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The ultimate guide to cavity wall insulation

If your home was built after the 1920s, the chances are that it’s got cavity walls. And unless it was built within the last 20 years, those cavities are probably empty. If so, filling them with wall insulation could be a very cost-effective way to retain heat in your home and save on your energy bills. 

Around one third of the heat loss from most homes is through the walls, so cavity insulation could save you up to £160 a year in heating bills. In fact, according to these figures from the Energy Saving Trust website it could pay for itself within less than 5 years.



       Type of property       


 Semi detached 

   Mid terrace   



Energy bill savings (£ pa)






Average payback period

32 months

36 months

43 months

47 months

45 months

Average payback period






CO2 savings per year






CO2 savings per year

1,100 kg

650 kg

430 kg

450 kg

360 kg




These are estimated figures for England, Scotland and Wales, based on insulating a gas-heated home. The actual payback time will depend on the date when the insulation is installed, as the amount saved each month will vary between winter and summer. The average installation cost shown here is unsubsidised.

What is a cavity wall?

A cavity wall is one made out of two separate thin walls (usually built of brick, and known as ‘skins’ or ‘leaves’) with a gap (or cavity) between them. They are held together by metal wall ties.

How can I tell if I have cavity walls?

As we mentioned above, the age of your home is the first clue. However, if you’re not sure how old it is, or you reckon it was built around 1930 so could be either a cavity or a solid wall, have a look at any exposed brickwork. If your home has cavity walls, the bricks will all look the same size, like this:

cavity wall

… but if the walls are solid, every other brick will probably have been placed end-on, like this:

Solid wall

If all the brickwork in your home has been rendered or cladded so you can’t see any actual bricks, you may be able to tell from the thickness of the outer walls. Check the windows and doorways: if a brick wall is more than 10 inches thick, it’s probably a cavity wall. However, solid stone walls can also be very thick.

My home doesn’t have brick walls – can I insulate it?

  • If your house has stone walls, they’re most likely to be solid, with no cavities to insulate. See our Ultimate guide to solid wall insulation <link to new guide> for alternative ideas.
  • If you live in a timber- or steel-framed building, or your home is built of pre-fab concrete, they won’t have cavity walls, but you may be able to insulate them in another way. To find a suitable local installer, get in touch with the National Insulation Association.
  • If a contractor suggests injecting wall cavity insulation between the outer brick leaf and the inner frame of your timber-framed home, don’t accept this as it can cause serious damage.

Is cavity wall insulation suitable for my home?

You should only consider cavity wall insulation if:

  • Your home has unfilled cavity walls made of brick.
  • The cavities are at least 2 inches wide.
  • The brickwork or masonry is in good condition.
  • Your external walls are accessible. If some are joined to a neighbouring house, the installer will need to insert a cavity barrier, which could add to the costs. Installers may also be reluctant to work around garages, conservatories or extensions.
  • Your home is less than 12 metres (about 4 storeys) high.
  • Your internal walls are dry. Wet wall insulation is worse than no wall insulation, so if you have any damp patches, you’ll need to get the cause sorted out before installing insulation. For the same reason, cavity insulation is not suitable if the walls are regularly exposed to driving rain.
  • There are no areas of steel- or timber-framed construction.

Cavity wall insulation is only suitable for your home if you can answer ‘yes’ to all these points,

Some timber-framed homes look exactly as though they’re built of brick – but of course they’re not. These buildings are not suitable for cavity wall insulation, as they need the cavity to allow moisture to escape. If you’re not sure whether your home is built in this style, check up in the attic. If your party or gable walls are made of timber instead of brick, you’ve got a timber-framed house.

How can I tell if my walls have already been insulated?

If your home was built in the last 20 years, the walls were probably insulated when it was built. If not, or if you want to make sure, you can:

  • Ask a registered installer to drill a small hole in the wall and let you know whether the wall is empty or insulated. This is called a boroscope inspection.
  • Check with the building control department of your local authority. They should have records if your walls have already been insulated.

There are also a couple of clues to look out for that could save you the trouble of a boroscope inspection:

  • Installers will have drilled 1-inch holes at regular intervals when inserting the wall cavity insulation. Although they’ll have filled these in, you should still be able to see faint marks – but don’t confuse them with the marks left by an injected damp proof course.
  • Check in your attic – the cavity insulation material may be spilling out at the top of the wall. However, this is not a good thing, so you should probably get a professional to clear it up and seal off the wall.

Can I install wall cavity insulation myself?

Only if you’re a trained and qualified wall insulation installer.

What does the work involve?

Your installer should start by checking the walls are suitable, in good condition and free from damp. They will then drill a series of small holes, blow the insulation into the cavity with special equipment and fill in the holes with mortar afterwards.

A professional installer should be able to complete the work in around 2 hours for an average-sized house with easy-to-access walls. They should ‘make good’ when they’ve finished and make sure you’re not left with any mess.

They should not need to enter your house for work purposes at all (although of course they may need to use your loo).

Not long after the work is complete, you should be sent a guarantee issued by the Cavity Insulation Guarantee Agency (CIGA), and your installer should give you written confirmation that the work complies with building regulations. Your local authority or CIGA may also come round to carry out a spot check to make sure the work is of an acceptable standard.

What kind of cavity insulation will they install?

Cavity wall insulation can be mineral fibre wool, polystyrene granules (also known as beads) or polyurethane foam. They should all be manufactured to British standards.


Mineral wool is used most often. It’s like the mineral ‘quilt’ insulation used in lofts but broken up into small tufts so it can be blown into the walls. It must be kept absolutely dry, or it loses its ability to insulate, and it may settle over time, creating air pockets at the top of the walls.

diy home insulation

Beads and granules are also popular, as they trap heat very efficiently and create gap-free wall insulation. However, loose granules have been known to escape through airbricks and can gush out if you ever need to have work done that involves drilling or cutting into the wall.

Foam offers the best thermal cavity wall insulation, but installation is tricky and needs expert attention, and some foams have been known to degrade in the long term.

How do I find a good installer?

Visit one of these websites:

  • The British Board of Agrement (BBA) – click on ‘installer search’.
  • The Cavity Insulation Guarantee Agency (CIGA) – click on ‘find an installer’.
  • The National Insulation Association (NIA) – click on ‘find your nearest installer’.

Remember, work like this only qualifies for a guarantee if it’s carried out by a fully qualified professional who has signed up to appropriate codes of practice. So before you confirm the booking, make sure that:

  • A careless installer could block flues or airbricks.
  • They might blow insulation materials out of the top of the walls into your loft, or even into next door.
  • If they fail to distribute the material evenly, it could create air pockets. These can lead to cold areas on your internal walls, causing patches of condensation and mould.
  • The cavity insulation might cause the wall ties holding your walls together to rust. However, this should only happen if damp gets in because your brickwork is crumbling or is often exposed to torrential rain – so you really shouldn’t have chosen cavity wall insulation to start with.

*Source and notes for graphs and table

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