Cavity wall insulation costs between £20 and £40 per square metre. That makes it the cheapest major retrofit.
If you have cavity walls, this is the most cost-effective insulation upgrade you can make. Cavity wall insulation typically costs £1,200–£2,800 for a semi-detached home, based on 30–70 m² of wall area (Energy Saving Trust, 2026). The per-square-metre figure is derived by dividing the total installed cost by the wall area, not the floor area.
Costs are lower for homes with easy access and standard brick/block cavity walls. They are higher for narrow cavities or walls with structural ties (MCS installer cost data, 2026). Payback is typically 2–5 years, depending on your current heating system and fuel costs (Energy Saving Trust payback calculator, 2026).
Solid wall insulation costs £80–£130 per square metre, with internal and external options differing by up to 50%.
Solid walls have no cavity to fill, so insulation must be added to the internal or external face. External wall insulation (EWI) costs £90–£130/m²; internal wall insulation (IWI) costs £60–£90/m² (DESNZ Home Insulation: Costs and Savings report, 2026). For a typical three-bedroom semi-detached with solid walls (approx. 60 m²), IWI costs £5,000–£9,000 and EWI costs £8,000–£15,000 (Energy Saving Trust, 2026).
EWI is more expensive but avoids losing internal floor space and reduces thermal bridging. IWI can reduce room size by 5–10 cm per wall. The U-value improvement is significant: from ~2.1 W/m²K (uninsulated solid wall) to ~0.3 W/m²K (insulated) (Building Regulations Approved Document L1A, 2026).
Loft insulation costs £15–£30 per square metre for DIY rolls, or £30–£50 per square metre for professional installation.
Loft insulation is the simplest and fastest insulation job. DIY mineral wool rolls (270 mm depth) cost £15–£25/m²; professional installation adds labour at £10–£20/m² (National Insulation Association, 2026). For a standard 50 m² loft, total cost is £750–£1,500 professionally installed (Energy Saving Trust, 2026).
Payback is 1–3 years, the shortest of any insulation type (Energy Saving Trust payback calculator, 2026). The U-value target is 0.16 W/m²K for 270 mm mineral wool (Building Regulations Approved Document L1A, 2026).
Quick numbers, insulation cost per square metre at a glance
| Insulation type | Typical cost per m² (low–high) | Typical total cost for a semi-detached home | Typical U-value after installation | Typical payback period |
|---|---|---|---|---|
| Cavity wall | £20–£40 | £1,200–£2,800 | 0.30 W/m²K | 2–5 years |
| Solid wall – EWI | £90–£130 | £8,000–£15,000 | 0.30 W/m²K | 10–20 years |
| Solid wall – IWI | £60–£90 | £5,000–£9,000 | 0.30 W/m²K | 10–20 years |
| Loft – DIY | £15–£25 | £750–£1,250 | 0.16 W/m²K | 1–3 years |
| Loft – professional | £30–£50 | £1,500–£2,500 | 0.16 W/m²K | 1–3 years |
| Floor – suspended timber | £40–£80 | £1,200–£2,400 | 0.25 W/m²K | 5–10 years |
| Floor – solid concrete | £50–£100 | £1,500–£3,000 | 0.25 W/m²K | 10–15 years |
Sources for all figures: Energy Saving Trust, DESNZ, NIA, MCS installer data (all 2026).
The cheapest insulation per square metre is cavity wall, but the biggest savings come from solid wall insulation.
Cavity wall insulation saves £200–£400 per year on heating bills for a semi-detached gas-heated home (Energy Saving Trust, 2026). Solid wall insulation saves £300–£600 per year, but costs 3–5 times more per square metre (Energy Saving Trust, 2026).
The best payback depends on your current wall type: cavity wall pays back in 2–5 years; solid wall in 10–20 years. Loft insulation saves £100–£200 per year for a typical home (Energy Saving Trust, 2026).
Insulation cost per square metre is the key number for comparing quotes and avoiding overpaying.
Most installers quote a total price, not a per-square-metre rate. Ask for the m² rate to compare like-for-like. The per-square-metre cost includes materials, labour, scaffolding (for EWI), and waste disposal (NIA, 2026).
A quote above £150/m² for solid wall or £50/m² for cavity wall should trigger a second opinion (Energy Saving Trust, 2026). VAT on insulation is 0% for installation in homes over two years old (GOV.UK, 2026).
how to compare insulation quotes from installers
The Great British Insulation Scheme closed on 31 March 2026. It funded insulation measures only and never covered windows or glazing.
The Great British Insulation Scheme closed on 31 March 2026. It funded insulation measures only and never covered windows or glazing. Installers must be MCS certified or TrustMark registered with a relevant certification body (MCS, 2026).
Without an MCS certificate, you cannot claim the GBIS grant (up to £1,500 for a semi-detached home) (DESNZ, 2026). Check the MCS installer database at mcs-certified.com before hiring (MCS website, 2026).
Frequently Asked Questions
Cavity wall insulation costs £20-£40 per square metre, solid wall insulation £80-£130, and loft insulation £15-£50 per square metre (Energy Saving Trust, 2026).
Yes, cavity wall insulation at £20-£40/m² is significantly cheaper than solid wall insulation at £80-£130/m², per Energy Saving Trust 2026 data.
External wall insulation costs £90-£130 per square metre installed, according to the DESNZ Home Insulation report (2026).
Professional loft insulation installation costs £30-£50 per square metre, including labour, per the National Insulation Association (2026).
Cavity wall insulation typically pays back in 2-5 years, depending on your heating system and fuel costs (Energy Saving Trust payback calculator, 2026).
