What is cavity wall insulation, and does your home have cavity walls?
Cavity wall insulation is material injected into the gap between the two layers of an outside wall, so less heat escapes through the brickwork. Energy Saving Trust says that if your house was built after the 1920s, it is likely to have cavity walls, while older homes are more likely to have solid walls with no gap to fill.
Heating a home with leaky walls is the Do-Nothing Default in bricks and mortar. Energy Saving Trust says about a third of all the heat lost in an uninsulated home escapes through the walls, so it is worth a look before winter really bites. Cavity wall insulation is one of the better known fixes, though it does not suit every house.
You can often tell your wall type from the outside. If the bricks sit in an even pattern, all laid lengthways, you probably have cavity walls. If some bricks are turned so you see their short ends, the wall is probably solid. Energy Saving Trust puts it in terms of thickness: a brick wall over 260mm thick probably has a cavity, and a narrower one is probably solid. Stone walls can be thicker than that and are usually still solid, so thickness alone does not settle it.
If your home was built in roughly the last 20 years, Energy Saving Trust says the cavity is probably insulated already. Worth checking before you pay for a survey.
Key takeaways
- Cavity wall insulation is for homes with a gap between the inner and outer wall, usually built after the 1920s.
- Energy Saving Trust does not publish a typical price, so get at least two quotes from registered installers rather than working to a figure you have read somewhere.
- Energy Saving Trust says the installation cost should be made back in five years or less through lower bills, though that is an estimate rather than a promise.
- If you already have damp on an internal wall, the wall should not be insulated until that is sorted out.
- ECO4 ends on 31 December 2026 and the Warm Homes Plan is replacing it, so check what is on offer before you pay.
How much does cavity wall insulation cost?
Energy Saving Trust does not publish a typical price for cavity wall insulation. Its guide says costs vary depending on the size of your home, and leaves it there. Anyone quoting you a single national average is working from their own figure, not Energy Saving Trust’s, so treat round numbers with care and get more than one quote.
What Energy Saving Trust does say about the money is this: you should be able to make back the installation cost in five years or less through savings on your yearly energy bills. That is an estimate, and your own saving will depend on your house, your heating and how much you use it.
If you are already doing building work, you might cut the cost by doing the job at the same time. Cavity wall insulation is not a DIY job. Energy Saving Trust says you will need to employ a registered installer.
Before you pay a deposit, ask what the survey covers and what guarantee you get. Energy Saving Trust says installers should belong to the National Insulation Association, the Cavity Insulation Guarantee Agency, known as CIGA, or the British Board of Agrement, and that the work should be guaranteed for 25 years, either by CIGA or by an insurance backed guarantee. Keep that paperwork safe, because 25 years is a long time and you may need it.
Is cavity wall insulation worth it?
For many homes with suitable walls, cavity wall insulation can be worth it, but the answer depends on your home and how you heat it. The five year payback above is the clearest thing Energy Saving Trust says on the subject, and it is a guide rather than a guarantee.
Energy Saving Trust adds that suitability is down to how your home is built and its condition. It also says cavity wall insulation helps tackle carbon emissions and is a relatively unintrusive home improvement.
It might not be suitable if your home is exposed to extreme weather or flooding, is in poor condition, or risks overheating. The cavity itself also has to be wide enough: Energy Saving Trust says it must be at least 50mm wide and clear of rubble. An installer or building professional should assess all of this. Any repairs to damaged brickwork or pointing should be done first.
Can cavity wall insulation cause damp?
Start with the point Energy Saving Trust makes first, because it is the one that catches people out. If you already have damp patches on an internal wall, that wall should not be insulated until the problem is resolved. Speak to a damp prevention specialist before you book an installer, not after. Insulating over an existing damp problem does not fix it and makes it harder to get at.
Where there is no existing problem, cavity wall insulation should not cause damp if it is installed correctly. A proper survey is what stands between you and a bad outcome, which is why the registered installer requirement matters more here than on most home improvements.
If you notice damp or mould after installation, call the company that did the work first. They should visit and check whether the original survey or the installation is part of the problem. If they will not help, or they have gone out of business, Energy Saving Trust says to contact CIGA to check whether you are covered by its 25 year guarantee. CIGA also has a form for reporting bad practice.
Damp can have other causes too, such as poor home maintenance, and installers or guarantors will not pay for repairs caused by that. If you are unsure, an independent assessment from a buildings surveyor is the way to settle it.
Is there a grant for cavity wall insulation?
Funding exists, but the schemes have moved on, and Energy Saving Trust’s cavity wall guide does not cover them. For that you need the government’s own pages.
The government says the ECO4 scheme, which funds insulation through energy suppliers, will end on 31 December 2026, after it extended the end date by nine months from the original deadline of 31 March 2026. Suppliers may already have stopped taking new applications, so ask before you rely on it.
In its place, the government’s Warm Homes Plan, announced on 20 January 2026, commits £15 billion of public investment to upgrading homes. The government says low income households and those in fuel poverty could receive support covering the full cost of having insulation installed, and that homeowners will be able to apply for government backed low and zero interest loans, with more detail to follow. Note the word could: the announcement does not guarantee it for any particular household. You can read the government’s Warm Homes Plan announcement in full.
Check the scheme’s own page before you sign anything, because rules and eligibility change. Insulation can lower how much energy you use, but it will not change your tariff. Cavity wall insulation and a good deal work best together, and comparing energy deals takes a few minutes.
Frequently asked questions about cavity wall insulation
- How do I know if I have cavity walls?
- Houses built after the 1920s are likely to have them. Brick laid in an even lengthways pattern, or a brick wall over 260mm thick, suggests a cavity wall. Stone walls can be thick and still be solid. If you are unsure, ask a specialist installer or surveyor.
- How much does cavity wall insulation cost?
- Energy Saving Trust does not give a typical price, only that costs vary with the size of your home. Get at least two quotes from registered installers rather than relying on a national average.
- Can I fit cavity wall insulation myself?
- No. Energy Saving Trust says it is not a DIY job and you will need to employ a registered installer.
- Does cavity wall insulation always work?
- It does not suit every home. Exposure to extreme weather or flooding, poor condition, overheating risk and a cavity narrower than 50mm or blocked with rubble are all reasons it may not be suitable.
- I already have damp. Can I still insulate?
- Not yet. Energy Saving Trust says a wall with damp patches should not be insulated until the problem is resolved. Speak to a damp prevention specialist first.
- Who do I contact if damp appears after cavity wall insulation?
- Call the installer first. If they will not help or no longer exist, contact CIGA to check whether you are covered by its 25 year guarantee, or check for an insurance backed guarantee.
Sources
- Energy Saving Trust, Cavity wall insulation supports the wall type and age tests, the 260mm thickness guide, the one third heat loss figure, the five year payback, the 50mm minimum cavity, the registered installer requirement, the NIA, CIGA and BBA memberships, the 25 year guarantee and the advice on damp. It publishes no cost figures.
- GOV.UK, Extending the ECO4 end date: government response supports the 31 December 2026 end date and the nine month extension from 31 March 2026.
- GOV.UK, Warm Homes Plan announcement supports the 20 January 2026 date, the £15 billion of public investment, the support for low income households and the low and zero interest loans.
Checked 4 November 2026.
