If your home has empty cavity walls, filling them is a substantial gain on an EPC, and the Energy Saving Trust puts the typical cost at around £2,200. Two things decide whether you get the benefit: whether your walls are suitable, and whether the assessor can see or prove the cavity is filled.
Do you have cavity walls?
Walls matter: in an uninsulated home about a third of the heat lost goes out through them. Three clues tell you what kind you have.
- Age. Homes built after the 1920s probably have cavity walls. Older homes are more likely to have solid walls, and the few with cavities usually have ones too narrow to fill. Homes built from the 1990s on generally have wall insulation already.
- The brick pattern. If every brick is laid lengthways, it is probably a cavity wall. If the short ends of some bricks show in a repeating pattern, it is probably solid.
- The thickness. Measure at a window or door: a brick wall 260mm to 350mm thick is likely to be a cavity wall, and a thinner one solid.
If your walls are rendered or clad, none of this shows from outside, and it is worth asking an installer or surveyor.
Is yours already filled?
It is easy not to know: the insulation may have gone in before you bought the home. A row of small filled holes, roughly a metre apart in the outside walls, is a sign it has been done. An installer can confirm it with a borescope inspection through a small hole, and your council’s building control may hold a record. Your current EPC may say too.
How it shows on your EPC
The assessment records how the walls are built and how they are insulated. The catch is evidence: if insulation inside a cavity cannot be seen or proved, the method records the worst reasonable case for the home’s age and construction. A cavity filled a decade ago with no paperwork can go down as empty.
So the paperwork is worth as much as the work. Keep the installer’s certificate and the guarantee, and have them ready for the visit. Since June 2025 the method can also use the actual thickness of wall insulation, and how well the material insulates, where documents show it. Preparing for your EPC lists the rest of the paperwork that counts.
Is your home suitable?
According to the Energy Saving Trust, standard cavity wall insulation suits a home where:
- the outside walls are unfilled cavity walls;
- the cavity is at least 50mm wide and clear of rubble;
- the brickwork or masonry is in good condition;
- the walls are not exposed to driving rain; and
- the home is not at risk of flooding.
It is not recommended for narrow or uneven cavities, exposed sites or homes at risk of flooding, because it can lead to damp. Repairs to brickwork, pointing, gutters and roofs should be done first, and any damp patches inside sorted before anything is injected. In a flat, the whole block has to be done, with everyone’s agreement. In a terraced or semi-detached house, the installer has to fit a barrier so the insulation stays on your side of the wall.
Cost, installation and guarantees
| Typical cost in Great Britain | Around £2,200 |
|---|---|
| Paying for itself | Likely in five years or less, through lower bills |
| How long the work takes | About two hours for an average home with walls that are easy to reach |
| How it is done | Holes of about 22mm, roughly a metre apart, insulation blown in, usually polystyrene beads, and the holes filled |
| Guarantee to ask for | 25 years from CIGA, or an independent insurance-backed guarantee |
Source: Energy Saving Trust, “Cavity wall insulation”, last updated 8 May 2026. Use a TrustMark registered installer, who will survey the walls first.
Before paying in full, check what help is available. The Energy Saving Trust notes that the government’s Warm Homes Plan is set to offer fully funded insulation for people on low incomes, and low and zero-interest loans for homeowners.
If your walls are solid
Solid walls cannot be filled. Solid wall insulation is a separate job, and it often appears in EPC recommendations even where it would make little sense, because the list is generated from the assessment data. For landlords, the government plans an exemption under the 2030 standard for choosing not to fit solid wall insulation where it is the only measure left; EPC C by 2030 has the details.
Common questions
Does cavity wall insulation improve an EPC rating?
Yes, where the cavities were empty it is a substantial gain. The assessor has to be able to see or prove the cavity is filled, so keep the certificate and guarantee.
How do I know if I already have cavity wall insulation?
Look for small filled holes about a metre apart in the outside walls, check your EPC, ask your council’s building control, or have an installer look with a borescope.
How much does cavity wall insulation cost?
Around £2,200 for a typical home in Great Britain, according to the Energy Saving Trust, which expects it to pay for itself in five years or less.
Can I get cavity wall insulation in a flat?
Not for one flat on its own. The whole building has to be insulated, with the agreement of the owner and the other flats.
What paperwork proves my cavity walls are insulated?
The installer’s certificate and the guarantee, such as a CIGA guarantee. Have them ready for the EPC visit.
Where to go next
Loft insulation and your EPC, how to improve your EPC rating, and an Improvement Plan for your home.