Installing Cavity Wall Insulation: The Ultimate Guide

An insulating material is inserted into the cavity between your home's external walls to help reduce heat loss from your home.

Cavity wall insulation can significantly improve the energy efficiency of your home and save you up to £250 on your energy bills every year.

It only takes a few hours to install and is designed to last for the entire lifespan of your property.


What's In This Guide to Cavity Wall Insulation?

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What Is Cavity Wall Insulation?

When homes were built more than 20 years ago, it was common to build 2 walls with a space or cavity between them. The idea was to prevent rain water from coming through the external wall and into the house. The trouble is that cavity walls are not good insulators and are likely to be a major cause of heat loss from your home.

Cavity wall insulation is designed to reduce heat loss from cavity walls by filling any gaps with an insulating material. If done well, cavity filling is effective at keeping the warmth in and making your home far more energy efficient.

cavity wall insulation


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Do I Need Cavity Wall Insulation?

If your home feels chilly and you find yourself always turning the central heating on, you might want to consider installing cavity insulation. It’s not right for every home, however. Check the list below to see if your home might be suitable for cavity wall insulation:

  • Your home has unfilled cavity walls which are made of brick at least 50 mm wide
  • Your home’s brickwork is in good condition
  • You can access the external walls of your property
  • Your property is not suffering from damp
  • Your walls aren’t exposed to heavy rain or strong winds
  • Your home isn’t made of any steel or timber-framing


Have My Walls Already Been Insulated?

Whether your home has cavity wall filling or not is most likely a question of age. Many homes built from the 1990s onwards are likely to have been insulated already. However, anything built before the 1990s is more likely to have empty cavity walls that would benefit from being filled in.

It’s possible to find clues as to whether your home has cavity walls by looking at any exposed brick work. If the bricks are all placed lengthways or look the same size, it’s likely that there is a cavity in between. If every other brick has been placed end-ways, it’s probably a solid wall.

You could also try measuring the width of the wall; a brick wall with a width of under 26cm is likely to be solid whereas anything wider may have a cavity. To be completely sure, it’s worth getting a professional opinion by contacting an installer who can drill a hole and check with a camera to see if a cavity exists and if it’s been filled or not. Alternatively, you can contact the building control department of your local authority who should have records on whether your walls have been insulated.


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Cavity Wall Insulation Types

There are a few different types of cavity wall insulation but blown mineral wool, polystyrene beads, and polyurethane foam are the most commonly used. All these materials have been approved by the British Board of Agrément or the British Standards Institution and are suitable for cavity insulation. An installer will be able to recommend the best type of insulation for your home.


Blown Mineral Wool

  • Made up of strands of fiberglass or mineral wool
  • Like a quilt that’s broken up into tufts and blown into your walls
  • Best used in wider cavities as it’s harder to fill consistently into smaller spaces
  • Has been known to absorb rainwater and promote damp


Polystyrene Beads or Granules (EPS)

  • The beads trap heat and are effective at filling all gaps
  • The beads expand when warm to promote insulation
  • Effective at reducing damp as resistant to water
  • Sometimes beads can escape from the walls if work is done that requires drilling


Polyurethane Foam (PU)

  • Created by mixing 2 chemical components
  • Good at filling all the gaps
  • Effective at reducing damp and heat insulation
  • Has been known to degrade over time


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How Long Does Cavity Wall Insulation Take to Install?

On average, it takes a couple of hours to install cavity wall insulation - but this will depend on the size of your property and how easy it is to access your cavity walls.

To fill your cavity walls, insulating material is blown or injected into the cavity through small holes which are drilled into the mortar between the bricks of your home’s exterior walls. Once the work has been completed, the holes are then filled with a material that matches your brickwork.

The installer will need to get to all of your property’s walls, including access to any garages, conservatories or lean-to sheds. It’s also a good idea to remove any delicate ornaments from shelves and mantelpieces, as drilling can cause vibrations.

Once installed, cavity wall insulation should last the for the entire lifespan of your property. Make sure your installation is guaranteed for 25 years by the Cavity Insulation Guarantee Agency (CIGA). Always ask your installer if they are members of CIGA before you book them.

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Cavity Wall Insulation Cost

The cost of your cavity wall insulation will depend on the material you use and the size of your property. For an average sized home, installation typically ranges between £370 and £475 . The most expensive insulating material is EPS beads, while blown mineral wool is normally the cheapest.

Type of Cavity Wall Insulation
Cost per m²
Blown Mineral Wool
£14 to £17
Polystyrene Beads or Granules
£18 to £21
Polyurethane Foam
£22 to £25


This table displays the average cost of cavity insulation depending on the size of a property:

Property Size
Average Cost
Detached House
£720
Semi-detached House
£475
Mid-terrace House
£370
Bungalow
£430
Flat
£330


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Savings from Cavity Wall Insulation

Given that about a third of your home’s heat loss happens through its walls, there are huge energy savings to be made by installing cavity wall insulation, particularly for larger properties. Over time, the cost of installation will pay for itself through savings you’ll make on your energy bills. The speed of this return on investment will depend on the size of your property, as the tables below show.


England, Scotland and Wales

Property Size
Annual Energy Bill Savings
Installation Cost
Profit After 5 Years
Detached House
£250
£720
+£500
Semi-detached House
£145
£475
+£250
Mid-terrace House
£90
£370
+£80
Bungalow
£100
£430
+£70
Flat
£65
£330
+£25


Northern Ireland

Property Size
Annual Energy Bill Savings
Installation Cost
Profit After 5 Years
Detached House
£215
£720
+£535
Semi-detached House
£130
£475
+£175
Mid-terrace House
£80
£370
+£30
Bungalow
£85
£430
-£5
Flat
£60
£330
-£30


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Cavity Wall Insulation Grants

It’s possible to get support from the Government for cavity wall insulation. The Energy Company Obligation (ECO) is a UK Government energy efficiency scheme to help reduce carbon emissions and tackle fuel poverty.

The current scheme will end in September 2018 and will be renewed (under the same name) with a scheme that prioritises the most fuel-poor households. It aims to help 900,000 households with energy-saving measures. Cavity wall insulation falls within the Carbon Emissions Reduction Obligation (CERO) part of the scheme.

If eligible, you could be entitled to free cavity wall insulation or support for part of the cost. The amount of funding you’ll receive will depend on your financial circumstances and the energy efficiency of your home.

Finding out whether you’re eligible for funding for the ECO scheme can be a little complex; it depends on different factors such as where you live, if you’re on benefits, and the potential carbon and cost savings that could be made by having cavity wall insulation. You may be eligible if you live in private housing and are on benefits, or if you live in social housing and your property’s energy efficiency rating is E, F or G.

Your energy supplier should let you know if you’re eligible for the ECO grant, which they may also refer to as the Affordable Warmth scheme. The 6 big energy firms offer ECO support: British Gas, EDF Energy, E.ON, npower, ScottishPower, and SSE.

You can also check your eligibility by calling the government’s Energy Saving Advice Service on 0300 123 1234.


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Finding Cavity Wall Insulation Installers

It’s important that your cavity wall insulation is installed by a qualified professional and that you do not attempt to carry out the work yourself. This is a highly technical procedure that requires particular expertise. When cavity insulation is done incorrectly it can damage your home by creating cold spots and damp in your walls.

Make sure your installer is a member of the National Insulation Association (NIA), the Cavity Insulation Guarantee Agency (CIGA) or the British Board of Agrement (BBA). You can find a local registered cavity wall installer by calling the Energy Saving Trust helpline on 0300 123 1234.


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Cavity Wall Insulation Removal

When cavity wall insulation fails it’s usually due to a historic job done poorly which needs to be removed and redone. Back in the 1980s there was a big push to insulate all cavity walls, but in many cases the job wasn’t effective as the work was rushed and not carried out properly.

Poorly installed cavity wall insulation has led to issues including:

  1. Cold spots where the insulating material was not inserted into the cavity wall evenly.
  2. The use of inappropriate material like urea-formaldehyde (no longer used) which can break down and fall to the bottom of the cavity, leaving the top part uninsulated once again.
  3. Damp issues caused by insulation that had moved and allowed water to seep into the cavity.
  4. In some cases the uPVC glue which is used to stick insulating beads together was not applied, causing the beads to drop to the bottom of the cavity and not insulate the top part of the wall.
  5. Some properties were simply unsuitable for cavity wall insulation and should never have been filled in the first place.

Most cavity wall insulation material can be removed pretty easily, but methods of extraction vary slightly. Wool insulation can simply be sucked out by a large vacuum, whereas glued beads, foam, or urea- formaldehyde need to be broken down first and then removed. This will require removing some of the external bricks to allow the insulation to be accessed.

The time it takes to remove the insulation depends on the size and type of the building. As a guide, removing cavity insulation from all of the walls of a detached house could take up to 2 days.


Cavity Wall Insulation Removal Cost

Cavity wall insulation removal costs about £21 to £22 per m2 but this will depend on the material that needs to be removed. This table shows the average cost* of cavity wall insulation removal for different sized properties.

Property Size
Average Cost
Detached House
£2,520 to £2,640
Semi-detached House
£1,680 to £1,760
Mid-terrace House
£1,470 to £1,540
Bungalow
£1,260 to £1,320
Flat
£1,050 to £1,100

*VAT will be added to the cost shown above.


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