How much does it cost to insulate a flat?
The short answer
- £950 for cavity wall insulation in a mid-floor flat, at Energy Saving Trust prices
- About £68 a year saved by it in a 2 bed flat on gas, a 14 year payback at full price
- £500 to £650 for loft insulation in a top floor flat
Updated 25 September 2026. Every figure on this page comes from our published model; see where our data comes from.
Insulating a flat costs about £950 for cavity walls, the Energy Saving Trust's figure for a mid-floor flat, up to £12,000 or more for solid walls. A top floor flat can also insulate its loft from about £500, and homes that had cavity wall insulation fitted used a median 12.0% less gas.
What each measure costs
Loft first, then the walls. For a 2 bed flat on gas, cavity wall insulation saves about £68 a year and pays back in 14 years; solid walls save more but cost far more.
| Measure | Typical cost | Saving a year | Payback |
|---|---|---|---|
| Loft insulation, bare loft to 270mm (top floor flat) | £650 | More than £18 | Under 36 years |
| Loft insulation, top-up from 120mm (top floor flat) | £500 | £18 | 28 years |
| Cavity wall insulation | £950 | £68 | 14 years |
| Solid wall insulation, internal | £12,000 | £98 | Over 50 years |
| Solid wall insulation, external | £15,000 | £98 | Over 50 years |
| Suspended timber floor (ground floor flat) | £1,400 to £2,500 | Not published | Not published |
The data behind this table
Costs: Energy Saving Trust, typical professional installation before any grant: loft and cavity figures for a mid-floor flat (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). The Trust prices no loft for a flat, so a top floor flat takes its mid-terrace loft figures. 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 2 bed flat 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 £98 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 (top floor flat)
About £1,600 for both. Homes that had loft and cavity wall insulation fitted together used a median 8.4% less gas, about £47 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 2 bed flat needs, from 6 kW at average insulation to 5 kW well insulated. Plan the whole job with the retrofit plan, or check your own home in the insulation calculator.
Savings by size of flat
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 |
|---|---|---|---|
| 1 bed flat | £14 | £53 | £77 |
| 2 bed flat | £18 | £68 | £98 |
Compare other types of home: semi-detached house, detached house, terraced house or bungalow, 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 flat?
About £950 for cavity wall insulation in a mid-floor flat, up to £12,000 for internal solid wall insulation, at Energy Saving Trust prices before any grant. A top floor flat can top up its loft from about £500.
Is cavity wall insulation worth it for a flat?
Homes that had it fitted used a median 12.0% less gas, about £68 a year for a 2 bed flat on gas, so at about £950 it takes 14 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 flat 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.