Internal wall insulation (IWI) costs £4,000-£8,000. External wall insulation (EWI) costs £8,000-£14,000. Both save around £455 per year on a semi-detached house with solid walls (Energy Saving Trust). EWI is more thermally effective because it wraps the entire building and eliminates thermal bridges. IWI is up to 50% cheaper and does not change your home’s exterior appearance — making it the only option for listed buildings and many conservation areas. Both methods are eligible for ECO4 funding until December 2026.
How Do Internal and External Wall Insulation Compare?
| Feature | Internal Wall Insulation (IWI) | External Wall Insulation (EWI) |
|---|---|---|
| Typical cost | £4,000-£8,000 | £8,000-£14,000 |
| Cost per m2 | £40-£100 | £85-£130 |
| Annual savings (semi-detached) | Up to £455 | Up to £455 |
| Thermal performance | Good (U-value ~0.30 W/m2K) | Better (U-value ~0.25 W/m2K) |
| Eliminates thermal bridges | No | Yes |
| Room space lost | 10-15cm per treated wall | None |
| Changes exterior appearance | No | Yes |
| Planning permission needed | Rarely | Often |
| Suitable for listed buildings | Sometimes (with consent) | Rarely |
| Interior disruption | High (1-3 weeks) | Low |
| Exterior disruption | None | High (scaffolding, 2-3 weeks) |
| Installation approach | Room by room possible | Whole house at once |
Sources: Energy Saving Trust, Checkatrade, Which?
What Is Internal Wall Insulation and How Does It Work?
Internal wall insulation is fitted to the inside face of your external walls, reducing heat loss without changing your home’s exterior. It is the more affordable of the two solid wall insulation methods and can be installed one room at a time.
There are two main approaches:
Insulated plasterboard (dry lining)
Pre-bonded insulation boards (typically PIR foam or phenolic foam backed with plasterboard) are fixed directly to the wall using adhesive or mechanical fixings. This is the quickest method, adding 50-100mm to the wall thickness.
Stud wall with insulation
A timber or metal stud frame is built in front of the wall and filled with mineral wool or wood fibre insulation, then covered with plasterboard. This adds 100-150mm but allows for services (wiring, pipes) to be run within the frame.
For Victorian and other pre-1920s solid wall homes, breathable insulation materials are essential. Wood fibre boards, mineral wool batts with lime plaster, and calcium silicate boards allow moisture to pass through the wall safely. Foil-backed boards and closed-cell foam can trap moisture, causing damp and mould in older properties.
Cost breakdown for a typical three-bedroom semi-detached:
| Item | Cost |
|---|---|
| Materials (insulation boards + plasterboard) | £1,500-£3,000 |
| Labour (plastering, fitting, making good) | £2,000-£4,000 |
| Ancillary work (moving radiators, sockets, skirting) | £500-£1,000 |
| Total | £4,000-£8,000 |
What Is External Wall Insulation and How Does It Work?
External wall insulation wraps your home’s exterior in a continuous layer of insulation, delivering superior thermal performance by eliminating cold bridges at junctions. The insulation is covered with a render finish that can transform the appearance of the property.
The standard installation process:
1. Scaffolding is erected around the property
2. Insulation boards (mineral wool, EPS or wood fibre) are mechanically fixed and adhesive-bonded to the external walls
3. A reinforcing mesh is embedded in a base coat over the insulation
4. A decorative render finish is applied (silicone, acrylic or mineral)
5. Window reveals, sills and roof edges are detailed to prevent water ingress
The total system thickness is typically 100-150mm, extending the building’s footprint outward. This means window and door reveals become deeper, and features like drainpipes, meter boxes and satellite dishes need repositioning.
Cost breakdown for a typical three-bedroom semi-detached:
| Item | Cost |
|---|---|
| Scaffolding | £1,000-£2,000 |
| Materials (insulation boards + render system) | £3,000-£5,000 |
| Labour | £3,500-£6,000 |
| Ancillary work (reveals, sills, detailing) | £500-£1,000 |
| Total | £8,000-£14,000 |
Which Method Saves More Energy?
EWI is more thermally effective than IWI in real-world conditions, even though laboratory U-values appear similar. The reason is thermal bridging. Internal insulation stops at every floor, ceiling, internal wall and window junction, creating cold bridges where heat escapes around the insulation. External insulation wraps continuously over these junctions, eliminating them.
In practice, this means:
- IWI achieves a wall U-value of around 0.30 W/m2K, but whole-wall performance is reduced by thermal bridges at junctions
- EWI achieves a wall U-value of around 0.25 W/m2K or better, with thermal bridging largely eliminated
The Energy Saving Trust estimates both methods save around £455 per year for a semi-detached house and up to £660 for a detached house (Energy Saving Trust). However, EWI typically delivers savings at the higher end of the range due to its superior real-world performance.
EWI also provides a weatherproof outer layer, reducing rain penetration and extending the life of the underlying masonry.
When Is Internal Wall Insulation the Better Choice?
Choose internal wall insulation when the exterior of your home cannot or should not be changed. IWI is the right choice if:
- Your home is a listed building — external changes are almost never permitted on listed properties, but internal insulation (with Listed Building Consent) may be acceptable if it does not damage original features
- You live in a conservation area where the front elevation cannot be altered
- Your property is a mid-terrace and you only need to insulate the front and rear walls (party walls are shared and do not lose heat)
- You have a limited budget and cannot afford EWI
- You want to insulate room by room as part of a phased renovation
The main trade-off is loss of room space. Each treated wall loses 10-15cm of depth. In a small Victorian terrace, this can reduce a 3.5m-wide room to 3.2m — a noticeable difference, particularly with furniture in place.
When Is External Wall Insulation the Better Choice?
Choose external wall insulation when thermal performance, weatherproofing and preserving interior space are priorities. EWI is the right choice if:
- Your home is not listed and not in a conservation area (or planning is obtainable)
- You want no loss of room space inside the property
- You want to avoid interior disruption — no need to move out, no replastering, no repositioning of sockets or radiators
- Your property has damp or cold wall issues that EWI will resolve by keeping the masonry warm and dry
- You are planning other exterior work (re-rendering, roof repair) and can combine costs
- You want to improve the appearance of a tired-looking property
EWI also benefits from the “thermal mass” effect: the masonry wall sits inside the insulation layer and acts as a heat store, releasing warmth slowly and keeping indoor temperatures more stable.
Do You Need Planning Permission?
External wall insulation almost always needs planning permission if it faces a public highway, and always if the property is listed or in a conservation area. Rear and side elevations may fall under permitted development in some cases, but this varies by local authority.
Internal wall insulation rarely needs planning permission unless it affects the character of a listed building (in which case Listed Building Consent is required).
Always check with your local planning authority before committing to either method. Your installer should be able to advise on the requirements for your area.
What Grants Are Available for Solid Wall Insulation?
ECO4 runs until December 2026 and can cover the full cost of both internal and external wall insulation for eligible households (GOV.UK).
Eligibility criteria:
- You receive a qualifying benefit (Universal Credit, Pension Credit, Income Support, JSA, ESA, Housing Benefit, Child Tax Credit or Working Tax Credit)
- Or your household income is below your council’s LA Flex threshold (typically £31,000-£38,000)
- Your property is rated EPC band D, E, F or G
Given the high cost of solid wall insulation — particularly EWI — grant funding makes a transformative difference. A £12,000 EWI installation that might never pay for itself at full price becomes one of the best investments available when fully or heavily subsidised.
Check our insulation grants page for eligibility details and application guidance.
Frequently Asked Questions
Can you combine internal and external wall insulation on the same house?
Yes. A common approach for mid-terrace houses is external insulation on the rear elevation (where planning is easier) and internal insulation on the front elevation (where the street-facing appearance must be preserved). This is also used where one elevation is accessible and another is not.
Does internal wall insulation cause condensation?
It can if installed incorrectly. The insulation moves the dew point (where warm moist air condenses) closer to the wall surface. Proper installation includes a vapour control layer on the warm side and, for older solid walls, the use of breathable insulation materials that manage moisture safely. Always use a TrustMark or MCS-certified installer.
How long does external wall insulation last?
A properly installed EWI system lasts 30-40 years. The render finish may need maintenance or recoating every 15-20 years depending on exposure and the type of render used. Silicone render is the most durable and self-cleaning option.
Will external wall insulation make my house look different?
Yes. EWI covers the original brickwork or stonework with a rendered finish. You can choose from a wide range of colours and textures, but the original character of exposed brick or stone will be hidden. For many homeowners, this is an improvement — for others, particularly in period properties, it is a drawback.
Is solid wall insulation worth it without a grant?
At full cost, the payback period is 15-30 years, which makes it a long-term investment rather than a quick win. However, it significantly improves comfort, eliminates cold walls, reduces condensation risk, and increases your EPC rating (important for property value and future regulations). If grant funding covers part or all of the cost, solid wall insulation becomes one of the most valuable upgrades available.
Find out which method suits your home and get accurate pricing.
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