Cavity Wall Insulation

According to the Energy Saving Trust, cavity wall insulation typically costs between GBP 500 and GBP 1,500 depending on property size. A mid-terrace with two exposed walls sits at the bottom of that range; a large detached house with four walls and difficult access sits at the top. Grant funding may cover it for eligible households, though the route changed in March 2026 when the Great British Insulation Scheme closed.

Three checks, in ascending order of reliability. Build date: properties built between the 1920s and 1990s typically have cavity walls, and post-1990 homes almost always do. Brick pattern: if every brick on the outer face is laid lengthways in a uniform pattern, the wall is likely cavity construction, whereas a repeating pattern that includes shorter brick ends indicates solid brick. Wall thickness: measure at an opening such as a window reveal or the front door — a cavity wall is usually over 260mm thick, a solid brick wall around 220mm. Your EPC report also states the wall type, and a surveyor confirms it with a borescope.

Typically a few hours for a standard property, and almost always within a single day. The work is done entirely from outside, so nobody needs to move furniture or take time off beyond letting the installer reach all the external walls. Scaffolding is rarely needed — most installers work from ladders or a tower.

It can, in the wrong property, which is why the survey exists. The risk is that fill bridges the cavity and gives moisture a path from the wet outer leaf to the dry inner one. That is why exposed locations facing driving rain, walls with cracked render or failed pointing, and properties with existing damp are screened out before any work is approved. In a suitable, well-maintained wall, correctly installed insulation should not cause damp — and a surveyor refusing your property is the system working rather than failing.

Three materials are in common use. Blown mineral wool is the most widely installed and the cheapest. Polystyrene bead (EPS) flows more freely and is often preferred for narrow or partly obstructed cavities, and is bonded with adhesive so it does not slump. Polyurethane foam sets hard and is used where the wall also needs stabilising; it is the most expensive and by some distance the hardest to remove later. The survey determines which is appropriate — this is not usually a homeowner choice.

Installations carried out by a CIGA-registered installer carry an independent 25-year guarantee from the Cavity Insulation Guarantee Agency. The guarantee applies to registered installers only, so confirm registration before work starts rather than after. CIGA does not publish the full terms and exclusions on its website — request the guarantee document itself, and note CIGA's own position that condensation or mould does not necessarily indicate a fault with the insulation.

Raise it with the installer first, then with CIGA at concerns@ciga.co.uk or 01525 853300 if the installation was registered. Where insulation has genuinely failed or went into an unsuitable property, the remedy is extraction — removing the fill entirely, usually by vacuum through the original holes or newly drilled ones. Extraction costs more than the original installation did, because it is slower and generates waste, and the wall then needs its underlying defect fixed before refilling is even a question.

Possibly, depending on where you live and your household income. The Great British Insulation Scheme, which funded most cavity wall insulation in Great Britain, closed on 31 March 2026. In England the live route is the Warm Homes: Local Grant through your local authority, with ECO Flex as a discretionary route. Scotland runs Warmer Homes Scotland, Wales runs Nest, and Northern Ireland runs the Affordable Warmth Scheme and NISEP.

Usually. Uninsulated cavity walls are one of the heavier penalties in an EPC assessment, and filling them commonly moves the rating by one or two bands. The improvement only shows on your certificate once a new EPC is carried out — the existing certificate does not update itself, and the assessor needs evidence the work was done.

No. The equipment injects material under pressure into a sealed void, the fill has to be distributed evenly with no gaps or over-packing, and the result is invisible once the holes are sealed — there is no way to inspect your own work. It is also the one insulation measure where getting it wrong damages the building rather than merely wasting money. Every grant scheme requires a certified installer, and the CIGA guarantee is only available through registered ones.

Sometimes, though it is less common than with loft insulation. A borescope inspection shows whether the existing fill has slumped, was only partly installed, or has degraded. Where a cavity was filled decades ago and the material has settled, leaving the upper portion empty, a top-up may be possible. Where the original fill is intact there is nothing useful to add — the cavity is a fixed volume.

Yes — the construction underneath is what matters, not the finish. The practical difference is cosmetic: drill holes are sealed and colour-matched to mortar on exposed brickwork, but on rendered or pebbledashed walls a perfect match is harder and the patches may stay faintly visible. Ask the installer what the finish will look like before work starts. Cracked or failed render is a separate problem and should be repaired first, since it lets water into the wall.