Facts last checked 17 September 2026
The short answer
Internal wall insulation (IWI) lines the inside of your outside walls. It can be done room by room and leaves the frontage untouched, but it takes floor space, disturbs every fitting on the wall and carries the higher moisture risk. External wall insulation (EWI) wraps the outside, keeps your rooms intact and deals better with cold spots, but it changes how the house looks and often needs planning permission. The Energy Saving Trust puts a typical three-bedroom semi at about £12,000 internally and £15,000 externally. On a traditional wall, getting moisture right matters more than which side the insulation goes.
What is the difference between internal and external wall insulation?
A solid wall has no cavity to fill, so insulation has to be added to one face of it. Our guide to telling a cavity wall from a solid wall covers how to check which you have.
- Internal wall insulation: insulation boards fixed to the inside face of the external walls, a timber or metal stud wall filled with insulation, or layers of insulating lime plaster. Skirting, sockets, radiators and door and window linings come off and go back on.
- External wall insulation: insulation boards or an insulating render fixed to the outside of the wall, finished with render, brick slips or cladding. Downpipes, meter boxes and anything else fixed to the wall have to be moved out, and the roof edge and window sills often need extending.
Both are judged against the same Building Regulations target. Approved Document L sets an improved U-value of 0.30 W/m²K for a solid wall insulated internally or externally, against 0.55 for a filled cavity. A U-value is how much heat passes through a square metre of wall for each degree of temperature difference, so lower is better.
How do internal and external insulation compare?
| Internal (IWI) | External (EWI) | |
|---|---|---|
| Energy Saving Trust typical cost, three-bedroom semi | About £12,000, including redecoration | About £15,000 |
| Smart Energie typical range | From £3,500 per elevation; £6,500 to £12,000 whole house | From £12,000; £18,000 to £32,000 whole house |
| Floor area | Reduced in every room treated | Unchanged |
| Disruption | High: rooms out of use, redecoration needed | Mostly outside, household can stay put |
| Appearance | Outside unchanged | Outside changes, often significantly |
| Cold spots at floors and internal walls | Harder to avoid | Reduced, as the whole wall is kept warm |
| Moisture risk to the wall | Higher: the wall becomes colder | Lower if permeable materials are used |
| Planning | Usually not needed; listed building consent almost always needed if listed | Often needed, especially in conservation areas; unlikely to be accepted on a listed building |
Why is moisture the biggest risk with solid walls?
Traditional solid walls of brick, stone and lime stay healthy by absorbing moisture and drying out again. The Energy Saving Trust explains that insulation changes that balance, and internal insulation makes the wall colder, so moisture can build up inside it. Condensation hidden inside the wall build-up is called interstitial condensation. Historic England warns that trapped moisture increases decay of the building fabric and can harm the health of the people living there.
The risk is well documented. Historic England cites government research from the Demonstration of Energy Efficiency Potential (DEEP) programme which found that installing internal wall insulation always increased moisture risk in its case studies, even with a thin layer. The same research found vapour-permeable (breathable) systems had lower risk, and that targeting a U-value of around 0.8 W/m²K instead of 0.30 reduced the condensation risk while still delivering about two thirds of the energy savings.
Historic England adds that where solid walls are thin or exposure to wind-driven rain is high, internal insulation may not be suitable at all, and that closed-cell foam and plastic-based insulations are not usually appropriate for traditional buildings.
What about thermal bridging and cold spots?
A thermal bridge is a route where heat bypasses the insulation, such as a window reveal, the point where a floor meets the wall, or an internal wall joining the outside wall. It wastes heat, and the cold patch it leaves is where condensation and mould appear.
- Internal insulation stops at every floor, ceiling and internal wall. Historic England notes it often has to be returned along party walls, internal walls, floors and ceilings to reduce bridging, and that window and door reveals and wall-to-floor junctions need particular care.
- External insulation wraps the building in one continuous layer. Historic England says it keeps structural elements warmer and reduces thermal bridging at internal walls and floors, though window positions and eaves still need detailing.
Do you need planning permission or other consent?
- Planning, in England: householder permitted development allows exterior work only where materials are of similar appearance to the existing house. On article 2(3) land, which includes conservation areas, National Parks and areas of outstanding natural beauty, cladding any part of the exterior with render, stone, timber, plastic or tiles is not permitted development. Historic England advises consulting your council before any external insulation work.
- Listed buildings: Historic England says external insulation is unlikely to be acceptable and will typically need listed building consent, and consent is almost always needed for internal insulation too.
- Building Regulations: Approved Document L says that when more than 50% of a wall's surface is renovated, for example by dry-lining or re-rendering it, the whole wall should be brought up to the upgrade target where that is technically feasible and pays back within 15 years.
- Neighbours: Approved Document L's guidance on external insulation asks for the impact of the thicker wall on adjoining buildings to be assessed, which matters on terraces and semis where the wall meets next door.
How much room do you lose with internal insulation?
Approved Document L says that, in general, it is reasonable to accept a loss of no more than 5% of a room's floor area, while taking the room's use and furniture into account. Where acoustic returns onto party walls matter, it allows a less demanding target of 0.35 W/m²K or above.
| Room | Floor area before | Floor area after | Loss |
|---|---|---|---|
| 4.0 m x 3.5 m, one external wall (4.0 m) | 14.0 m² | 13.6 m² | 2.9% |
| 4.0 m x 3.5 m corner room, two external walls | 14.0 m² | 13.26 m² | 5.3% |
Small rooms and corner rooms lose proportionally more, and returns along party walls take more again.
So when is each the better choice?
External insulation usually makes more sense when:
- the outside can change, the house is not listed and planning is achievable
- the render or brickwork needs work anyway, or scaffolding is going up for a roof, windows or solar, which the Energy Saving Trust says cuts the cost
- rooms are small and floor space matters
Internal insulation usually makes more sense when:
- the frontage is attractive, in a conservation area or part of a matching terrace
- you are already stripping rooms for redecoration, so much of the disruption and redecoration cost is already committed
- you want to phase the work room by room, starting with the coldest rooms
Neither is right while a wall is damp, badly pointed, leaking from gutters or heavily exposed to driving rain. Those need fixing first. Smart Energie assesses moisture risk wall by wall before recommending either, and our insulation service page sets out where we decline the work.
What should you check before going ahead?
- Repairs first: the Energy Saving Trust lists damaged brickwork or pointing, roofs and overhangs, and gutters and downpipes as work to complete before any wall insulation.
- Exposure and flood risk, which the Energy Saving Trust says your installer should check.
- Materials matched to the wall: permeable insulation and finishes on traditional construction, with a written explanation of how condensation risk was assessed.
- Ventilation: the Energy Saving Trust says installers should assess whether extra ventilation is needed and never block airbricks or trickle vents.
- A whole-house view. PAS 2035, the British Standards Institution's retrofit specification, sets out assessment and design for insulation alongside ventilation and heating, and TrustMark oversees its retrofit roles.
- A 25-year guarantee: the Energy Saving Trust says TrustMark solid wall insulation businesses offer one, insurance-backed or through the Solid Wall Insulation Guarantee Agency (SWIGA).
Installation of insulation in homes is zero-rated for VAT until 31 March 2027. The retrofit planner shows how wall insulation fits with loft, floors and heating, and how we price explains what moves a quote.
Common questions
Is external wall insulation always better than internal?
Thermally it is usually more robust, because it avoids most thermal bridges and keeps the wall warm. But it costs more, changes the look of the house and is often restricted by planning, so on a listed building or protected frontage a careful breathable internal system is often the only realistic option.
Can I insulate just one wall or one room?
Yes, internally. Room-by-room internal insulation is one of its main advantages. Take extra care at the edges, where the insulated wall meets uninsulated walls and floors, because those junctions are where cold spots and condensation tend to form.
Will solid wall insulation cause damp?
It can if the wrong materials are used or the wall is already wet. Government DEEP research cited by Historic England found internal insulation always increased moisture risk, less so with vapour-permeable systems. Fix repairs and leaks first, use breathable materials on traditional walls, and do not insist on the thickest possible build-up.
Is solid wall insulation worth the money?
The Energy Saving Trust estimates about a third of an uninsulated home's heat is lost through the walls, so the saving can be substantial, but at £12,000 to £15,000 the payback is long. It makes most sense when combined with work you are doing anyway, such as re-rendering, re-roofing or redecorating, or as part of a plan for a heat pump.