Around a third of UK homes — most built before 1920, plus many post-war concrete-frame and steel-frame properties — have solid walls. That means no cavity to fill. Cavity wall insulation, the cheap and quick fix that works for most houses built after 1930, simply isn’t an option. For solid-wall homes, external wall insulation (EWI) or internal wall insulation (IWI) is the main route to meaningful heat retention improvement.
EWI is more disruptive and more expensive than cavity fill. It’s also considerably more effective, and in 2026 there are grants that can cover part or all of the cost if you qualify. Here’s a clear-eyed look at the numbers.
What external wall insulation actually is
EWI involves fixing insulation boards directly to the outside of your walls, then applying render, cladding, or a specialist finish over the top. The result is a permanently warmer, draughtier-free wall with a noticeably improved U-value (the measure of how readily heat passes through a building element — lower is better).
A standard solid brick wall has a U-value of around 2.0 W/m²K. With 100mm of EPS (expanded polystyrene) insulation — the most common choice — that drops to roughly 0.28 W/m²K, just inside the building regulations target for new builds. Mineral wool boards achieve similar results with better fire performance; phenolic foam boards can reach the same U-value with 60–70mm thickness, which matters on narrow-frontage terraces where width is constrained.
The insulation layer is finished with a thin-coat render system, brick-slip cladding (which mimics the original masonry), or occasionally timber or metal cladding. A good brick-slip finish is nearly indistinguishable from the original at normal viewing distances.
What it costs
EWI is priced per square metre of external wall, excluding windows and doors. The market rate in 2026 runs from around £80 to £150 per m² installed, depending on:
- Insulation type: EPS is cheapest, phenolic foam most expensive
- Finish: thin-coat render is more affordable than brick-slip or decorative cladding
- Access: scaffolding for a two-storey property adds £1,500–3,500
- Property condition: repairing cracked render or repointing before installation adds cost
- Location: London and South East typically run 15–25% above the national average
For a typical 1930s semi-detached with around 70m² of external wall area:
| Specification | Cost range |
|---|---|
| EPS 90mm + thin render | £7,000–£9,000 |
| Mineral wool 100mm + render | £8,500–£11,000 |
| Phenolic foam 70mm + brick slip | £11,000–£16,000 |
These include scaffolding, insulation, render/finish, window reveals, soffits, and making good. They do not include redecorating, replacing fascias/gutters disturbed in the process, or any associated planning costs.
Planning permission
EWI changes the external appearance of a building and can add 80–150mm to the wall thickness, which slightly reduces the footprint of the property. For most detached and semi-detached houses this falls within permitted development rights, but there are exceptions:
- Conservation areas: EWI is almost always restricted; you’ll typically need planning permission and may be refused
- Listed buildings: EWI is rarely permitted; IWI is usually the only option
- Terraced houses: if you’re on a corner plot, the side elevation may face a public road, which can trigger planning requirements
- Flats and maisonettes: usually need planning permission regardless
Check with your local authority before committing. Most EWI contractors will advise on this, but the planning risk sits with the homeowner.
What does it do for your energy bills?
The Energy Saving Trust estimates solid-wall insulation saves the average household around £280–440 per year on gas heating bills, depending on property size and heating habits. With energy prices elevated through 2025–2026, the higher end of this range is more realistic for larger homes.
In terms of EPC impact: a typical solid-wall home rated E or F will jump to C or D after EWI installation. This matters if you’re subject to the government’s proposed minimum EPC C standard for rented properties, currently scheduled to apply to new tenancies from 2028.
U-value improvement also has a secondary benefit that bills don’t capture: the internal surface of a solid wall stays warmer, which reduces condensation and significantly improves perceived comfort. Many EWI installers report that homeowner satisfaction relates as much to the elimination of damp and cold spots as to the bill reduction.
Available grants in 2026
Two main schemes can fund EWI for eligible households:
ECO4 (Energy Company Obligation)
ECO4 is administered through large energy suppliers, who are obligated by Ofgem to deliver energy efficiency upgrades to low-income and fuel-poor households. For owner-occupiers in properties rated EPC D, E, F, or G:
- Income criteria: households receiving means-tested benefits (Universal Credit, Pension Credit, Working Tax Credit, Child Tax Credit, Income Support, income-related ESA/JSA) qualify automatically
- Local Authority Flex: councils can refer households in low-income areas without needing to meet the benefit criteria — worth checking with your council if you’re near a qualifying area
- Coverage: ECO4 can fund EWI fully for qualifying households, with no cap on what it pays towards the insulation
The ECO4 scheme runs through 2026. The successor ECO5 scheme is expected to launch in 2027 with similar but potentially expanded eligibility criteria.
To apply, contact a Trustmark- and PAS 2030-certified installer directly, or use the Simple Energy Advice helpline (0800 444 202) to be matched with an eligible installer.
Great British Insulation Scheme (GBIS)
The GBIS targets households in EPC D–G properties and has two routes:
Group A (lower income): If you receive means-tested benefits, GBIS can fully fund EWI. This group overlaps significantly with ECO4, and households may be assessed for both to determine which delivers the better outcome.
Group B (council tax bands A–D): If you’re in a lower council tax band but don’t receive benefits, GBIS offers a partial contribution — currently covering 40–50% of installation costs for EWI on eligible properties. The remainder falls to the homeowner.
GBIS has a lower remaining budget than ECO4 and is delivering at slower pace than originally announced. Apply via Simple Energy Advice or directly through an approved installer; demand significantly exceeds supply in most regions.
Warm Homes: Local Grant
From late 2025, the government’s Warm Homes: Local Grant has been deploying through local authorities, targeting owner-occupiers and private renters in EPC D–G properties with household incomes below £36,000. This can fund EWI fully for qualifying households. Check your council’s website for local delivery timelines — availability varies significantly by region.
The payback calculation for self-funding households
If you don’t qualify for any grant funding, EWI is a long-payback investment. At current energy prices:
| Property type | Installation cost | Annual saving | Payback period |
|---|---|---|---|
| 2-bed terrace | £7,000–9,000 | £180–250 | 30–45 years |
| 3-bed semi-detached | £9,000–12,000 | £260–370 | 30–40 years |
| 4-bed detached | £14,000–20,000 | £400–550 | 30–40 years |
These payback periods are long — longer than for heat pumps or solar, and much longer than for loft insulation. The financial case for self-funding EWI is most compelling if:
- You plan to stay in the property for 15+ years
- You’re doing other external works (render repair, reclad) where EWI can be added for marginal extra cost
- The EPC improvement is strategically important (rental compliance, mortgage product, resale value)
- The thermal comfort improvement matters significantly for your health or quality of life
EWI vs internal wall insulation
Where EWI is impossible (conservation areas, listed buildings, some flats), internal wall insulation (IWI) is the alternative. IWI involves dry-lining the inside of external walls with insulated plasterboard or a frame filled with mineral wool.
IWI costs less per m² (typically £50–90/m²), doesn’t require planning permission or scaffolding, and can be done room by room. The downsides: it reduces floor area (typically 100–120mm per wall), requires moving skirting boards, radiators, plug sockets, and kitchen/bathroom fittings, and is more disruptive to live around. Heat performance is slightly lower than external insulation because cold bridges at the floor/ceiling junction are harder to address.
For most properties with genuine external access, EWI is preferred: better performance, longer-lasting finish, no internal disruption.
Finding a qualified installer
EWI installed poorly performs poorly. Critically, badly installed EWI can trap moisture and cause significant structural damage. Use only Trustmark-registered installers who hold PAS 2030 certification and, where relevant, PAS 2035 retrofit coordination oversight.
The National Insulation Association (NIA) member directory and Trustmark’s installer search are the most reliable sources. Always get three quotes, ask for references from recent jobs within 10 miles, and ensure the specification (insulation type, thickness, finish) is in writing before signing a contract.
Most reputable EWI installations come with a 25-year guarantee on the render system; check this is backed by a third-party warranty provider, not just the installer.