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Reducing home heat loss

Cavity wall insulation

About a third of all the heat lost in an uninsulated home escapes through the walls. By properly insulating cavity walls, you will save energy and cut costs off your heating bill.

In general, 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 at all.

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

  • If your house was built after the 1920s, it is likely to have 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.
  • Pre-1920 older houses are more likely to have solid walls. A solid wall has no cavity; each wall is a single solid wall, usually made of brick or stone.

Working out your wall type

The first thing you need to find out is what sort of walls you have. If you can see the brickwork on the outside of the house, look at the pattern of the bricks.

Cavity wall

If your home has cavity walls, the bricks will usually have an even pattern with all the bricks laid lengthways.

Solid wall

If your home has solid walls, the bricks will have an alternating pattern, with some bricks laid across the wall so you can see the smaller ends from the outside.

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.

Some houses have a different type of wall structure altogether. If your house is a steel-frame or timber-framed building, or is made from pre-fabricated concrete, then you will need to ask a specialist insulation installer to advise you.

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.

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.

England, Scotland and Wales

Northern Ireland

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.

If you live in a flat then you won’t be able to have just your flat insulated – you will usually need to get agreement from everyone to insulate the whole block.

Find an installer

Your installer should be a member of one of these 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.

What to do if you are experiencing issues with your cavity wall insulation

If you notice problems with your walls after having cavity wall insulation installed, such as damp and mould, you should first call the company who carried out the installation. They should discuss the problem with you and revisit your property to establish whether the original survey or installation work is contributing to the problem. If it is, they should arrange for remedial work to fix the problem or for removal of the insulation if required.

If the installer is not willing to help or no longer exists then contact CIGA to establish whether you have a 25 year CIGA Guarantee. If you do, they should be able to help resolve the issue under the terms of the guarantee. If you don’t have a CIGA guarantee, check your paperwork from time of installation to see if you have an independent insurance backed guarantee which provides a similar level of cover. CIGA also have a contact form to report bad practice.

The symptoms of damp in a home can be very similar and could be caused by poor or inappropriate insulation, or by other issues such as inadequate home maintenance. Neither your installer nor a guarantee provider will agree to carry out or pay for remedial work if they determine that the problems were not caused by inappropriate insulation, bad workmanship or poor materials. If you believe your installer or guarantee provider is unreasonably refusing to help you, you should follow their complaints procedure.

If you are unable to get help from your installer or a guarantee provider, your final option is to speak to a company that can carry out the necessary work. We recommend that you use a company that is accredited with an appropriate scheme. If you are asking them to remove the insulation, you should use a specialist in cavity wall insulation removal accredited with a cavity wall insulation scheme such as CIGA or BBA.


Air needs to flow in and out of your house, so it stays fresh, dry and healthy. A good installer will be sure not to block or seal any intentional ventilation, including:

  • Underfloor grilles or airbricks, which help keep wooden beams and floors dry.
  • Wall vents, which let small amounts of fresh air into rooms.
  • Trickle vents – modern windows often have small vents above them to let fresh air trickle in.

In Northern Ireland, building regulations require adequate ventilation when installing wall insulation. This usually means installing more vents if there are not enough. Check with your local building control office to find out what you need to do for your home.

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Last updated: 4 July 2024