Foil roof insulation: the pitched-roof build-up that keeps headroom

The build-up, cold side to warm

1. Tiles and battens, then the existing felt or breathable membrane.
2. Ventilated gap — 50mm to a non-breathable felt, 25mm to a breathable membrane.
3. Optional PIR between rafters (50–75mm) where the certificate calls for it.
4. Counter-battens forming a 25mm gap.
5. Multifoil quilt across the rafters, stapled, joints taped.
6. Battens forming a 25mm service gap.
7. Plasterboard and skim.

Total depth below the rafters is typically 65–80mm — against 100mm or more of PIR under the rafters for the same result. In a loft room that is the difference between a usable ceiling and a cramped one.

U-value target

For a pitched roof upgraded in England, Approved Document L sets 0.18 W/m²K for the insulated slope. A certified multifoil with 50mm PIR between rafters, or two quilts, typically achieves it. The product certificate tells you the exact combination. Building control will ask for it.

Where foil on a roof goes wrong

• Fitted tight to the felt with no ventilated gap — condensation on the felt and rafters within a winter.
• Joints taped with the wrong tape — the layer opens up, the vapour control fails.
• Foil pushed into the eaves, blocking the airflow the roof relied on.
• Single-layer bubble foil sold as a full roof insulation — it is not.

Cost

£45–£80 per m² of roof slope installed, or £2,000–£6,500 for a typical semi, more with a plastered finish. A full loft conversion is priced separately. Cost breakdown.

Related routes

• Room-in-roof insulation — the whole-room approach
• Rafter insulation — all materials, not just foil
• Loft conversion insulation

Questions homeowners ask