Glossary · Controls, energy efficiency and insulation
What is cavity wall insulation?
Cavity wall insulation is insulating material put into the gap between the inner and outer leaves of a cavity wall. Filling that gap slows the heat passing out through the walls, so the heating has less heat to replace. Whether a wall is suitable depends on how it is built and its condition.
Key points
- It fills the gap between the two layers of a cavity wall to slow heat loss through the wall.
- In a government evaluation, gas-heated homes that fitted cavity wall insulation used on average 1.9 kWh a day less gas, a 5.8% reduction.
- An estimated 71% of British dwellings had cavity wall insulation by the end of 2024.
- Insulation fitted under ECO should come with a guarantee. If the installer cannot help, contact the guarantee provider.
- Not every wall is suitable. Building Regulations guidance applies its cavity insulation figures only to walls that are suitable for it.
How cavity wall insulation works
Many British homes have outer walls built as two layers with a gap between them. Cavity wall insulation fills that gap with insulating material, so less heat passes from the warm inside to the cold outside. The heating then has less heat to replace.
Government research gives a sense of the effect. In an evaluation of the Green Homes Grant, homes with gas central heating that installed cavity wall insulation used on average 1.9 kWh a day less gas, a 5.8% reduction. Modelled national estimates were higher, at 8.1%.[1]Department for Business, Energy and Industrial Strategy, Evaluation of the Green Homes Grant Voucher Scheme (GHGV): Final outcome and economic evaluation report The real figure depends on the house, how the heating is used and what insulation was there before.
It is a common upgrade. The Warm Homes Plan estimates that by the end of 2024, 71% of British dwellings had cavity wall insulation and a further 10% had solid wall insulation.[2]Department for Energy Security and Net Zero, Warm Homes Plan (HTML) A government consumer guide describes cavity wall insulation as one of the most cost-effective ways to improve a home’s energy efficiency when it is installed in suitable homes with appropriate installation standards and guarantees.[3]Department for Energy Security and Net Zero, Cavity wall insulation (CWI): consumer guide to issues arising from installations
Suitability, standards and when it goes wrong
Not every wall can take cavity insulation. Approved Document L applies its cavity wall figures only to a wall that is suitable for cavity insulation. A wall that is not suitable is treated as one that needs internal or external insulation instead.[5]Ministry of Housing, Communities and Local Government, Approved Document L, Conservation of fuel and power, Volume 1: Dwellings, 2021 edition incorporating 2023 amendments For an existing home, the guidance sets a threshold U-value of 0.70 W/(m2.K) and an improved target of 0.55 for cavity insulation.[5]Ministry of Housing, Communities and Local Government, Approved Document L, Conservation of fuel and power, Volume 1: Dwellings, 2021 edition incorporating 2023 amendments
| If | Do this |
|---|---|
| You suspect faulty installation | Contact the installer first, who should investigate and rectify defects |
| The installer has gone out of business or does not act | Contact the guarantee provider named in your paperwork |
| It was fitted under the Energy Company Obligation | Check Ofgem’s guidance on complaints about cavity wall insulation |
| A claims management company calls you | Check them before agreeing to anything, and do not hand over personal or financial details |
Government guidance says genuine issues have arisen in a small number of instances. It tells householders who suspect a failed installation, or who are unsure whether it has caused condensation or mould, to contact the installer first.[3]Department for Energy Security and Net Zero, Cavity wall insulation (CWI): consumer guide to issues arising from installations Ofgem says that cavity wall insulation fitted under ECO should be covered by a guarantee, and that if it was handed over after 1 January 2020 and was TrustMark registered, TrustMark may be able to help.[4]Ofgem, Complaints about standard of cavity wall insulation installation
The Energy Company Obligation can still help with insulation such as cavity walls for people on certain benefits. It is closing on 31 December 2026.[6]GOV.UK, Help from your energy supplier: the Energy Company Obligation
Example
Raj lives in a 1970s semi in Bradford. His EPC says the walls have no insulation, and the surveyor confirms the cavity is suitable. His installer fills the cavity and gives him a guarantee certificate. A year later he notices damp patches on an inside wall. He contacts the installer, who inspects it and puts things right. Had the installer disappeared, Raj would have contacted the guarantee provider named on his certificate.
What people get wrong
People often assume cavity wall insulation is risk-free or, the other way round, that every installation causes damp. Government guidance does neither. It says issues have arisen in a small number of instances, and that claims management companies sometimes contact householders about compensation that may not be genuine.
The second mistake is to lose the paperwork. The guarantee, installer details and any survey are what you need if something goes wrong. Keep them with your EPC and your other property records. A government guide also says to avoid signing documents on the doorstep.[3]Department for Energy Security and Net Zero, Cavity wall insulation (CWI): consumer guide to issues arising from installations
The bottom line
Cavity wall insulation fills the gap in a cavity wall to slow heat loss. Government research found average gas savings of 5.8% in homes that fitted it, with modelled estimates higher. It only suits walls that can take it, so it needs an assessment first and a guarantee afterwards. If anything looks wrong, go to the installer first, then the guarantee provider. See also loft insulation and the Warm Homes Plan.
Common questions
How much does cavity wall insulation save?
A government evaluation found gas-heated homes that installed it used 1.9 kWh a day less gas on average, a 5.8% reduction. Modelled national estimates were higher at 8.1%, so the saving in your home depends on the house and how it is heated.
Is my wall suitable for cavity wall insulation?
Only a wall that is suitable for cavity insulation should be filled. An installer must assess it first. Building Regulations guidance treats walls that are not suitable as walls needing internal or external insulation instead.
What if I think my cavity wall insulation is faulty?
Contact the installer first. If they have gone out of business or do not act, contact the guarantee provider named in your paperwork. For ECO installations, Ofgem also publishes guidance on complaints.
Can I get cavity wall insulation free?
Possibly. The Energy Company Obligation can help with cavity wall insulation if you get certain benefits and your home has an EPC rating in the eligible bands. The scheme closes on 31 December 2026.
Sources
We cite the original source for every figure: government, regulators, standards bodies and manufacturers' own documents. Each one shows the date we last checked it.
- Department for Business, Energy and Industrial Strategy. Evaluation of the Green Homes Grant Voucher Scheme (GHGV): Final outcome and economic evaluation report. Checked 11 November 2026. ↩
- Department for Energy Security and Net Zero. Warm Homes Plan (HTML). Checked 11 November 2026. ↩
- Department for Energy Security and Net Zero. Cavity wall insulation (CWI): consumer guide to issues arising from installations. Checked 11 November 2026. ↩
- Ofgem. Complaints about standard of cavity wall insulation installation. Checked 11 November 2026. ↩
- Ministry of Housing, Communities and Local Government. Approved Document L, Conservation of fuel and power, Volume 1: Dwellings, 2021 edition incorporating 2023 amendments. Checked 11 November 2026. ↩
- GOV.UK. Help from your energy supplier: the Energy Company Obligation. Checked 11 November 2026. ↩