Short answer
A multifoil quilt is 30–45mm (thicker products up to ≈100mm) but installs at about 80–95mm once its air spaces are counted — still far less than the board or wool needed for a similar target. Physical thickness is not thermal equivalence.
- Quilt 30–45mm, installed ≈90mm
- Thin ≠ equivalent
- Shallow rafters push the quilt under
- Loft floors: 270mm wool
Product thickness vs installed depth
| Product | Product thickness | Installed depth (with gaps/battens) |
|---|---|---|
| Bubble foil | 3 – 8mm | 45 – 55mm |
| Multifoil quilt | 30 – 45mm | 80 – 95mm |
| Multifoil + 50mm PIR between rafters | 80 – 95mm | 130 – 145mm |
| PIR alone, rafter level to a 0.18 target | 120 – 150mm | 145 – 175mm incl. ventilation gap |
| Mineral wool, rafter level to a 0.18 target | 200 – 250mm | 250mm+ incl. ventilation gap |
Physical thickness comparison — not equivalent thermal performance
These bars illustrate representative physical depths only. They do not imply that the products provide equivalent U-values or thermal resistance. Required thickness depends on the entire construction and target performance.
Representative installed depths for a pitched-roof upgrade towards a similar target. Physical thickness is not thermal equivalence: 40mm of multifoil does not automatically equal any given depth of PIR or wool, and the actual performance of each depends on the certified complete build-up and the timber bridging in your roof. U-value guide.
See how rafter depth changes the route in a typical cold-loft project →
Shallow rafters
Many older UK roofs have 75–100mm rafters. Options, in order of preference: multifoil under the rafters with the gap above formed by the rafter depth; counter-battening to deepen the rafters; PIR between rafters plus multifoil under; or, where headroom allows, sacrificing ceiling height for board. The survey measures rafter depth and available headroom and the specification chooses.
Loft floors
Thickness is not the constraint on an open loft floor: 270mm of mineral wool is the standard and there is room for it. Foil on a loft floor is only about boarding height. Loft depth guide.
The survey measures rafters, headroom and existing layers so the right thickness can be specified.
Questions homeowners ask
How thick is multifoil insulation?
Is 40mm of multifoil enough?
Can I use foil if my rafters are only 75mm?
- GOV.UK — Approved Document L, Volume 1: Dwellings (current edition, as amended) — checked 2 September 2026
- Energy Saving Trust — Roof and loft insulation — checked 2 September 2026
- SuperFOIL — product range — checked 2 September 2026