Cavity wall insulation

Cut away of cavity wall insulation

If your home was built after 1920, the chances are that its external walls are made of two layers with a gap or cavity between them. Cavity wall insulation fills that gap, keeping the warmth in to save energy. It can also help reduce condensation inside the house if this is a problem on your external walls.

Not sure whether you have cavity walls? Work out what sort of walls you have.

How much could you save?
Is cavity wall insulation suitable for your home?
How is insulation installed?
What is the insulation made of?


  How much could you save?



Semi detached

Mid terrace



Annual saving  £250  £145  £95  £100  £75
Installation cost  £720  £475  £370  £430  £330
Payback time 5 Years or fewer
Carbon dioxide saving per year  1040kg  600kg  395kg  410kg  325kg

These are estimated figures based on insulating a gas-heated home. The average installed cost is unsubsidised.


  Is cavity wall insulation suitable for your home?

Your home will usually be suitable for cavity wall insulation if:

  • its external walls are unfilled cavity walls
  • your cavity is at least 50mm wide
  • the masonry or brickwork of your property is in good condition
  • it is more than ten years old (most newer houses will have insulation already)
  • the walls are not exposed to driving rain.

Are your cavity walls unfilled?

If your house was built in the last ten years or so, its walls are probably insulated already. To find out whether they are: 

  • ask a registered installer for a boroscope inspection. They 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 - they might know if your cavity walls have been filled already.

Are your external walls easy to access?

Cavity wall insulation is blown into the cavity from the outside of a house. Every part of the wall must be filled with insulation, so it's important that the installer can reach all your external walls.

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.


What if there is damp?

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


How is insulation installed?   

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. With specially designed equipment, they then blow insulation into the cavity. 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 find a registered installer. A professional can do the job in around two hours for an average house with easily accessible walls; it should be simple, quick - and make no mess.


Finding an installer

Your installer should be a member of one of these organisations:

Remember to also check that:

  • your installer is signed up to a code of professional practice like the one provided by the NIA
  • the installation is guaranteed for 25 years by CIGA.

What is the insulation made of?  

Cavity wall insulation can be made out of three types of materials:

  • mineral wool
  • beads or granules
  • foamed insulants.

All three are manufactured according to British standards. Foam Insulation systems should be certified by the British Board of Agrément and installed according to strict guidance laid out in the associated BBA Certificates.

Find out more at the British Board of Agrément website.


What we recommend

Your installer will suggest the most suitable type of insulation for you but check that the installation is covered by under the Cavity Insulation Guarantee Agency (CIGA): this means it complies to British standards and it has a 25-year guarantee. Ask your installer to make sure you make the best choice.


Help for landlords

Until April 2015, the Landlord’s Energy Saving Allowance lets you claim up to £1,500 against tax for energy-saving improvements you have made to each house or flat you rent out. Find out more at the Directgov website.


Save money!

You’ll be charged a lower rate of VAT when you have energy-saving work done to your house, both for the materials and equipment, and for the labour. If the house is new, you pay no VAT at all. Find out more at the HMRC website.


Planning a home improvement project?