Side by side
| Multifoil | PIR board | |
|---|---|---|
| Form | Flexible quilt, 30–45mm | Rigid foil-faced board, 25–150mm |
| Per-millimetre performance | Lower; relies on air gaps | Higher; conductivity around 0.022 W/mK |
| Depth below rafters for 0.18 | 65–80mm (with 50mm PIR between) | 100mm+ (PIR between and under) |
| Fit | Rolled across rafters; continuous | Cut to fit between rafters; gaps must be foamed and taped |
| Speed | Fast | Slower |
| Vapour control | Built in | Foil facing; joints must be taped |
| Cost installed | £45 – £80 per m² | £40 – £75 per m² |
Choose multifoil when
- Headroom in a loft room is tight.
- The roof is awkward — dormers, valleys, uneven rafters.
- Speed matters and the roof is being insulated from inside.
Choose PIR when
- Rafters are deep enough to take 100mm+ between them.
- You are building a warm roof from outside during re-roofing.
- Floors and walls where a rigid board is easier to fix and finish.
Not sure which your roof needs?
Check my homeSix questions, then a likely route and an indicative cost before any contact details.
Why installers combine them
PIR between the rafters uses the rafter depth; multifoil across them closes the thermal bridge at every rafter and provides the vapour control layer in one continuous sheet. The combination reaches 0.18 W/m²K with less depth below the rafters than PIR alone, and fewer cut-board gaps than PIR alone.
Questions homeowners ask
Is PIR better than multifoil?
Per millimetre, yes. Per centimetre of lost headroom, no. Choose on what the roof has room for.
Can I use multifoil instead of PIR under the rafters?
Yes — that is the standard combination. PIR between, multifoil under.
Sources and when they were last checked
- GOV.UK — Room in roof insulation best practice — checked 2 September 2026
- SuperFOIL — product range — checked 2 September 2026