How Much Roof Insulation Do I Need?

Loft Insulation (Cold Roof)

For a standard cold loft (insulation at joist level, ventilated loft space):

• Mineral wool (glass or rock): 270mm total — typically 100mm between joists + 170mm cross-laid on top
• Rolls per loft: ~6 rolls (1200mm × 8m) for a 40–50m² loft area
• Coverage per roll varies — check the packaging for m² per roll

Room-in-Roof Insulation (Warm Roof)

For a habitable loft or loft conversion (insulation between and below rafters):

• PIR rigid boards (e.g., Celotex, Kingspan): 120–150mm between rafters + 25–50mm below
• Mineral wool between rafters: 200–250mm, but requires a vapour control layer
• Leave a 50mm ventilation gap above the insulation if using a breathable membrane, or use the manufacturer’s guidance for non-ventilated warm roofs

Part L 2021 U-Value Targets

• New build roof: 0.16 W/m²K
• Existing roof (renovation): 0.16 W/m²K where practically achievable
• Flat roof (new or replacement): 0.18 W/m²K

Tips

Do not compress mineral wool — it relies on trapped air for insulation value. If you need to store items in the loft, use loft legs to raise boarding above the insulation layer. Ensure no gaps around hatches, pipes, and cables.

Last updated: April 2026. Thickness requirements depend on the insulation material’s thermal conductivity (λ value). Always check Part L and the product data sheet.