Three loft situations, three answers
| Situation | Foil? | Better answer |
|---|---|---|
| Open loft, nothing stored, insulating the floor | No | Mineral wool to 270mm between and over joists |
| Loft used for storage with boarding | Sometimes | Wool between joists, raised boarding; a foil layer over the wool can add value without height |
| Insulating at rafter level to make a warm loft | Yes | Multifoil across rafters, or multifoil plus PIR between |
Why the floor is the wrong place
A loft floor has space. Space is exactly what multifoil is designed to save, so paying a premium for thinness where it is not needed makes no sense. The physics also works against it: the reflective faces need air gaps, and on a loft floor the “gap” above is the whole loft, which is fine, but the gap below is the ceiling plasterboard with no air movement — so half the product's performance is lost. Wool between the joists and a second layer across them is cheaper, warmer and what the 270mm standard assumes.
Tell us what the loft is for and we will show the right route before asking for contact details.
Foil between or under rafters
This is the good case. Insulating the slope turns a cold loft into a warm one: the tanks and pipes stop freezing, the space becomes usable, and a future conversion is already half done. Multifoil across the rafters with a 25mm gap each side, taped continuous, achieves a warm-roof build-up in under 100mm. Combine with 50mm PIR between rafters to hit the 0.18 W/m²K target if the certificate requires it. See foil roof insulation for the full detail.
Foil under loft boarding
Where a loft is boarded for storage, the board usually crushes the wool beneath it. Raised loft-leg boarding is the proper fix. Where height is genuinely tight, a certified multifoil laid over the joists beneath the boards, with the board battens providing the air gap, adds performance without adding height. It is a compromise and should be priced as one.
Cost
Rafter-level multifoil in a typical semi: £2,000–£6,500 installed. Loft floor wool to 270mm: £450–£1,600. The difference is the price of a warm loft rather than a cold one. Cost breakdown.
Questions homeowners ask
Can I put foil on top of existing loft insulation?
Is foil loft insulation cheaper than wool?
Which foil is best for a loft?
- Energy Saving Trust — Roof and loft insulation — checked 2 September 2026
- SuperFOIL — product range — checked 2 September 2026