What Insulation Thickness Do I Need?

Part L 2021 Limiting U-Values (England)

• Walls: 0.18 W/m²K (new build), 0.30 W/m²K (existing)
• Pitched roof (between rafters): 0.15 W/m²K
• Flat roof: 0.15 W/m²K
• Ground floor: 0.13 W/m²K (new build), 0.25 W/m²K (existing)

Thickness by Insulation Type

To achieve a wall U-value of 0.18 W/m²K:

• PIR board (Celotex/Kingspan, λ 0.022): 90–100mm
• Mineral wool (λ 0.035): 140–160mm
• EPS (λ 0.032): 130–150mm
• Phenolic foam (λ 0.020): 80–90mm

Roof Insulation Thickness

To achieve 0.15 W/m²K:

• PIR between rafters: 120–150mm
• Mineral wool at ceiling level: 270mm (two layers, 100mm + 170mm cross-laid)

Floor Insulation Thickness

• PIR below screed: 70–100mm (depends on floor perimeter-to-area ratio)
• PIR between joists: 100mm

Key Considerations

• Always check with Building Control for your specific project
• The actual U-value depends on the whole wall/roof build-up, not just the insulation
• Thermal bridging at junctions reduces overall performance
• Scotland, Wales, and Northern Ireland have different regulations

Last updated: April 2026