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Roof Insulation Calculator — U-Value Target & Thickness
Calculate insulation thickness to achieve your target U-value. Supports between-rafter and over-rafter buildup for mineral wool, PIR and EPS.
Total roof slope area to insulate
Depth of existing rafters
Price for one layer's worth of material per m²; cost is multiplied by the number of layers in the buildup
Thickness of any existing insulation
Gap above between-rafter insulation
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How We Calculate This
This calculator determines the insulation thickness needed to achieve your target U-value, based on the thermal conductivity (lambda, λ) of your chosen material and the standard roof surface resistances.
The formula
Per BS EN ISO 6946, total thermal resistance Rtotal = Rsi+ ΣRlayers + Rse, and U = 1 ÷ Rtotal. For a roof/ceiling the heat flow is upward, so Rsi = 0.10 and Rse = 0.04 m²K/W (combined 0.14).
Required total R = 1 ÷ Target U-value
Insulation R needed = Required total R − 0.14 − R(existing insulation)
Insulation thickness (mm) = Insulation R needed × λ × 1000 (rounded up)
Worked example — mineral wool (λ 0.044) to 0.16 W/m²K: required total R = 1 ÷ 0.16 = 6.25; insulation R = 6.25 − 0.14 = 6.11; thickness = 6.11 × 0.044 × 1000 ≈ 269 mm. The buildup is then split into layers (e.g. between- and over-rafter) and the pack/board count multiplies the per-layer coverage by the number of layers.
This is a simplified single-material method that excludes thermal bridging through the rafters/joists. A full BS EN ISO 6946 combined-method or SAP U-value calculation, allowing for the framing fraction, typically requires roughly 5–10% more insulation. Use it for sizing and follow the manufacturer’s data for compliance.
Working figures for this are collected in our Insulation U Values reference table.
Working figures for this are collected in our UK Insulation Lambda Values Reference reference table.
Frequently Asked Questions
Under the current in-force England Building Regulations (Approved Document L 2021, Volume 1: Dwellings), the limiting U-value for new roofs is 0.16 W/m²K — Table 4.1 sets this for ALL roof types, so pitched and flat roofs in a dwelling share the same 0.16 limit (there is no separate flat-roof figure; the 0.18 W/m²K value applies only to non-domestic buildings under AD L Volume 2). A new roof element in an existing dwelling or an extension must reach 0.15 W/m²K (Table 4.2), and a renovated roof is upgraded to 0.16 W/m²K (Table 4.3). The notional new-build design target is 0.11 W/m²K.
Thickness depends on the insulation type. Mineral wool at 0.044 W/mK needs approximately 269mm to reach a 0.16 W/m²K U-value; PIR board at 0.022 W/mK achieves the same with about 135mm. The calculator subtracts the roof surface resistances (Rsi 0.10 + Rse 0.04 = 0.14 m²K/W, upward heat flow per BS EN ISO 6946) and accounts for each material’s thermal conductivity. These are simplified single-material figures and exclude rafter thermal bridging, so a full SAP/U-value calculation typically needs around 5–10% more.
Between-rafter insulation fits in the space between rafters (typically 100–150mm deep). If this isn’t enough, additional insulation is added over the rafters (or under, at ceiling level). For a cold pitched roof BS 5250:2021 requires a minimum 50mm ventilated air gap above between-rafter insulation.
Yes. For cold pitched roofs, a minimum 50mm ventilation gap is required above the insulation (between rafters) to prevent condensation. This reduces the available space for insulation. Warm roof designs may not need this gap. Use our Roof Ventilation Calculator to work out vent quantities for your roof.
Lambda (λ) is the thermal conductivity of an insulation material, measured in W/mK. Lower lambda means better insulation. Mineral wool is ≈0.044, PIR is ≈0.022 (twice as effective per mm).
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Last updated: March 2026
Verified against UK standards · estimates only, confirm with your supplier.