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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 phenolic board.
Total sloping ceiling area to insulate
Depth of existing rafters
AD L Vol 1 2021 (inc. 2023 amendments) Table 4.2: 0.15 W/m²K for a new sloping ceiling in an existing dwelling
Material cost per m²
Vapour control layer cost
1 standard, 2 for fire rating
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How We Calculate This
This calculator gives an indicative insulation thickness to achieve the target U-value, based on the declared thermal conductivity (λ) of the chosen insulation.
The formula (single-layer estimate)
Thickness (mm) = (λ / Target U-value) × 1000
Between rafters = min(Rafter Depth, Total Thickness)
Over rafters = Total Thickness − Between Rafters
This is a quick rule-of-thumb that treats the build-up as one homogeneous layer of insulation. It ignores the internal/external surface resistances (roof Rsi ≈ 0.10, Rso ≈ 0.04 m²K/W), the plasterboard and vapour control layer, any ventilation gap, and — most importantly — thermal bridging through the timber rafters, which carry roughly 12–15% of a between-rafter area at λ ≈ 0.13 W/mK and raise the effective U-value. The actual U-value must be confirmed by a full BR 443 calculation (the method Approved Document L requires), and in practice you will usually need a layer thicker than the bare λ/U figure once bridging is accounted for. Always follow the manufacturer's certified system build-up and confirm with Building Control.
Insulation types (for U=0.15, single-layer estimate)
- Phenolic board (Kingspan K107): λ 0.019 — ≈127mm
- PIR board (Celotex/Kingspan): λ 0.022 — ≈147mm
- Mineral wool: λ 0.035 — ≈233mm
Frequently Asked Questions
A new sloping ceiling created in a loft conversion is a new thermal element in an existing dwelling, so under Approved Document L Volume 1 (2021 edition incorporating 2023 amendments), Section 10 and Table 4.2, it must achieve a roof U-value of 0.15 W/m²K. As a single-layer estimate, PIR insulation (λ=0.022) needs about 147mm to reach 0.15; the exact thickness must be confirmed by a BR 443 U-value calculation that accounts for surface resistances, plasterboard and timber rafter bridging.
Ideally both. Fill the rafter depth with insulation between rafters, then add a layer over the rafters (or under, using counter-battens) to reach the required U-value and reduce thermal bridging through the timber. A continuous layer over the rafters is more thermally efficient than between alone.
Yes. A vapour control layer must be installed on the warm side (room side) of the insulation to prevent moisture from the room reaching cold surfaces where it could condense. Use a minimum 500-gauge polythene sheet or proprietary VCL, lapped 150mm and sealed at all edges with tape.
For the thinnest build-up, phenolic board (Kingspan Kooltherm K107, λ=0.019) reaches U=0.15 in roughly 127mm, and PIR rigid board (Celotex GA4000, Kingspan TP10, λ=0.022) in about 147mm — both as single-layer estimates. Mineral wool is cheaper but needs around 233mm. These are indicative; confirm the real thickness with a BR 443 calculation and always check Building Control approval.
If the roof has a breathable membrane (modern felt), no ventilation gap is needed above the insulation. If the roof has a non-breathable traditional bitumen felt, a 50mm ventilation gap must be maintained between the insulation and felt, with ventilation at ridge and eaves.
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Last updated: March 2026
Verified against UK standards · estimates only, confirm with your supplier.