The Short Answer
The Energy Saving Trust puts a typical installation at around £2,200 in Great Britain and around £1,000 in Northern Ireland, and says it will likely pay back in 5 years or less. DESNZ's measured figures are less rosy: gas use fell by a median 12% in the year after cavity insulation, which for a semi on today's gas price is about £101 a year, or roughly 22 years to pay back £2,200.
On the measured figures that's a long payback, so in our view it's best value when a grant pays. It's still worth considering on a suitable wall: it's a quick job (the EST says about 2 hours on an easy house), the house feels warmer, and CIGA guarantees work by registered installers for 25 years. Get a proper survey first: exposed, damp or narrow cavities are where it goes wrong.
Can Your Walls Be Filled?
Cavity wall checker
Can your walls be filled, and what will it cost?
Roughly when was the house built?
How It Works
Many homes built after the 1920s have two layers of brick or block with a gap between them. An installer drills small holes in the outside wall, blows insulation into the gap and fills the holes. The EST says that for an average home with easy-to-reach walls it takes about 2 hours and shouldn't make any mess. By its research about 33% of the heat lost from an uninsulated home goes through the walls.
Do You Have Cavity Walls?
Measure the wall at an open door or window, from inside to outside. The EST says a brick wall 260mm to 350mm thick is likely to be a cavity wall; narrower is probably solid. Older houses are more likely to have solid walls, and houses built from the 1990s generally have wall insulation already. Your EPC usually says what the walls are and whether they're insulated. Solid walls need different insulation: see our solid wall guide.
What It Costs and Saves
| Home | Cost (EST, previously published) | Saving a year (DESNZ measured) | Payback |
|---|---|---|---|
| Flat | £950 | £57 | about 17 years |
| Mid-terrace | £1,100 | £87 | about 13 years |
| Bungalow | £1,700 | £98 | about 17 years |
| Semi-detached | £2,200 | £101 | about 22 years |
| Detached | £3,900 | £132 | about 30 years |
Costs are the Energy Saving Trust's earlier figures by type of home in Great Britain (its page now gives only the £2,200 typical cost). Savings are DESNZ's median 12% cut in gas use, measured in homes in England and Wales in the year after insulation, applied to each type of home's typical gas use at 7.97p a kWh. In a quarter of homes, gas use didn't fall against similar homes; DESNZ notes some people take the benefit as a warmer home rather than a lower bill. So the EST's 5 years is the hopeful end, and our table the cautious one.
When It Isn't Suitable
The EST says cavity insulation might not be suitable if your home is:
- exposed to extreme weather or flooding
- in poor condition
- at risk of rooms overheating
- built with narrow cavities, under 50mm (usually pre-1920s homes)
Damaged brickwork or pointing, roofs and overhangs, and gutters and downpipes need fixing first, or you might trap damp or the insulation may fail. A good installer checks all this and shouldn't block airbricks, vents or other intentional ventilation. Timber-frame, steel-frame and prefabricated concrete homes need a specialist assessment.
Damp and the CIGA Guarantee
The EST says cavity insulation shouldn't cause damp if installed correctly. When fills do go wrong it's usually rain getting across a filled cavity on an exposed or poorly maintained wall, showing as damp patches on the inside of outside walls. If that happens: fix gutters and pointing, find your guarantee, and contact the installer. The Cavity Insulation Guarantee Agency provides independent 25-year guarantees for cavity insulation fitted by registered installers in the UK and the Channel Islands, and you can buy a duplicate if you've lost yours. If the installer won't help, contact CIGA. Don't let anyone extract a fill under a "free survey" before checking whether the guarantee covers it.
Grants
- ECO4 (England, Scotland, Wales) can fully fund insulation for households on qualifying benefits, until 31 December 2026.
- England: the Warm Homes: Local Grant, for household incomes usually £36,000 or less in homes rated EPC D, E, F or G.
- Scotland: Home Energy Scotland's grant covers cavity insulation (up to £1,500 for loft, cavity or floor).
- Wales: Nest, for eligible households.
- Northern Ireland: Affordable Warmth (income under £23,000) and NISEP.
The Great British Insulation Scheme closed on 31 March 2026: anyone cold-calling with "GBIS funding" is out of date at best. Our insulation costs guide and the grant pages have the detail.
What You Don't Need
- A quote over the phone, in our view: insist on a survey of the walls first.
- A second fill in a cavity that's already been done.
More: insulation savings calculator | insulating before a heat pump
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