Cavity Wall Insulation — Keep Heat From Escaping Through Your Walls
Many homes built after the 1920s have cavity walls: two layers of masonry with a gap between them. Filling that gap with insulation reduces the heat lost through the walls.
Around a third of the heat lost from an uninsulated home escapes through the walls. Houses built from the 1990s onwards usually already have wall insulation, but many older cavity-wall homes do not. Where a wall is suitable, filling the cavity is usually quick and done from outside — but suitability matters, because the wrong wall or a poor installation can let in damp.
- Typical yearly saving
- £110–£380
- Typical unsubsidised cost
- £950–£3,900
- Guarantee to ask for (CIGA or insurance-backed)
- 25 years
- Heat lost through uninsulated walls
- ~33%
What Is Cavity Wall?
An installer drills small holes in the outside wall, blows or injects insulation — such as mineral wool or polystyrene beads — into the cavity, then fills the holes. Before any work, the installer should survey the walls to confirm the cavity is suitable.
How It Works
Survey
The installer checks the cavity width and condition, the brickwork, exposure to wind-driven rain and flooding, and any existing damp.
Drilling
Small holes are drilled in a pattern across the outside walls.
Filling
Insulation is blown or injected into the cavity until it is filled.
Making Good
The holes are filled to match the wall, and you should receive guarantee paperwork.
Is It Right for My Home?
If your house was built after the 1920s, its external walls are probably cavity walls. The Energy Saving Trust says standard cavity wall insulation may not be suitable if the walls are exposed to driving rain or extreme weather, the home is at risk of flooding, the cavity is narrower than 50mm, or there is existing damp. Solid-walled homes (mostly built before the 1920s) need solid wall insulation instead.
Key Benefits
Lower Heating Bills
Around £110 a year for a mid-terrace house and £380 for a detached house, according to the Energy Saving Trust.
Done From Outside
The work is carried out from outside, so there is little disruption indoors.
Fewer Cold Spots
Warmer walls mean more even temperatures in rooms against outside walls.
Long Guarantee
Work should be guaranteed for 25 years by the Cavity Insulation Guarantee Agency (CIGA) or an independent insurance-backed guarantee.
Lower Carbon Emissions
Less heating demand means lower emissions from your home.
Better Energy Rating
Insulated walls can improve your home's Energy Performance Certificate rating.
Ask for a Free Cavity Wall Quote
Typical Costs and Savings
Typical figures to help you compare, not a quote for your home. Your costs and savings depend on the property, how you heat it and how you use energy.
| Example | Typical Cost | Typical Saving a Year |
|---|---|---|
| Detached house | £3,900 | £380 |
| Semi-detached house | £2,200 | £200 |
| Mid-terrace house | £1,100 | £110 |
| Detached bungalow | £1,700 | £160 |
| Mid-floor flat | £950 | £110 |
Energy Saving Trust estimates for a gas-heated home, based on fuel prices as of July 2026. Installation costs are unsubsidised averages. Source: Energy Saving Trust — cavity wall insulation. Last reviewed September 2026.
Help With the Cost
Funding depends on where you live, your income and your home, and schemes change. We are not a government body — check the official scheme before relying on it.
Warm Homes: Local Grant (England)
Council-run funding for wall, loft and floor insulation in lower-income households living in homes rated EPC D–G.
Read more on our grants guide →0% VAT until 31 March 2027
Professionally installed insulation in a home is zero-rated for VAT until 31 March 2027.
Read more on our grants guide →Frequently Asked Questions
Your next step
Thinking about Cavity Wall Insulation?
It is free to ask and there is no obligation.