What U-value do I need for a roof?

Current UK Building Regulations (Part L 2021) require a maximum U-value of 0.16 W/m²K for new pitched roofs. The 2025 update targets 0.11 W/m²K. Renovations may use a relaxed target of 0.20 W/m²K where space is constrained.

How thick does roof insulation need to be?

Thickness depends on the insulation type. Mineral wool at 0.044 W/mK needs approximately 275mm for 0.16 U-value. PIR board at 0.022 W/mK achieves the same with only 140mm. The calculator accounts for these different thermal conductivities.

What is between-rafter and over-rafter insulation?

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). A 50mm air gap above between-rafter insulation is required for ventilation.

Do I need a ventilation gap above 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.

What is lambda value?

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).

TradeCalculator

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.

Roof Area

Total roof slope area to insulate

Insulation Type

Affects required thickness

Target U-Value

Building Regs requirement

Rafter Depth

Depth of existing rafters

Price Per m\u00b2 (optional)

How We Calculate This

This calculator determines the insulation thickness needed to achieve your target U-value, based on the thermal conductivity of your chosen material.

The formula

Required R-value = 1 ÷ Target U-value

Insulation thickness (mm) = Required R × Lambda × 1000

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.