How much does cavity wall insulation cost in the UK? (2026)
Verified UK cavity wall insulation costs in 2026 by property size, the ECO4 and GBIS grant landscape that often covers the work in full, the borescope check that decides suitability, the materials (bead, mineral wool, foam), and the scope gaps that catch homeowners out.
Cavity wall insulation in a typical UK 3-bed semi costs £450 to £950 non-grant in 2026, and often £0 under ECO4 or GBIS for qualifying households (Energy Saving Trust, Checkatrade, GOV.UK). The borescope survey before injection is what decides whether the job is right for your walls.
Quick answer
UK cavity wall insulation cost in 2026 by property size, non-grant: 1 to 2-bed terrace £370 to £600, 3-bed semi £450 to £950, 4-bed detached £700 to £1,500. Per m² of wall area: £15 to £25. Borescope survey: £100 to £200 (often free with a quote). Many qualifying households pay £0 under ECO4 or GBIS through approved installers. CIGA 25-year guarantee is standard from registered installers.
How to read this guide#
Two kinds of figures appear below:
- Headline price ranges (non-grant cost by property size, per-m² rate, survey fee): cross-referenced against Energy Saving Trust, Checkatrade, and GOV.UK grant guidance.
- Practical guidance (grant eligibility, materials, suitability, red flags): drawn from standard UK insulation industry practice and CIGA registered-installer norms.
Headline ranges (verified)#
Non-grant cost by property size#
| Property | Range |
|---|---|
| 1 to 2-bed terrace | £370 – £600 |
| Small 3-bed terrace | £400 – £800 |
| 3-bed semi-detached | £450 – £950 |
| 4-bed semi or small detached | £600 – £1,200 |
| 4-bed detached | £700 – £1,500 |
| 5-bed detached | £1,000 – £1,800+ |
Per-m² rate#
| Wall type | Range per m² |
|---|---|
| Standard masonry cavity | £15 – £25 |
These figures cover injection through small drilled holes in the mortar joints, filling the cavity with the chosen material, and making good the drill holes with matching mortar. They assume a straightforward survey result.
Survey and assessment#
| Item | Range |
|---|---|
| Borescope cavity check | £100 – £200 (often free with quote) |
| Damp check before install | £100 – £250 (separate trade) |
| EPC if needed for grant eligibility | £60 – £120 |
Grants (where eligible)#
| Scheme | Coverage |
|---|---|
| ECO4 (Energy Company Obligation) | Often 100% for qualifying households |
| GBIS (Great British Insulation Scheme) | Often 100% or heavily subsidised |
Practical guidance (industry standard)#
The grant landscape, in plain terms#
Most UK households who pay for cavity wall insulation in 2026 either do not qualify for ECO4 or GBIS, or have not checked. The schemes are worth a quick look before paying:
- ECO4 is funded by the major energy suppliers and administered by Ofgem. Eligibility is mostly means-tested (qualifying benefits) or based on an EPC rating of D to G in poorly performing properties. Apply through GOV.UK or an approved installer.
- GBIS is a wider scheme aimed at less well-insulated owner-occupied and privately rented homes in council tax bands A to D in England (A to E in Scotland and Wales). Less means-tested than ECO4 and easier to qualify for.
Both schemes pay the installer directly. You do not advance the cost and reclaim it. An installer offering "free cavity wall insulation" is using a grant; ask which scheme so you can verify eligibility on GOV.UK rather than rely on the installer's word.
Materials, in plain terms#
Three common UK fills, each with a trade-off:
- Mineral wool (rock or glass fibre): cheapest, good thermal performance, blown in dry. Sensitive to wet walls because it holds moisture if water tracks into the cavity. Best on sheltered locations and walls in good condition.
- Bonded polystyrene bead (EPS): the most popular UK fill in 2026. Drains rather than absorbs, which suits exposed locations. Slightly better thermal performance than mineral wool. Mid-priced.
- Polyurethane foam: the highest performance, particularly on narrow cavities or walls with poor air-tightness. Most expensive, needs specialist install, and some installers do not offer it.
The installer should recommend the material after the borescope survey, based on wall exposure, cavity width, and any debris found.
When the borescope check matters#
The borescope is the single most important step in a cavity wall insulation job. Skipping it is the most common cause of a job that should not have been done. It checks:
- Whether the wall actually has a cavity (some walls that look like cavity walls are solid masonry, particularly pre-1920 properties).
- Cavity width (under 40mm, the work is borderline; under 25mm, often refused).
- Rubble, brick fragments, or debris that would block the fill or bridge to the inner leaf and transmit damp.
- Existing damp or moisture in the cavity itself.
A registered installer (CIGA-affiliated) will refuse the job if the borescope shows any of these. A non-registered installer might proceed anyway, which is when you end up with the headlines about cavity insulation causing damp.
CIGA: the 25-year guarantee#
The Cavity Insulation Guarantee Agency (CIGA) is the industry body that backs cavity wall insulation work with an insurance-backed 25-year guarantee. The guarantee covers the materials and the installation. CIGA-registered installers should be the default choice; the certificate transfers with the property and matters when you sell.
A non-CIGA installer might be £100 to £200 cheaper but leaves you with no recourse if the work causes damp or fails. The guarantee is worth more than the saving.
When cavity wall insulation is a bad idea#
The Energy Saving Trust and CIGA both flag a few cases where the work should not be done:
- Severely exposed locations, particularly the west coast of the UK, Northern Ireland, and parts of west Wales and west Scotland where driving rain regularly saturates the outer leaf. Map zones are published; a competent surveyor knows the local exposure rating.
- Existing damp: penetrating damp through the outer leaf, rising damp at floor level, or persistent condensation. Insulation traps the moisture, makes the inner wall colder, and worsens the problem.
- Cracked or failing mortar joints that admit water. Repoint first, then insulate.
- Timber-framed houses with brick outer leaf: the cavity is designed to drain. Filling it can cause the timber frame to rot.
- Cavity widths under 25mm, where the fill cannot reach the back of the cavity reliably.
A fair survey identifies any of these and reports honestly. A quote that ignores them in pursuit of a sale is the more common version of the failure pattern.
Regional variation#
Material costs are nationally priced. Labour and survey fees vary slightly:
- Inner London: ~10 to 20% above national
- Outer London / South-East: ~5 to 15% above
- Midlands and East: close to national
- North of England, Wales: ~5 to 10% below
- Northern Ireland, rural Scotland: ~10 to 15% below
The grant-funded versions have no regional variation because the installer is paid by the scheme at a national rate.
Red flags in cavity wall insulation quotes#
Beyond standard quote red flags (covered separately), some are insulation-specific:
No borescope check. The single most important step has been skipped. Either the installer is rushing or is not equipped.
Not CIGA-registered. No 25-year insurance-backed guarantee. The saving is small; the risk is not.
Material not named. "Cavity wall insulation" without bead, mineral wool, or foam specified leaves room to substitute downwards.
No grant offered on a property that obviously qualifies. A 1990s ex-council 3-bed in a band-A council-tax area is almost certainly GBIS-eligible. An installer quoting full price without mentioning the scheme is either uninformed or hoping you will not check.
No exposure assessment on a coastal or west-facing property.
Quote drops if you "pay cash". Cash-only avoids the paper trail. The CIGA certificate needs the paperwork, so cash payments rarely come with one.
No making-good of the drill holes. A proper finish matches the mortar colour and is barely visible. A quote that does not mention making good is leaving you with 30 to 50 visible drill marks.
Sequence of work on a typical install#
- Survey. Installer measures the walls, checks exposure, drills a small inspection hole, runs the borescope, checks cavity width and condition.
- Material recommendation. Based on the survey, the installer recommends bead, mineral wool, or foam.
- Grant check. If applicable, the installer confirms eligibility under ECO4 or GBIS and handles the paperwork.
- Drilling. Small holes (around 22mm) are drilled through the mortar joints in a pattern across the wall, usually one per square metre.
- Injection. The chosen material is blown or pumped into the cavity through the holes.
- Making good. The drill holes are filled with matching mortar.
- Certification. The installer issues a Building Regs compliance certificate and registers the work with CIGA. The CIGA certificate arrives by post within a few weeks.
A typical job takes half a day on site, one day for a larger detached.
Comparing your cavity wall insulation quote#
The reliable way to know if a cavity wall insulation quote is fair is to check it against the bands above and the grant landscape. The easier way is to paste or upload your quote into Check the Quote, where we check every line against current UK rates, verify the installer is CIGA-registered, flag grant eligibility you may not have been told about, and tell you what is missing. Your first check is free. For loft insulation and external wall insulation, see the related guides.
Got a quote you want checked?
Paste any UK contractor quote and Check the Quote compares every line item against current market rates, flags missing scope, and runs a Companies House check on the contractor. Free on your first project.
Frequently asked questions
- How much does cavity wall insulation cost in the UK in 2026?
- A typical 3-bed semi costs £450 to £950 non-grant in 2026 (Checkatrade, Energy Saving Trust). A small 1 or 2-bed terrace runs £370 to £600. A 4-bed detached runs £700 to £1,500. Per m² of wall area, the rate is roughly £15 to £25 supplied and installed. Many qualifying households pay nothing under the ECO4 or GBIS schemes; non-grant homeowners pay the bands above.
- Can I get cavity wall insulation free under ECO4 or GBIS?
- Often, yes. ECO4 fully funds cavity wall insulation for households on qualifying benefits or in EPC band D to G properties through approved installers. The Great British Insulation Scheme (GBIS) targets band D to G homes in council tax bands A to D and is usually free or heavily subsidised. Apply through GOV.UK or a registered installer; the installer handles the paperwork. Non-qualifying households pay the full price.
- What is the borescope check before installation?
- A borescope is a small camera the installer pushes into a drilled hole to inspect the cavity. It confirms the wall actually has a cavity (some "cavity" walls turn out to be solid), measures the cavity width, and checks for rubble, debris, or damp that would make injection a bad idea. The check is £100 to £200 if paid separately, often free as part of a quote visit. A quote with no survey or borescope mentioned is skipping the only step that prevents an expensive mistake.
- Which insulation material is best for my walls?
- Three common UK materials: mineral wool (cheapest, good thermal performance, sensitive to wet walls), bonded polystyrene bead (most popular, drains better, suits exposed locations), and polyurethane foam (highest performance, most expensive, needs specialist install). The installer should recommend a material based on your wall type, exposure, and the borescope findings. A quote that does not name the material is leaving room to substitute downwards.
- When is cavity wall insulation a bad idea?
- Three cases. First, severely exposed locations on the west coast where driving rain saturates the outer leaf; the insulation can transmit damp to the inner wall. Second, properties with existing damp issues (penetrating damp, rising damp, or condensation) that should be fixed first. Third, walls with debris in the cavity, which most CIGA-registered installers will refuse to fill. The borescope and survey identify all three.
- What is normally left out of a cavity wall insulation quote?
- Common gaps: the post-installation guarantee (CIGA insurance is standard from registered installers; check it is included), making good the drilled holes externally (usually included but worth confirming), removal of pebbledash or render to access the brickwork (a major extra), repointing of the brickwork where mortar joints are failing, and post-install ventilation upgrades if the property has condensation issues. A good installer flags all of these in the survey.