How much does it cost to insulate a detached house?
The short answer
- £800 to £1,100 for loft insulation and £3,900 for cavity walls in a detached house
- About £139 a year saved by cavity wall insulation in a 4 bed detached house on gas, a 28 year payback at full price
- £12,000 to £15,000 for solid walls, which pays best with a grant such as ECO4
Updated 25 September 2026. Every figure on this page comes from our published model; see where our data comes from.
Insulating a detached house costs from about £800 to top up the loft to £12,000 or more for solid walls. Cavity wall insulation costs about £3,900, the Energy Saving Trust's figure for a detached house, and homes that had it fitted used a median 12.0% less gas afterwards.
What each measure costs
Loft first, then the walls. For a 4 bed detached house on gas, cavity wall insulation saves about £139 a year and pays back in 28 years; solid walls save more but cost far more.
| Measure | Typical cost | Saving a year | Payback |
|---|---|---|---|
| Loft insulation, bare loft to 270mm | £1,100 | More than £37 | Under 29 years |
| Loft insulation, top-up from 120mm | £800 | £37 | 21 years |
| Cavity wall insulation | £3,900 | £139 | 28 years |
| Solid wall insulation, internal | £12,000 | £201 | Over 50 years |
| Solid wall insulation, external | £15,000 | £201 | Over 50 years |
| Suspended timber floor | £1,400 to £2,500 | About £85 (Trust estimate) | 16 to 29 years |
The data behind this table
Costs: Energy Saving Trust, typical professional installation before any grant: loft and cavity figures for a detached house (updated 8 May 2026), solid wall figures for a typical home (18 June 2026), suspended floors £1,400 to £2,500 depending on the house (19 May 2026). Savings: the median fall in gas use measured in real homes after each measure (DESNZ NEED Impact of Measures 2026, Table 1: loft 3.2%, cavity wall 12.0%, solid wall 17.3%), applied to a 4 bed detached house with average insulation at the October 2026 cap price of 7.97p per kWh. The loft figure is the median across all loft jobs, most of them top-ups, so insulating a bare loft saves more. Floor savings are the Trust's own estimates. Payback is cost divided by the yearly saving, at full price. Homes heated by oil, LPG or electricity save more in pounds.
Cavity or solid walls?
Check before you price anything. Homes built from the 1930s usually have cavity walls, which are far cheaper to insulate; brickwork showing the short ends of some bricks means solid walls. Your EPC records the wall type.
Solid wall insulation costs about £12,000 fitted inside or £15,000 outside, and saves about £201 a year here, so at full price it only pays with a grant. Read is cavity wall insulation worth it and solid wall insulation cost.
Loft and cavity together
About £5,000 for both. Homes that had loft and cavity wall insulation fitted together used a median 8.4% less gas, about £98 a year here; that is a different group of homes from the single-measure figures, so the two cannot be compared directly.
In our EPC model the loft adds about 7 points and cavity walls about 8, often enough to lift a home a band. Insulating first also shrinks the heat pump a 4 bed detached house needs, from 12 kW at average insulation to 10 kW well insulated. Plan the whole job with the retrofit plan, or check your own home in the insulation calculator.
Savings by size of detached house
Bigger homes save more. These are the measured savings a year for homes heated by gas, with average insulation to start with.
| Home, heated by gas | Loft | Cavity walls | Solid walls |
|---|---|---|---|
| 3 bed detached house | £30 | £112 | £161 |
| 4 bed detached house | £37 | £139 | £201 |
| 5 bed detached house | £50 | £186 | £269 |
Compare other types of home: semi-detached house, terraced house, bungalow or flat, or see them side by side in insulation cost by house type.
Grants that can pay for it
ECO4 until 31 December 2026, then council grants. ECO4 funds insulation for households on qualifying benefits, in homes rated D to G if owned or E to G if privately rented, and the work must be finished by then.
In England a Warm Homes: Local Grant through your council can help households on lower incomes, and insulation carries 0% VAT until 31 March 2027. Landlords can check the 2030 band C rules with the landlord EPC calculator.
Frequently asked questions
How much does it cost to insulate a detached house?
From about £800 to top up the loft to £12,000 for internal solid wall insulation, at Energy Saving Trust prices before any grant. Cavity wall insulation costs about £3,900 for a detached house.
Is cavity wall insulation worth it for a detached house?
Homes that had it fitted used a median 12.0% less gas, about £139 a year for a 4 bed detached house on gas, so at about £3,900 it takes 28 years to pay back at full price. It can be free through ECO4 for households on qualifying benefits, and it adds about 8 EPC points in our model.
Can I get insulation for my detached house free?
Possibly. ECO4 funds insulation for households on qualifying benefits until 31 December 2026, in homes rated D to G if owned or E to G if privately rented, and in England a Warm Homes: Local Grant through the council can help households on lower incomes. Insulation carries 0% VAT until 31 March 2027.
Data sources
- Energy Saving Trust, cavity wall, loft, solid wall and floor insulation advice, May and June 2026.
- DESNZ, NEED Impact of Measures 2026, 11 June 2026.
- Ofgem, energy price cap, October to December 2026.