INSULATION GUIDE
Solid Wall Insulation 2026: Grants, Costs & Savings
Solid wall insulation costs £8,000 to £22,000 depending on whether you go internal or external, and it's the single biggest energy upgrade most pre-1920s homes can make.
Solid wall insulation costs £8,000 to £22,000 depending on whether you go internal or external, and it's the single biggest energy upgrade most pre-1920s homes can make.
Answer a few quick questions to see which government energy grants you're eligible for. Free, instant results.
Solid wall insulation is a layer of insulating material fitted to walls that don't have a cavity. About a third of UK homes have solid walls, mostly anything built before 1920.
If your house is Victorian, Edwardian, or interwar terraced, the walls are almost certainly solid brick or stone. You can usually tell by looking at the brickwork. Cavity walls show a repeating pattern of long bricks. Solid walls show alternating long and short bricks, the short ones being the ends of bricks turned sideways to tie the wall together.
Why does this matter? Because solid walls lose around 35% of a home's heat. That's more than the roof, the floor, or the windows. It's the single biggest hole in your home's thermal envelope.
And here's the honest bit. If you've already done loft insulation, draughtproofing, and a smart thermostat and your bills are still painful, solid walls are almost certainly the reason. There isn't a cheap fix. But there are grants, and the payback is real.
You need solid wall insulation if your home was built before 1920, your EPC rating is D or below, and your gas bill regularly tops £150 a month in winter. If any two of those apply to you, it's worth getting a survey. The third one will follow.
Worth saying upfront: this is not the same as cavity wall insulation, which is a much cheaper job on post-1920s homes. The two are often confused. If you're not sure which you have, your EPC report will tell you, look for the "walls" line in the recommendations section.
Internal wall insulation costs £8,000 to £15,000. External wall insulation costs £12,000 to £22,000.
Those are the realistic 2026 ranges for a typical three-bedroom semi-detached. Bigger detached houses can push £25,000 for external. Small mid-terrace flats can come in under £7,000 for internal. The variation is genuinely massive.
What drives the price:
A few specifics from recent quotes we've seen. A two-bed terrace in Manchester for internal insulation came in at £9,200 including replastering. A four-bed detached in Hampshire for external came in at £19,800 with silicone render. Same product category. Wildly different prices.
Get at least three quotes. Two will usually be within £1,500 of each other and the third will be £4,000 higher or lower. The outlier tells you something is off, either they've missed something, or the two cheaper ones have.
Two schemes currently fund solid wall insulation: ECO4 and the Warm Homes: Local Grant. Both can cover the full cost.
ECO4 is the big one. If you receive qualifying benefits (Universal Credit, Pension Credit, Income Support, ESA, JSA, Child Benefit on a low income, and a few others) and your home is rated D, E, F or G on its EPC, ECO4 can fund solid wall insulation in full. No income contribution, no upfront payment, no recovery of costs later.
The scheme is administered by the major energy suppliers (British Gas, Octopus, OVO, EDF, Scottish Power, and EON Next all run ECO4 programmes), and it runs until December 2026. We've covered ECO4 eligibility in full elsewhere, so the short version here: if you're on benefits and your walls are solid, this is your route.
Warm Homes: Local Grant is different. It runs through local authorities, not energy suppliers, and it's aimed at non-benefits households on low incomes with poor-rated homes. Eligibility varies wildly by council. Some authorities pay the full cost. Some cap at £10,000 with a homeowner contribution. Some don't run the scheme at all. We dig into this in our Warm Homes Local Grant guide.
And here's the thing nobody tells you. The Great British Insulation Scheme closed in March 2026. It used to be the route for households who didn't qualify for ECO4. That option is gone. If you're not on benefits and your council doesn't run a Warm Homes scheme, you're paying for solid wall insulation yourself, or waiting to see what replaces GBIS.
There's also a 0% VAT relief on energy-saving materials including insulation, which runs until 31 March 2027. That saves you roughly £1,500 to £4,000 on a self-funded install depending on the project size. Make sure your installer applies it, some still default to charging 20% and you'll need to challenge it.
One thing worth flagging on ECO4: the installer is appointed by the scheme. You don't get to choose them. They're MCS-accredited and meet the scheme's quality requirements, but you don't have the freedom you'd have if you were paying yourself. Mostly this works fine. Occasionally we hear about timing frustrations, surveys booked weeks out, install dates moved. Patience helps.
External insulation is better in almost every technical way, but internal is what most people end up choosing. The reasons are practical, not technical.
External wraps your house in insulation board, then renders over it. Your internal rooms stay the same size, you don't lose any floor space, your walls stay dry because the original brickwork is protected from rain, and your home's external appearance gets a fresh finish. The performance is typically better because there are fewer thermal bridges (the gaps where heat sneaks through around windows and door frames).
Internal insulation goes on the inside of your external walls. You lose roughly 80mm to 120mm off every external wall, which translates to about 5% of your floor area in a typical room. Skirting, radiators, sockets, and coving have to come off and be refitted. You'll lose access to the room while the work happens. And you need to do it one room at a time unless you want to move out.
| Factor | Internal | External |
|---|---|---|
| Cost | £8,000 to £15,000 | £12,000 to £22,000 |
| Floor space lost | 4 to 6% per room | None |
| Disruption | High (one room at a time) | Low (works outside) |
| Planning permission | Not usually needed | Often needed in conservation areas |
| Conservation issues | Fine for most listed buildings | Often refused on listed buildings |
| Performance | Good | Better |
| Aesthetic change | None outside | Changes facade entirely |
So why do people pick internal? Three reasons. It's cheaper. It doesn't need planning permission in most cases. And if you live in a listed building or a conservation area, external is often refused.
If you can afford external and your council allows it, do external. If you can't, or you can't, internal is genuinely fine. It's just more of a faff. We've covered both in dedicated guides, internal wall insulation here and external wall insulation here.
One caveat on internal. Get the moisture management right. Solid walls breathe, and if you trap moisture between the new insulation and the old brickwork, you get damp and mould. A good installer will use a vapour-permeable system (like wood fibre board or a calcium silicate board with the right plaster finish). A bad installer will slap on PIR foam with foil backing and create a problem you'll be paying to fix in five years. Ask specifically what membrane and what insulation board they're using.
Most households save £400 to £700 a year after solid wall insulation, depending on house size and starting EPC rating.
The Energy Saving Trust puts the typical saving for a gas-heated semi-detached at £450 a year. A detached house can save £700+. A mid-terrace flat saves less, around £250, because two of the four walls are shared with neighbours and don't lose much heat.
Those numbers assume you keep heating the house to the same temperature you did before. Most people don't. After insulation, the house holds heat for longer, so you start heating it less aggressively. Real-world savings are often higher than the modelled figures because of this behavioural shift.
Payback if you're paying yourself: 12 to 20 years on internal, 15 to 25 years on external. Slow, by domestic upgrade standards. But here's where it gets more interesting.
If you get it on ECO4, the payback is immediate. The grant pays for the install, your bills drop the next month, and the savings are pure gain from day one. Same logic for Warm Homes: Local Grant where it covers the full cost.
Also worth factoring in: solid wall insulation typically jumps your EPC by two bands. A house going from E to C unlocks (sorry, accesses) eligibility for other grants you weren't previously qualified for. It also adds to your home's resale value, somewhere between 3% and 6% depending on local market norms, though the data on this is patchy and varies a lot by region.
Start with the eligibility checker. Two minutes, no signup, tells you which scheme applies to your home.
The order we'd suggest:
A practical tip on quotes. Ask each installer for a U-value calculation showing the wall's thermal performance after the install. A good installer will produce one. A bad one will look at you blankly. The target U-value is 0.30 W/m²K or lower for solid wall insulation to count properly under building regs.
And one final thing. Don't sign anything on the doorstep. Solid wall insulation has a long, slightly grim history of doorstep sales operators, particularly in the ECO scheme era, and while the major suppliers and councils now run things properly, the doorstep cowboys are still out there. If someone knocks asking about "government grants for your walls" and pressures you to sign that day, close the door. Real grant routes don't work that way.
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