The Short Answer
Solid wall insulation costs around £12,000 internally or £15,000 externally for a typical 3-bed semi (Energy Saving Trust), and saves somewhere around £280 a year on gas. That's a 40-year payback, so don't do it for the money. Do it if a grant pays (ECO4 until 31 December 2026, the Warm Homes: Local Grant, Home Energy Scotland's £7,500, Nest or Affordable Warmth), if you're already re-rendering or replastering, or if cold walls and mould are making the house miserable. Choose external when the outside needs work anyway and you're not in a conservation area; choose internal room by room as you redecorate. Whichever you pick, the installer matters more than the material: the National Audit Office found major faults in 98% of external wall jobs done under ECO.
Solid wall insulation calculator
What it costs, what it saves, and how much room you lose
Your home
How it's heated
Internal wall insulation
Typical cost: £12,000
Saving: about £280 a year
Payback: about 43 years
External wall insulation
Typical cost: £15,000
Saving: about £280 a year
Payback: about 54 years
On those numbers it won't pay for itself in the life of the insulation. Do it for comfort, to cure cold walls and mould, to get ready for a heat pump, or because a grant or a renovation already in hand takes most of the cost.
Energy Saving Trust costs for a 3-bed semi (around £12,000 internal, £15,000 external), scaled to other house types by wall area, and its previously published saving for a gas-heated home, rescaled for your fuel at 1 October to 31 December 2026 prices. Internal costs assume every room done in one go and redecorated; doing it room by room as you redecorate costs much less.
Do You Actually Have Solid Walls?
Check before anyone quotes you, because a solid wall price on a cavity wall house is an expensive mistake. Three quick tests:
- Measure the wall at a window or door reveal. The Energy Saving Trust's rule of thumb is that a brick wall 260mm to 350mm thick is probably a cavity wall; a solid brick wall is usually narrower, around 220mm to 230mm. Stone walls are solid whatever their depth, and can be up to 500mm thick.
- Look at the brick pattern. If you can see the short ends of bricks (headers) mixed in with the long sides, it's almost certainly solid. If every brick shows its long side, it's probably a cavity, unless it's been rendered or pointed in a way that hides it.
- Think about the age. Most UK houses built before the 1920s have solid walls: Victorian and Edwardian terraces, older cottages, stone farmhouses. Most built from the 1930s onwards have cavities.
Your EPC usually says too, under "Walls". If it says "Cavity wall, as built, no insulation", you want cavity wall insulation instead, which costs about a fifth as much.
Internal or External: Which Is Right for Your House?
| Internal wall insulation | External wall insulation | |
|---|---|---|
| Typical cost, 3-bed semi | Around £12,000 (whole house, redecorated) | Around £15,000 |
| What happens | Insulated boards fixed inside the outside walls, then plastered | Insulation boards fixed to the outside, then rendered or clad |
| Room size | Each outside wall comes in by the thickness of the build-up | No change inside |
| Disruption | Room by room: skirting, sockets, radiators and window boards come off and go back | Scaffolding outside; you can live normally inside |
| Planning | Not normally needed | Needed in conservation areas, National Parks, AONBs and the Broads; consent for listed buildings |
| Can you do it in stages? | Yes, one room at a time as you redecorate | Realistically, the whole elevation at once |
| Looks | Unchanged outside | The house gets a new face: render, brick slips or cladding |
| Best when | You're redecorating, the front is attractive, or you're in a conservation area | The render or brick needs work anyway, or you're having a new roof and the scaffolding is up |
Our view: on a typical brick terrace or semi, internal, done room by room as you decorate is the realistic option for most people paying their own way, because it turns a £12,000 project into a few thousand pounds per room spread over years. External is better insulation (it wraps the floor and wall junctions that internal can't reach) and better for the house if the outside is already tired, but it's an all-at-once job and it changes how your home looks.
What Solid Wall Insulation Costs
The Energy Saving Trust puts a typical 3-bedroom semi in Great Britain at around £12,000 for internal and £15,000 for external wall insulation, based on a company doing the whole house in one go and, for internal, fully redecorating afterwards. It no longer publishes figures for other house types, so the table below scales the semi by wall area (using its previously published savings, which track wall area). Treat the other rows as estimates, not quotes.
| Home | Internal | External | Saving a year (gas) |
|---|---|---|---|
| Mid-floor flat | £6,000 | £7,500 | £140 |
| Mid-terrace house | £6,500 | £8,000 | £150 |
| Detached bungalow | £9,500 | £12,000 | £220 |
| Semi-detached house | £12,000 | £15,000 | £280 |
| Detached house | £21,500 | £27,000 | £500 |
Because cost and saving both rise with the area of wall, the payback barely changes with house size: roughly 43 years for internal and 54 for external, whatever the home. Savings are the Energy Saving Trust's previously published figures for a gas-heated home. On oil, LPG or electric heating the same insulation saves more, because each kWh of heat costs more; the calculator above works it out for your fuel.
How to pay a lot less
- Do internal room by room when you're redecorating anyway. You're paying for the plastering and decorating either way; the insulation becomes the extra, not the whole bill.
- Do external when the scaffolding is already up: a new roof, new windows, solar panels, or repairs to the gutters. The Energy Saving Trust makes the same point.
- Fix the outside first. Damaged brickwork or pointing, leaking gutters and downpipes, and roof overhangs all need sorting before any wall insulation goes on, or you risk trapping damp behind it.
- Check the grants below before you pay anything.
Will It Pay for Itself?
Not on the bill alone, unless something else is paying. A semi saving around £280 a year against a £12,000 internal job is about a 43-year payback, and external is longer. Even a large detached house is around 40 years.
That doesn't make it a bad idea, it makes it a different kind of decision:
- Comfort and health. Uninsulated solid walls are cold to the touch, so rooms feel chilly even at 20C and condensation forms on them. Insulating cures the cold-wall feeling and, done properly, the black mould in the corners.
- A heat pump. Cutting wall heat loss means a smaller heat pump running at a lower flow temperature, which is cheaper to buy and to run. It isn't a condition of the £7,500 Boiler Upgrade Scheme grant, though: BUS has no insulation requirement.
- A renovation already happening. If you're replastering or re-rendering, building regulations may require you to insulate anyway (below), and the extra cost is a fraction of the headline figure.
- A grant. With ECO4 or the Warm Homes: Local Grant paying, the payback is immediate.
What Goes Wrong, and How to Avoid It
This is the part most solid wall pages skip, and it's the part that matters most.
In October 2025 the National Audit Office reported that an estimated 22,000 to 23,000 homes given external wall insulation under the government's ECO scheme, 98% of them, need work to correct major issues that will cause problems such as damp and mould, along with 9,000 to 13,000 homes (29%) given internal wall insulation. Between 900 and 2,000 of the external jobs posed an immediate health and safety risk, from things like exposed live cabling and blocked boiler ventilation. It blamed poor installation, weak oversight and inadequate auditing.
If you had solid wall insulation through ECO4 or the Great British Insulation Scheme, gov.uk says every home with external wall insulation will be offered a free home check by Ofgem, TrustMark and the installer's certification body, faults will be put right by the installer, and you shouldn't be asked to pay. Internal wall insulation showed fewer issues, so no action should be needed unless you're worried.
The four failures to guard against
- Moisture trapped in the wall. Warm, damp indoor air reaching a cold wall behind internal insulation condenses where you can't see it. The fix is a properly designed and continuous vapour control layer, or a breathable system, matched to the wall. Old stone and lime-mortared walls in particular need materials that let them dry out, not ones that seal them.
- Cold spots at the edges. Window and door reveals, floor and ceiling junctions and where internal walls meet the outside wall stay cold if the insulation stops short. Cold spots are where mould forms. Ask how each junction will be detailed.
- Water getting behind the render. External systems fail at the tops, bottoms, sills and around pipes and vents, where poor detailing lets rain in. Gutters, downpipes and roof overhangs have to be right first, and often need extending.
- Ventilation blocked or removed. A warmer, tighter house needs its ventilation to work: extractor fans, trickle vents and any air bricks or flues serving appliances must be kept clear and extended through the new layer.
Questions to ask before you sign
- Are you TrustMark-registered, and who certifies your work?
- Who has assessed my walls for damp, and what system are you proposing for this wall type?
- How will you detail the window reveals, the floor and ceiling junctions, and the eaves or base of the wall?
- What happens to my extractor fans, boiler flue, air bricks, gas meter and satellite dish?
- Who handles building control, and will I get the completion certificate?
- What guarantee do I get, how long is it, and is it insurance-backed if your company stops trading?
An installer who's vague on these is the one to avoid, however good the price.
Building Regulations: When Insulating Becomes Compulsory
In England, Approved Document L treats re-rendering, re-cladding, dry-lining or stripping a wall back to the brick as renovating a thermal element. If you renovate more than 50% of an individual wall, or more than 25% of the whole outside of the house, the whole wall is expected to be brought up to a U-value of 0.30 where that's technically and functionally feasible and pays back within 15 years. If it doesn't, you insulate to the best standard that does, generally no worse than 0.7.
There's a sensible exception for internal insulation: if reaching 0.30 would shrink a room's floor area by more than 5%, a lesser standard may be acceptable. The calculator's room space tab shows where your rooms sit. Wales and Scotland have their own building regulations, so check with your local building control there.
Do You Need Planning Permission?
Usually not, for internal insulation, or for external insulation that doesn't change how the house looks. The Planning Portal says planning permission isn't normally required for insulation where there's no change in external appearance, but that if you're in a conservation area, National Park, Area of Outstanding Natural Beauty or the Broads, you need to apply before cladding the outside of a house in stone, render, timber, plastic or tiles. A listed building needs listed building consent for significant work inside or out. If in doubt, ask your council's planning team before you pay a deposit.
Grants for Solid Wall Insulation
| Where | Scheme | What it gives |
|---|---|---|
| England, Scotland, Wales | ECO4 | Can fully fund solid wall insulation for households on qualifying benefits; ends 31 December 2026 |
| England | Warm Homes: Local Grant | Council-funded wall insulation for incomes usually up to £36,000, EPC D to G |
| Scotland | Home Energy Scotland Grant and Loan | Grant up to £7,500 plus an interest-free loan up to £2,500 for solid wall, not means tested (overall grant up to 75% of the cost) |
| Wales | Nest; Green Homes Wales | Nest is free for eligible lower incomes; Green Homes Wales lends £1,000 to £25,000 interest-free to any homeowner |
| Northern Ireland | Affordable Warmth; NISEP | Affordable Warmth up to £10,000 for a solid wall home under £23,000 income, detached only; NISEP's Whole House Warmer Homes NI funds it in full for owners under £28,000 single or £35,000 for a couple or family, if the walls pass its survey |
See our grant guides for England, Scotland, Wales and Northern Ireland, each with a checker.
Old Houses: Victorian Terraces and Stone Cottages
Most solid-walled homes are old, and old houses were built to breathe: soft bricks and lime mortar let moisture in and dry it back out. Seal that up with the wrong materials and you move the damp somewhere worse. So in an old house:
- Do the cheap things first. A bare loft to 270mm, draught proofing, and suspended floors from below all cost a fraction of wall insulation and pay back far faster. See our home insulation costs guide for the order.
- Sort damp and repairs before insulating. Penetrating damp, failed pointing and blocked gutters have to be fixed first.
- Use a breathable system on stone or lime walls, and an installer who has done this wall type before.
- Consider internal insulation on the back and sides only if the front is the attractive bit, or the house is in a conservation area.
Our Verdict
If someone else is paying, through ECO4, the Warm Homes: Local Grant, Nest, Affordable Warmth or a NISEP scheme, take it, but vet the installer as hard as if you were paying, because that's exactly where the faults happened. If you're paying yourself, don't insulate solid walls for the bill saving. Do it room by room as you redecorate, or externally when the outside needs work anyway, and do the loft, draughts and floors first.
