Facts last checked 17 September 2026
The short answer
For a gas-heated semi-detached house, the Energy Saving Trust puts insulating an empty loft at about £750, saving about £190 a year, and filling cavity walls at about £2,200, saving about £200 a year. Solid wall insulation costs around £12,000 inside or £15,000 outside and saves about £280 a year. Loft and cavity insulation pay for themselves in roughly 4 to 11 years; solid wall insulation takes decades, so do it for comfort and alongside other work. Installation is zero-rated for VAT until 31 March 2027, and low-income households in England may get it fully funded through the Warm Homes: Local Grant.
How much does each type of insulation cost?
Two sets of numbers help here: an independent national average, and a real installer's published prices. Start with the national picture from the Energy Saving Trust, an independent organisation that publishes typical unsubsidised installed costs and bill savings for gas-heated homes in England, Scotland and Wales, based on fuel prices as of July 2026.
| Home | Cavity wall: cost | Cavity wall: saving a year | Empty loft to 270mm: cost | Empty loft to 270mm: saving a year |
|---|---|---|---|---|
| Detached house | £3,900 | £380 | £1,100 | £360 |
| Semi-detached house | £2,200 | £200 | £750 | £190 |
| Mid-terrace house | £1,100 | £110 | £650 | £140 |
| Detached bungalow | £1,700 | £160 | £1,200 | £350 |
| Mid-floor flat | £950 | £110 | n/a | n/a |
| Home | Loft top-up: cost | Loft top-up: saving a year | Solid wall: saving a year |
|---|---|---|---|
| Detached house | £800 | £35 | £500 |
| Semi-detached house | £600 | £17 | £280 |
| Mid-terrace house | £500 | £13 | £150 |
| Detached bungalow | £800 | £35 | £220 |
| Mid-floor flat | n/a | n/a | £140 |
Here are the prices we publish for the same work, so you can see how one installer's figures sit against the averages.
| Measure | Typical price | What it covers |
|---|---|---|
| Cavity wall fill, 3-bed semi | £800 to £2,200 | Includes a moisture-risk assessment and a 25-year CIGA guarantee |
| Loft top-up, standard home | From £450 | Topping up existing insulation |
| Loft, full re-lay | £650 to £1,200 | A full re-lay of the loft insulation |
| Internal wall insulation, 3-bed Victorian terrace | From £3,500 per elevation | £6,500 to £12,000 for the whole house, depending on walls and finish |
| External wall insulation | From £12,000 | £18,000 to £32,000 for a whole house, depending on render, scaffolding and detailing |
| Draught-proofing | From £280 | £450 to £900 for the whole house |
| Underfloor insulation | Priced at survey | Depends on access and how the floor is built |
Rendered or clad walls, partial old fill that has to come out, or damp that needs fixing first will push a cavity price up. A clear cavity, confirmed with a borescope (a small camera fed through a drilled hole), holds it down.
How long does insulation take to pay for itself?
Simple payback is the installed cost divided by the yearly saving. It ignores future price changes, so treat it as a guide, not a promise. Using the Energy Saving Trust's own figures for a semi-detached house:
| Measure | Cost | Saving a year | Simple payback |
|---|---|---|---|
| Empty loft to 270mm | £750 | £190 | About 4 years |
| Cavity wall | £2,200 | £200 | About 11 years |
| Loft top-up, 120mm to 270mm | £600 | £17 | About 35 years |
| Internal solid wall | £12,000 | £280 | About 43 years |
| External solid wall | £15,000 | £280 | About 54 years |
The Energy Saving Trust says cavity insulation will likely pay back in five years or less, but its own semi-detached figures work out closer to 11. At the lower end of our cavity price, £800 against a £200 saving, it is about 4. Gas unit rates rose from 7.33p to 7.97p per kWh under the October 2026 price cap, which nudges every saving up slightly.
Is your home suitable for cavity or solid wall insulation?
Most homes built after the 1920s have cavity walls and older ones usually have solid walls. If you are not sure which you have, our guide to cavity or solid walls shows you how to tell. Then check the risks, because the wrong insulation in the wrong wall can trap damp.
- Cavity fill may not be suitable if your walls face driving rain or extreme weather, the home is at risk of flooding, the brickwork is in poor condition, or the cavity is narrower than 50mm or full of rubble.
- Any damp patches on inside walls must be traced and fixed before insulating. So must damaged brickwork or pointing, leaking gutters and downpipes, and roof defects.
- In a flat, you cannot usually fill the cavity of your flat alone. The building owner and the other residents need to agree to insulating the whole block.
- Cavity insulation should come with a 25-year guarantee from the Cavity Insulation Guarantee Agency (CIGA) or an independent insurance-backed guarantee.
- Solid walls built from brick, stone or lime manage moisture by absorbing and releasing it. Insulation changes that balance, so the installer should use breathable (vapour-permeable) materials where needed and keep ventilation such as airbricks and trickle vents clear.
- External wall insulation changes how the house looks. Check with your council whether it needs planning permission, especially in a conservation area or on a listed building.
For the choice between inside and outside, see internal or external solid wall insulation.
Which insulation should you do first?
Fabric first: reduce the heat your home needs before you choose the system that supplies it. A smaller heat loss means a smaller boiler or heat pump, and a heat pump that can run at a lower, more efficient water temperature.
- Repairs: gutters, pointing, roof and any damp. Insulating over a problem can make it worse.
- The loft, if it has little or no insulation. It is the lowest cost and fastest payback. Our loft insulation guide covers depths and materials.
- Draught-proofing around doors, windows and floors, while keeping deliberate ventilation open.
- Cavity walls, if they are unfilled and suitable.
- Solid walls and floors, ideally timed with redecorating, a new roof, new windows or other work that already needs scaffolding.
- Then size the heating. If a heat pump is on your list, get its heat loss calculation done after the fabric work is planned; our guide to what a heat pump costs explains why that changes the price.
What grants and funding can lower the cost?
- 0% VAT. Installing insulation for walls, floors, ceilings, roofs, lofts and pipes, and draught stripping, in a home is zero-rated from 1 May 2023 to 31 March 2027. The reduced 5% rate is due to return after that. It only applies when the installer supplies and fits the materials, so buying materials yourself does not qualify.
- Warm Homes: Local Grant (England). Run by councils, it can fully fund upgrades including insulation for households with an income usually of £36,000 or less, someone receiving certain means-tested benefits, or an address in an eligible low-income postcode. The home must have an EPC rating of D to G and be privately owned, either by you or a private landlord. Owner-occupiers are not expected to contribute. You apply on GOV.UK and the council usually contacts you within 10 working days. The scheme is funded from April 2025 to March 2028, and what is available depends on your council's allocation.
- Warm Homes Plan. The government has said it intends to merge the Warm Homes: Local Grant and its social housing fund into a single low-income scheme from 2027/28. Its new Warm Homes Loan Scheme for households who can pay covers heat pumps, solar panels and batteries, but its current rules state it will not fund insulation.
- The Boiler Upgrade Scheme does not pay for insulation, and there is no insulation condition to get it.
Any grant for insulation that arrives through an unsolicited call or a knock on the door deserves a second look. Our guide to home energy grants in 2026 explains what is genuinely available and how to spot a scam. When you are ready, book an insulation survey and you will get a moisture-risk check before any price.
Common questions
How much does loft insulation cost in 2026?
The Energy Saving Trust puts insulating an empty loft to 270mm at about £650 for a mid-terrace, £750 for a semi and £1,100 for a detached house. Topping up from 120mm costs £500 to £800 but saves only £13 to £35 a year, so a top-up rarely pays back on bills.
Is cavity wall insulation worth it?
Usually, if your walls are suitable. For a semi-detached house the Energy Saving Trust puts it at about £2,200, saving about £200 a year, so roughly 11 years to pay back at those figures, and less at a lower price. It is not worth the risk on exposed, flood-prone or damaged walls, or narrow cavities.
Why is solid wall insulation so expensive?
It is a building project rather than a quick fill. External insulation needs scaffolding and a new render or cladding finish. The Energy Saving Trust's internal figure assumes the whole house is done in one go and fully redecorated afterwards. That is why fitting it when you already have scaffolding up, or are redecorating anyway, brings the cost down.
Can I get free insulation in 2026?
Possibly, if you live in England, your household income is usually £36,000 or less or you receive certain benefits, and your home has an EPC of D to G. Apply for the Warm Homes: Local Grant through GOV.UK and your council will assess you. Be wary of anyone who calls you first offering it.
Do I pay VAT on insulation?
Not on insulation supplied and installed in your home before 31 March 2027. It is zero-rated. Materials bought without installation are charged at the standard rate.
Should I insulate before getting a heat pump?
It is not required for the Boiler Upgrade Scheme, but it is usually wise. Lower heat loss means a smaller heat pump, fewer radiator changes and lower running costs. At minimum, deal with an empty loft and unfilled, suitable cavity walls first.