Cavity wall insulation typically costs £1,500 to £3,500 for a 3 bed semi, £1,000 to £2,200 for a mid terrace and £2,500 to £5,000 for a detached house, which works out at about £18 to £30 per m² of wall. It is the cheapest wall insulation there is, charged at 0% VAT, and it comes with a 25 year CIGA guarantee. The catch is that not every cavity should be filled, and removing a failed fill costs more than putting it in.
Key takeaways
- Indicative cost: mid terrace £1,000 to £2,200; 3 bed semi £1,500 to £3,500; detached £2,500 to £5,000; about £18 to £30 per m².
- Installation is 0% VAT until 31 March 2027 as an energy-saving material.
- Fill comes with a 25 year CIGA guarantee when installed by a registered installer.
- Bonded polystyrene bead is the usual choice for exposed walls and narrow cavities; blown mineral wool is the older, cheaper method.
- Do not fill a cavity that is narrower than about 50 mm, damp, exposed to driving rain without render, or in a timber or steel framed house.
- Extracting failed insulation costs £20 to £40 per m², £1,500 to £4,000 for a semi, and is 20% VAT.
How much does cavity wall insulation cost?
Cavity wall insulation is priced on the area of external wall, so the shape of the house matters more than the number of bedrooms. A mid terrace has two outside walls and is cheap; a detached house has four and costs the most. The ranges below are indicative London and South-East prices for a standard fill, including the survey, drilling, filling, making good the drill holes and the CIGA guarantee.
| House type | Indicative cost | Why |
|---|---|---|
| Mid terrace | £1,000 to £2,200 | Front and back walls only |
| End terrace or 3 bed semi | £1,500 to £3,500 | Three exposed walls |
| Detached | £2,500 to £5,000 | Four walls, often larger |
| Flat | Usually the whole block | Cavities run past flat boundaries, so fill is done building-wide |
| Rate per m² of wall | £18 to £30 | Bead at the upper end, mineral wool at the lower |
For a quick estimate, measure the outside walls (length times height, minus windows and doors) and multiply by £25. A semi with about 90 m² of wall comes out at roughly £2,250. Compare that with external wall insulation at £110 to £140 per m², which is what solid walls need. If you are not sure which wall type you have, read cavity wall vs solid wall first.
Bead or mineral wool: which should I have?
Two materials cover almost every job. Both are blown into the cavity through holes drilled in the mortar joints, roughly one metre apart, and the holes are filled afterwards.
Bonded polystyrene bead
Small expanded polystyrene beads are blown in with a binding adhesive so they set into a stable mass. Bead flows around wall ties and debris, fills narrow cavities, does not hold water and does not slump, and it can be extracted cleanly if that is ever needed. It is the material we would specify for most homes in London and the South-East, at the upper end of the price range.
Blown mineral wool
Loose mineral fibre blown in under pressure. It is cheaper and has been used for decades, but it can hold moisture if the outer wall lets rain through, and it can settle and leave cold gaps near the top of walls. It is fine in sheltered locations with sound brickwork. Most of the failed cavity insulation that gets extracted is old mineral wool or urea formaldehyde foam.
When should a cavity not be filled?
Filled cavities go wrong when the wall was never suitable. A registered installer should refuse the job in these cases, and the CIGA guarantee depends on the wall having been assessed properly.
- Narrow cavities. Below about 50 mm the fill cannot be blown evenly and bridges form. Many 1920s and 1930s homes have cavities of this size.
- Exposed walls. Walls facing driving rain on high ground or open sites, without render or cladding, can let water cross the fill to the inner leaf. The insulation then acts as a bridge for damp.
- Existing damp, poor brickwork or debris in the cavity. Cracked render, failed pointing, leaking gutters and rubble at the base of the cavity must be dealt with first. Filling a wet wall makes it worse.
- Timber or steel framed houses. The cavity is there to keep the frame dry. Never fill it.
- Already filled. Homes built after about 1990 usually had insulation built in. A borescope check takes minutes.
Where a cavity is unsuitable, the wall can still be insulated. Internal wall insulation or external wall insulation over the outer leaf both work on a cavity wall; see cavity wall insulation problems for what goes wrong and the alternatives.
What does it cost to remove failed cavity wall insulation?
If a past fill has gone wrong, with damp patches on the inner face, cold spots or a wall that never dries, the answer is extraction: bricks are removed at intervals, the old material is sucked or raked out, the cavity is cleared and checked, and the bricks are replaced. Indicative cost is £20 to £40 per m², or £1,500 to £4,000 for a 3 bed semi. Extraction is not an energy-saving material, so it is charged at 20% VAT.
The wall then needs to dry out before any refill, which can take a season. Many owners refill with bead once the cause of the failure has been dealt with; others leave the cavity clear and insulate externally instead. See the cavity wall extraction service page for the process.
VAT, the CIGA guarantee and grants
Cavity wall insulation is an energy-saving material under HMRC VAT Notice 708/6, so installation is 0% VAT until 31 March 2027 and 5% after that. Extraction and any brickwork or render repairs are 20%.
A fill carried out by a CIGA registered installer comes with a 25 year independent guarantee covering the material and workmanship. Keep the certificate; a buyer's solicitor will ask for it. Check any quote states CIGA, the installer's PAS 2030 cavity wall certification and TrustMark registration.
There is no grant for most homeowners. GBIS closed on 31 March 2026 and ECO4 ends on 31 December 2026 with no successor. Low income owner occupiers in EPC D to G homes can apply to their council under the Warm Homes: Local Grant until March 2028, and the council appoints the installer. Everyone else pays privately at 0% VAT. Our insulation grants guide has the full picture.
How Stay Warm handles cavity walls
Our own team installs external and internal wall, loft and floor insulation under PAS 2030:2023. Cavity wall fill and extraction are arranged and project-managed by Stay Warm and carried out by accredited installers holding PAS 2030 cavity wall certification and TrustMark registration, with the CIGA guarantee on the fill.
- Free survey, including a borescope check of the cavity, the wall condition and exposure.
- A straight answer: fill, do not fill, or insulate a different way. Where the cavity is unsuitable we quote solid wall insulation from our own team instead.
- Written quote with the material, the m² rate and the guarantee stated.
- Installation, usually one day for a semi, from outside.
- CIGA certificate and the installer's paperwork sent to you for the deeds.
See the cavity wall insulation service page or book a free heat loss survey and we will check the walls alongside the loft, floor and windows.
Frequently asked questions
How much does cavity wall insulation cost for a 3 bed semi?
Indicatively £1,500 to £3,500 including 0% VAT, the survey and the CIGA guarantee. A mid terrace is £1,000 to £2,200 and a detached house £2,500 to £5,000. The per m² rate is about £18 to £30.
Is cavity wall insulation 0% VAT?
Yes. Cavity wall insulation is an energy-saving material under HMRC VAT Notice 708/6, so installation is 0% VAT until 31 March 2027, then 5%. Extraction of failed insulation is 20%.
How do I know if my house has a cavity wall?
Most homes built between the 1920s and 1990s have cavity walls. Look at the outside brick pattern: if every brick is laid lengthways (stretcher bond) the wall is probably a cavity; if you see the short ends of bricks in a regular pattern it is probably solid. A wall thicker than about 250 mm at a door or window is another sign. A borescope survey confirms it.
Is bead better than mineral wool for cavity walls?
For most homes, yes. Bonded polystyrene bead does not hold water, does not slump and flows around obstructions and into narrow cavities. Mineral wool is cheaper and adequate in sheltered locations with sound brickwork.
Can cavity wall insulation cause damp?
It can if the wall was unsuitable: too exposed, narrow, full of debris or already damp. That is why the survey matters and why a registered installer should refuse an unsuitable wall. Properly assessed and installed fill does not cause damp.
How much does it cost to remove cavity wall insulation?
Indicatively £20 to £40 per m², or £1,500 to £4,000 for a 3 bed semi, at 20% VAT. The wall needs to dry before any refill.