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Cavity wall

 About a third of all the heat lost in an uninsulated home escapes through the walls.Heat will always flow from a warm area to a cold one. In winter, the colder it is outside, the faster heat from your home will escape into the surrounding air.

Most houses built from the 1990s onwards have wall insulation to keep the heat in, but if your house is older than that it may not have any wall insulation. If this is the case then you may be heating the outside air, instead of just heating your home. Most types of wall can be insulated in one way or another. If you have a typical house with cavity walls, you could save up to £140 per year in heating bills just from insulating the walls.

The first thing you need to find out is what sort of walls you have.

Cavity and solid walls

Houses in the UK mostly have either solid walls or cavity walls:

  • A cavity wall is made up of two walls with a gap in between, known as the cavity; the outer leaf is usually made of brick, and the inner layer of brick or concrete block.
  • A solid wall has no cavity; each wall is a single solid wall, usually made of brick or stone.

Working out your wall type

If your house was built after the 1920s, it is likely to have cavity walls. Older houses are more likely to have solid walls.

If you can see the brickwork on the outside of the house, look at the pattern of the bricks. If your home has cavity walls, the bricks will usually have a regular pattern like this:

If your home has solid walls, the bricks will have an alternating pattern like this:

If the brickwork has been covered, you can also tell by measuring the width of the wall.  examine a window or door on one of your external walls. If a brick wall is more than 260mm thick then it probably has a cavity; a narrower wall is probably solid. Stone walls may be thicker still but are usually solid.

Non-standard wall types

If your house is a steel-frame or timber-framed building, or is made from pre-fabricated concrete different rules apply for insulation.

Generally these houses don't have a cavity to fill, but it may be possible to insulate them in the same way as a solid wall. However, you may need a specialist company to insulate a non-standard wall. For further advice or to find an installer who can help you, contact the National Insulation Association.

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 in between. 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.

You can use the Energy Saving Trust’s Home Energy Check tool to work out what sort of walls you have.

How much you can save

England, Scotland and Wales

Building

Detached

Semi detached

Mid terrace

Bungalow

Flat

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

Northern Ireland

Building

Detached

Semi detached

Mid terrace

Bungalow

Flat

Annual saving £350 £200 £135 £130 £110
Installation cost £870 £415 £220 £270 £146
Payback time 3 Years or fewer
Carbon dioxide saving per year 1250 kg 720 kg 475 kg 495 kg 390 kg

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 it meets these criteria:

  • 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 10 years old (most newer houses will have insulation already).
  • The walls are not exposed to driving rain.

Are your cavity walls filled?

If your house was built in the last 10 years or so, the walls are probably insulated. To find out whether they are you can do the following: 

  • Ask a registered installer for a boroscope 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.

Are your external walls accessible?

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 your house is damp?

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. 

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. 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.

Finding an installer

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

  • the National Insulation Association (NIA) (Go to the NIA website)
  • the Cavity Insulation Guarantee Agency (CIGA) (Go to the CIGA website)
  • the British Board of Agrément (BBA) (Go to the BBA website).

Remember to also check whether the installer is signed up to a code of professional practice like the one provided by the NIA and that the installation is guaranteed for 25 years by CIGA.

What is insulation made of?

Cavity wall insulation can be made from three types of material: mineral wool, beads or granules, or 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 certificates.

Read more at the British Board of Agrément site. 

What the Energy Saving Trust recommends

Your installer will suggest the most suitable type of insulation but check the installation is covered under CIGA, which means it complies with British standards and it has a 25-year guarantee. Ask your installer to make sure you make the best choice.