Side by side
| Multifoil | Mineral wool | |
|---|---|---|
| Thickness for a loft floor | Not recommended | 270mm |
| Thickness for a roof slope | 30–45mm + gaps (+ PIR) | 200mm+ between and under rafters |
| Cost, material | £15 – £30 per m² | £3 – £8 per m² at 270mm |
| Breathability | Vapour control layer | Fully breathable |
| DIY | Outbuildings only | Loft floors, yes |
| Under boarding | Can be used over joists | Crushes unless boards are raised |
Loft floor
Wool. Every time. The 270mm standard assumes it, it costs a fraction of foil, and a cold ventilated loft above it is exactly the condition wool likes.
Roof slope
If the rafters are deep and headroom is not an issue, wool between and PIR under is the budget route. If the loft is becoming a room, multifoil keeps the ceiling where you want it. Multifoil vs PIR covers the slope decision in more depth.
Floor or slope? Storage or room?
Check my homeTell us what the loft is for and we will show the right material before anyone sells you one.
Questions homeowners ask
Can I use multifoil on top of mineral wool?
On a loft floor, only under raised boarding with an air gap. On a slope, multifoil across the rafters over wool between them is a valid build-up if the ventilation is kept.
Sources and when they were last checked
- Energy Saving Trust — Roof and loft insulation — checked 2 September 2026