The short answer
Spray foam loft insulation in the UK typically costs around £30 to £50 per square metre, so a whole loft often falls between £1,500 and £5,000 depending on area, foam type and access. Closed-cell foam costs more than open-cell. It is far dearer than rolled mineral wool, and the cost is not the main concern: spray foam applied to the underside of the roof can cause condensation and timber problems if ventilation is wrong, and many mortgage lenders and surveyors now flag or refuse properties with it. Take independent advice before committing.
Spray foam is heavily marketed but widely misunderstood. Here is what it costs, how the two types differ, and the practical risks that make many surveyors wary.
Spray foam cost at a glance
- Typical rate£30–£50/m²
- Whole loft£1,500–£5,000
- Open vs closed cellClosed cell costs more
- Mortgage riskSome lenders decline
- Removal cost (if needed)Often several thousand £
What spray foam costs and what affects the price
Spray foam is a two-part polyurethane mix sprayed onto a surface where it expands and sets. In lofts it is usually applied to the underside of the roof slope (creating a warm roof) rather than across the joists. Because it needs a trained operator, specialist equipment and protective measures, the installed rate sits well above mineral wool — commonly £30–£50 per square metre of sprayed area. The sprayed area follows the pitched roof, which is larger than the loft floor, so totals climb quickly.
The main price drivers are the type of foam (closed-cell is denser, more expensive and more rigid; open-cell is lighter, cheaper and more breathable), the thickness applied, the size and pitch of the roof, and how much preparation and masking the job needs.
Open-cell versus closed-cell foam
The two foam types behave very differently, and the choice affects both cost and risk.
- Open-cell foam is softer and more vapour-open, allowing some moisture movement. It is cheaper per square metre but offers a lower insulating value for a given thickness.
- Closed-cell foam is rigid, denser and acts as a vapour barrier. It insulates more per millimetre and adds some rigidity, but trapping moisture against roof timbers is exactly the problem surveyors worry about.
| Type | Typical rate | Characteristics |
|---|---|---|
| Open-cell | £30–£40/m² | Softer, vapour-open, lower R-value per mm |
| Closed-cell | £40–£50/m² | Rigid, vapour barrier, higher R-value per mm |
| Mineral wool (for comparison) | £5–£12/m² | Cheaper, breathable, the conventional choice |
Indicative UK figures for guidance, 2026. Spray foam totals depend on sprayed roof area, not loft floor area.
The mortgage and removal cost you should factor in
The sticker price is not the whole cost picture. A growing number of mortgage lenders and valuers treat spray foam in the roof as a defect, particularly closed-cell foam sprayed onto rafters, because it can obscure the condition of the timbers and may indicate ventilation problems. That can make a property harder to remortgage or sell, and some buyers ask for the foam to be removed as a condition of sale.
Removal is costly — often running into several thousand pounds — because cured foam bonds tightly and may have to be stripped along with damaged felt or battens, sometimes requiring a partial re-roof. For most homes seeking straightforward energy savings, conventional 270mm mineral wool at the joists delivers the insulation benefit at a fraction of the cost and without the resale complications. If you are considering spray foam, get an independent survey and check your lender's position first.
Why mineral wool usually wins on total cost
It is worth separating the headline cost of spray foam from its lifetime cost. On the headline alone, spray foam is already several times dearer per square metre than mineral wool, and because it is applied to the larger pitched-roof area rather than the flat loft floor, the totals are higher again. Add the realistic possibility of paying for removal later — to satisfy a lender, a surveyor or a buyer — and the lifetime cost can dwarf the upfront figure.
By contrast, 270mm of mineral wool laid across the joists is cheap, breathable, easy to inspect over, and entirely uncontroversial with lenders and surveyors. It leaves the roof timbers visible and ventilated, so a future survey can assess them normally. For the great majority of UK homes whose aim is simply to cut heat loss and lower heating bills, the conventional floor-level layer achieves that at low cost with none of the resale or ventilation risks that come with foam. Spray foam may suit specific situations — for example certain non-traditional roof structures — but those are the exception, and even then an independent specialist and your lender should be consulted before any foam is applied.
Frequently asked questions
Is spray foam better value than mineral wool?
On insulation alone, no — mineral wool at 270mm costs far less per square metre and meets the recommended standard. Spray foam is dearer and carries mortgage and ventilation risks that mineral wool does not.
Will spray foam stop me getting a mortgage?
It can. Several lenders decline or restrict lending on homes with certain spray foam installations, especially closed-cell foam on rafters. Check with your lender and get an independent roof survey before installing it.
How much does it cost to remove spray foam?
Removal is labour-intensive and often costs several thousand pounds, sometimes more if roof felt or battens are damaged in the process and need replacing. That potential future cost is worth weighing against any upfront saving.
Sources & further reading
- Property Care Association — Spray foam insulation guidance
- Energy Saving Trust — Roof and loft insulation
Figures on this page are typical UK ranges drawn from published sources and depend on your specific loft. They are guidance, not a quotation.