity wall insulation is not the only way to cut your energy bills. Find out more about energy saving at home, including loft insulation, solid wall insulation, draught proofing and exclusion, and hot water tank insulation.