Internal vs External Wall Insulation: A Practical Comparison
Solid walls can be insulated on the inside (internal wall insulation, IWI) or the outside (external wall insulation, EWI). This guide compares cost, disruption, floor space, moisture risk and planning, using published figures where they exist.
If it was external wall insulation, TrustMark's free Find & Fix audit comes first, and any repairs should not cost you anything (how the Find & Fix audit works). If it was internal wall insulation and you have concerns, contact Ofgem's ECO contact centre on 0808 169 4447 or ECOhelp@ofgem.gov.uk.
At a Glance: IWI vs EWI
| Factor | Internal (IWI) | External (EWI) |
|---|---|---|
| Typical cost | Around £12,000 (typical 3-bed semi, includes redecoration) | Around £15,000 (typical 3-bed semi) |
| Floor space | Some lost (rooms slightly smaller, depending on system) | None lost (added to the outside) |
| Disruption | Higher (rooms out of use while work is done) | Lower inside (scaffolding and outside work) |
| Appearance | Outside unchanged (inside redecorated) | Outside changes (render, cladding or brick slips) |
| Moisture risk | Wall behind gets colder (moisture can build up inside the wall if badly designed) | Depends on detailing (poor detailing at windows, roofline and base lets water in) |
| Planning | Not usually needed (listed buildings may need consent) | Check with your council (extra care in conservation areas) |
| Weather protection | No change (outer face still exposed) | Should improve (helps protect brickwork) |
| Repair cost if faulty | £250 - £6,000 (NAO, ECO4/GBIS audits) | £5,000 - £18,000 (NAO, ECO4/GBIS audits) |
Typical costs: Energy Saving Trust (June 2026), typical three-bedroom semi-detached house in Great Britain, whole house done by a company; the IWI figure includes full redecoration. Repair costs: National Audit Office report HC 1334 (14 October 2025), for ECO4/GBIS installations repaired before major damage. Typical figures, not quotes. Last reviewed September 2026.
When Internal Wall Insulation (IWI) May Suit
IWI is often chosen where external insulation is not possible or not wanted, for example to keep the look of the outside of the house. The Energy Saving Trust notes it is generally cheaper than EWI, but it is disruptive to fit and slightly reduces the floor area of each room insulated.
Situations where IWI is often considered:
- ✓
Listed buildings and conservation areas
IWI keeps the outside appearance, but listed buildings may still need consent for internal work, and traditional walls need permeable materials. Check with your local planning department first.
- ✓
Brickwork or stonework you want to keep
EWI covers the outside face of the wall, including decorative brick and stone details.
- ✓
Semis and terraces where neighbours aren't insulating
EWI on one house can leave an awkward step and junction with next door. IWI does not affect the outside.
- ✓
Walls next to a boundary, footpath or narrow passage
EWI adds thickness to the outside of the wall, which may not be possible where there is no room.
- ✓
Insulating only some rooms
IWI can be fitted room by room, but junctions with uninsulated walls need careful detailing to avoid cold spots where condensation can form.
- ✓
Budget
Generally cheaper than EWI (Energy Saving Trust: around £12,000 against £15,000 for a typical 3-bed semi), and can be timed with redecorating.
Advantages of IWI:
Practical:
- • No scaffolding needed
- • Can be timed with redecorating to reduce cost
- • Doesn't affect gutters, rainwater pipes or meter boxes
- • Should improve sound resistance
Planning and appearance:
- • Outside appearance unchanged
- • Not usually a planning matter (listed buildings may need consent)
- • Neighbours don't need to take part
- • Can be done room by room, with careful detailing
- Any damp has been found and fixed first. Wall insulation should not be installed over an existing damp problem.
- You are prepared to lose some floor area in each room that is insulated.
- The system is designed for your wall, with a moisture risk assessment. Traditional brick, stone or lime walls usually need permeable (vapour-open) materials and finishes.
- Ventilation has been assessed, and airbricks, wall vents and trickle vents are not blocked.
- You have allowed for radiators, sockets, skirting boards and door frames on those walls to be moved or refitted.
- You can manage rooms being out of use while the work is done.
When External Wall Insulation (EWI) May Suit
EWI fixes insulation to the outside of the wall, finished with render or cladding. The Energy Saving Trust says it doesn't reduce floor area, can be fitted without disruption inside the home and should improve weatherproofing. It usually costs more than IWI and depends heavily on detailing: the NAO found 98% of homes with EWI fitted under ECO4 and GBIS had major issues requiring remediation.
Situations where EWI is often considered:
- ✓
Detached houses with good access all round
Easier to insulate every external wall without awkward junctions with a neighbour's property.
- ✓
Rendered or pebbledashed walls with no features to preserve
A new finish can be chosen, such as render, brick slips, tiles or cladding.
- ✓
Homes needing outside work anyway
Costs can be lower if EWI goes on alongside a new roof, windows, solar panels or gutter repairs while scaffolding is up. Damaged brickwork, pointing, roofs and gutters must be repaired first.
- ✓
Terraces where neighbours insulate at the same time
Avoids steps between houses and may allow scaffolding to be shared.
- ✓
Rooms that can't spare floor space
EWI does not reduce room sizes.
- ✓
Older or listed buildings, where permitted
Needs permission and specialist advice. Permeable systems, such as wood fibre boards with a lime or silicate render, are often considered for traditional walls.
Advantages of EWI:
Thermal and technical:
- • Doesn't reduce room sizes
- • Wraps the outside, so fewer breaks at floors and internal walls
- • Keeps the masonry on the warm side of the insulation
- • Can reduce condensation on internal surfaces
Practical:
- • Most work happens outside
- • Renews the appearance of the outer walls
- • Should improve weatherproofing and sound resistance
- • Should help protect the brickwork
- Whether you need planning permission. Ask your local council, especially in a conservation area or for a listed building.
- Access for scaffolding, including narrow side passages.
- Whether roof overhangs need extending and rainwater pipes and meter boxes moving.
- How window and door reveals, sills and the base of the wall will be detailed to keep water out.
- How the insulation will finish where it meets a neighbour's wall on a semi or terrace.
- Ventilation. EWI can reduce air flow into rooms, so the installer should assess what is needed.
- The guarantee. The Energy Saving Trust says TrustMark solid wall insulation companies offer a 25-year guarantee, backed by an insurer or by SWIGA.
Typical Costs and What Affects Them
The Energy Saving Trust puts typical solid wall insulation costs between £12,000 and £15,000. Its figures assume a company insulates the whole house in one go, and for IWI they include fully redecorating afterwards. Your own quote will depend on wall area, access, the system chosen and any repairs needed first.
| Measure | Typical cost | Source |
|---|---|---|
| Solid wall insulation (typical range) | £12,000 - £15,000 | Energy Saving Trust, June 2026 |
| External wall insulation, typical 3-bed semi (GB) | Around £15,000 | Energy Saving Trust, June 2026 |
| Internal wall insulation, typical 3-bed semi (GB), including redecoration | Around £12,000 | Energy Saving Trust, June 2026 |
| External wall insulation, installed price per m² | £150 - £200/m² | Checkatrade, July 2026 |
Extra work often involved with IWI:
- • Skirting boards, door frames and fittings removed and refitted
- • Radiators, sockets and switches on external walls moved
- • Redecoration of each room (included in the Energy Saving Trust figure)
- • Special fixings for heavy items such as shelves and wall cupboards
Ways EWI cost can come down:
- • Fitting it alongside a new roof, new windows, solar panels or gutter repairs, when scaffolding is already up
- • Coordinating with neighbours on a terrace so scaffolding can be shared
- • Planning ahead so exterior repairs are done in the same programme
- • Getting at least three quotes, as the Energy Saving Trust recommends
What about energy savings?
The Energy Saving Trust estimates that around a third of the heat lost from an uninsulated home escapes through the walls, and it publishes estimated bill savings by house type on its solid wall insulation page. Actual savings depend on your home, how you heat it and energy prices, so treat any payback or property value figure in a sales quote with caution and ask how it was worked out.
Typical ranges, not quotes. Full source links are listed at the end of this page. Last reviewed September 2026.
Considerations by Property Type
Older Solid Wall Homes (Usually Built Before the 1920s)
Things to get right:
- • Traditional brick, stone, earth and lime walls absorb and release moisture, so insulation can upset that balance. Permeable materials and finishes are usually needed.
- • IWI makes the wall behind it colder, so moisture can build up inside the wall if the design is wrong.
- • Either option can work when it is designed for the building. Get advice from someone experienced with your building type.
- • Listed buildings and conservation areas: some kind of permission is likely to be needed. See Historic England's insulation advice.
Where solid wall insulation may not be suitable:
- • Homes exposed to extreme weather or flooding
- • Walls in poor condition that have not been repaired
- • Where there is a risk of rooms overheating
- • Either way, loft insulation and draught-proofing are lower-cost measures worth looking at first
Cavity Wall Homes (Usually Built After the 1920s)
If the cavity is unsuitable or already has failed insulation:
Get an independent survey before any refill, extraction or switch to IWI or EWI. If existing cavity wall insulation is causing damp, start with the installer and the guarantee (see cavity wall insulation damp).
Where cavity fill isn't suitable, IWI or EWI may be options, with the same checks described above.
Homes Built From the 1990s
Wall insulation is probably already there:
The Energy Saving Trust says homes built in the 1990s or later are likely to already have wall insulation, so adding IWI or EWI is rarely the first step. Consider:
- • Topping up loft insulation (Energy Saving Trust: around £600 in Great Britain to go from 120mm to 270mm)
- • Draught-proofing, while keeping intentional ventilation clear
- • Getting advice before adding further wall insulation
Decision Helper: Questions to Work Through
Did you have EWI or IWI fitted under ECO4 or GBIS, and are you worried about it?
NO → Continue to the next question.
Is your home listed or in a conservation area?
NO → Continue to the next question.
Is there any damp, damaged brickwork or pointing, or a roof or gutter problem?
NO → Continue to the next question.
Do you want to keep the outside appearance, or is there no room outside (boundary, footpath, narrow passage)?
NO → Continue to the next question.
Can you give up some floor area and cope with rooms being out of use?
NO → EWI may suit better, subject to planning, access and cost.
Whichever you choose, check the installer's TrustMark registration on TrustMark's register, their insurance and who backs the guarantee.
Still Unsure Which to Choose?
Tell us about your home. Better Home Hub may introduce a specialist who can look at your walls, and that specialist may pay us a fee.
Sending an enquiry is free. Surveys and installation are usually paid. Call 03330 505 281 or email hello@betterhomehub.co.uk.
Related Information
Sources
- Energy Saving Trust: Solid wall insulation (updated 18 June 2026)
- Energy Saving Trust: Cavity wall insulation (updated 8 May 2026)
- Energy Saving Trust: Roof and loft insulation (updated 8 May 2026)
- Checkatrade: External wall insulation cost (July 2026)
- National Audit Office: Energy efficiency installations under the Energy Company Obligation, HC 1334 (14 October 2025)
- TrustMark: Free Find & Fix audit offer (updated 29 April 2026)
- Historic England: Upgrading thermal elements and installing insulation
Last reviewed September 2026.