Loft Conversion Insulation Calculator — U-Value & PIR Thickness
Calculate insulation thickness between and over rafters to achieve Part L U-value of 0.15 W/m²K using PIR, mineral wool or multifoil.
Total sloping ceiling area to insulate
Depth of existing rafters
AD L Vol 1 2021 Table 4.2: 0.16 W/m²K for extension/conversion pitched roof
Material cost per m²
How We Calculate This
This calculator determines insulation thickness needed to achieve the target U-value based on the chosen insulation type.
The formula
Thickness (mm) = (Lambda / Target U-value) × 1000
Between rafters = min(Rafter Depth, Total Thickness)
Over rafters = Total Thickness - Between Rafters
Insulation types
- PIR board: λ 0.022 — ~150mm for U=0.15
- Mineral wool: λ 0.035 — ~233mm for U=0.15
- Multifoil hybrid: λ 0.030 — ~200mm for U=0.15
Frequently Asked Questions
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Last updated: March 2026
All calculations are estimates. Verify with your supplier.