Cavity Wall Insulation

Cavity Wall Insulation Explained

Many cavity walls can be insulated by injecting insulation material into the cavity from the outside. A specialist company will drill holes in the outside walls, inject insulation through the holes and then seal them with cement. The insulation material is usually either mineral wool or polystyrene beads, but polyurethane foam may sometimes be used instead.

To insulate your cavity walls, the installer drills small holes around 22mm in size at intervals of around 1m in the outside wall of your home. The installer then blows insulation into the cavity using special equipment. Once all the insulation is in, the installer fills the holes in the brickwork so you’ll barely notice them.

Filling cavity walls is not a job you can do yourself, you will need to employ a registered installer. A professional can do the job in around two hours for an average house with easily accessible walls. It shouldn’t make any mess.

Costs and savings

Typical installation costs of cavity wall insulation vary depending on the size of your home. But whether you live in a large detached house or small flat, you should be able to make back the installation cost in five years or less due to the yearly energy bill savings you will make. As a rough guide, £800 is what you can expect to pay to install cavity wall insulation in a detached house. Annual energy savings could be as much as £200/ year on a detached home (EST).

You might be able to reduce these costs by carrying out the work at the same time as other home improvements or by not tackling the whole house at once.

Is cavity wall insulation right for your home?

If your house was built in the last 20 years or so, the walls are probably already insulated. To find out whether they are, you can do the following:

  • ask a registered installer for a borescope inspection. The installer will drill a small hole in your external wall to see if your walls are hollow or filled
  • check with your local authority’s building control department

Your home will be suitable for standard cavity wall insulation if it meets the following criteria:

  • its external walls are unfilled cavity walls
  • your cavity is at least 50mm wide, and is clear of rubble
  • the masonry or brickwork of your property is in good condition
  • the walls are not exposed to driving rain
  • your house is not at risk of flooding

You will need an installer to carry out a survey to check that your house is suitable. If so, they will then be able to insulate your walls using mineral wool or polystyrene beads.

If your house has narrow or uneven cavities, is in an exposed site or there is a risk of flooding, then it may be possible to fill the cavity with polyurethane foam. This is more expensive than standard cavity wall insulation but is a particularly effective insulator. You will need a specialist foam insulation installer to survey your home for this, and to carry out the work if suitable.

If you have any damp patches on your internal walls then they should not be insulated until the problem is resolved. Speak to a builder who specialises in damp prevention.

If your home’s external walls are joined to another house, the installer will need to insert a cavity barrier to contain the insulation so your neighbours aren’t affected.


The installer should be a member of one of the following organisations:

Check whether the installer is signed up to a code of professional practice and that the installation is guaranteed for 25 years by CIGA, or through an independent insurance-backed guarantee.

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