What Insulation Thickness Do I Need?
Quick Answer
Walls: 90–100mm PIR • Roofs: 120–150mm • Floors: 70–100mm
Based on PIR board (λ = 0.022 W/mK) achieving Part L 2021 notional design targets (walls 0.18, roof 0.11–0.15, floor 0.13 W/m²K)
Part L 2021 U-Values (England)
Limiting values are the maximum permitted; notional values are the design targets used in the SAP calculation to demonstrate overall compliance.
- Walls: 0.26 W/m²K (new-build limiting), 0.18 W/m²K (notional target), 0.30 W/m²K (existing/renovation limiting)
- Pitched roof (at ceiling): 0.16 W/m²K (limiting), 0.11 W/m²K (notional)
- Flat roof: 0.18 W/m²K (limiting for new-build)
- Ground floor: 0.18 W/m²K (new-build limiting), 0.13 W/m²K (notional target), 0.25 W/m²K (existing/renovation limiting)
Thickness by Insulation Type
To achieve a wall U-value of 0.18 W/m²K (notional target):
- 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