Short answer
Between-rafter fixing keeps the ceiling at the rafter face and is chosen for headroom. It needs deep rafters — roughly 140mm for quilt, two air spaces and a ventilated void — and brings more joints and timber bridging than under-rafter fixing.
- For headroom, not performance
- ≈140mm rafters minimum
- Every bay is a joint
- Bridging through every rafter
When between-rafter makes sense
- Loft rooms where finished headroom is already at the 2.0–2.2m limit.
- Deep rafters (150mm+) where the quilt, both gaps and a ventilated void all fit inside the rafter depth.
- Re-roofing from above, where the quilt is laid from the outside.
The build-up inside the rafter
- Ventilated gap of 50mm below the membrane (25mm with a breathable membrane, depending on the certificate)
- 25mm still gap
- Multifoil quilt, cut to rafter spacing plus 100mm, folded and stapled to the rafter sides
- 25mm still gap to the plasterboard line, formed with battens or the remaining rafter depth
- Plasterboard
Add the numbers for a typical certificate: quilt 40mm + two air spaces of about 25mm + a 50mm ventilated void ≈ 140mm. Rafters shallower than that cannot take a between-rafter multifoil without counter-battening — at which point under-rafter is usually simpler.
Bridging and joints
The rafters themselves conduct heat through the layer. On 400mm centres timber can be 10–12% of the roof area, and a proper U-value calculation includes it. Every rafter bay is also a joint: each edge is stapled and taped to the rafter side. Continuity across dormers and at the ridge needs a tape strip over the rafter face.
Compared with under-rafter
| Between rafters | Under rafters | |
|---|---|---|
| Headroom lost | 0 – 25mm | 65 – 90mm |
| Thermal bridging | Through every rafter | Minimal |
| Joints | Every bay | Every roll width |
| Ventilation detail | Critical inside the bay | Rafter depth is the gap |
| Minimum rafter depth | 140mm+ | Any |
Depth and headroom are measured on the survey and the specification chooses the detail.
Questions homeowners ask
Can multifoil go between rafters?
Is between-rafter worse than under?
What ventilation gap is needed above it?
- SuperFOIL — product range — checked 2 September 2026
- GOV.UK — Approved Document L, Volume 1: Dwellings (current edition, as amended) — checked 2 September 2026
- BSI — BS 5250:2021 Management of moisture in buildings — checked 2 September 2026