Short answer
Under-rafter fixing runs the quilt across the underside of the rafters: continuous layer, fewer joints, rafter depth forming the upper air space, about 65–90mm of headroom used. It is the standard multifoil detail for UK loft rooms.
- Continuous, fewer joints
- Rafter depth = upper space
- ≈25mm battens inside
- Add PIR to hit a target
The build-up, room to sky
- Plasterboard on 25mm battens
- 25mm still air gap (the batten depth)
- Multifoil quilt, joints overlapped and taped, tensioned across the rafters
- Rafter depth: still air gap of 25mm minimum, then either nothing (cold roof) or PIR between rafters leaving a ventilated gap above
- Ventilated void and membrane, roof covering
Why under rather than between
Under-rafter fixing covers the rafters, so the timber is not a thermal bridge through the foil layer; joints are fewer and easier to tape; and the rafter depth becomes the upper gap for free. It costs 65–90mm of headroom. Between-rafter fixing preserves headroom but multiplies joints and bridging. Between rafters.
Fixing sequence
- Check the roof: ventilation path, membrane, timber condition, services.
- Fit PIR between rafters first if specified, leaving the ventilation gap above.
- Roll the quilt horizontally across the rafters from the eaves up, stapling every 100–150mm, 50–100mm overlaps on joints, all taped.
- Seal to walls, dormer cheeks and around penetrations with tape.
- Fix 25mm battens through the quilt into the rafters, plasterboard onto battens.
Adding PIR
Quilt alone typically gives a roof around 0.3 W/m²K; with 50mm PIR between rafters the build-up can reach 0.18. Whether PIR is needed depends on the target and the rafter depth — with 100mm rafters, 50mm PIR still leaves a 50mm ventilated gap. U-value guide.
Cost
Whether Building Regulations approval is required depends on the nature and scope of the work. Some insulation work within existing roof spaces may not itself require an application, while renovation of a thermal element, structural alterations, conversions or other controlled work can trigger requirements. Transitional arrangements and the date and scope of a project can affect which edition of the guidance applies. Check property-specific projects against current Building Regulations guidance and, where appropriate, local building control.
Multifoil under rafters, installed: roughly £45–£80 per m²; with PIR between, £60–£110 per m². Cost guide.
Survey measures rafters, headroom and ventilation; the quote sets out this build-up with a named product and certificate.
Questions homeowners ask
Does multifoil go under or between rafters?
How much height do I lose?
Can I fit plasterboard straight onto the foil?
- SuperFOIL — product range — checked 2 September 2026
- YBS Insulation — SuperQuilt technical information — checked 2 September 2026
- TLX Insulation — TLX Silver technical information — checked 2 September 2026
- GOV.UK — Approved Document L, Volume 1: Dwellings (current edition, as amended) — checked 2 September 2026