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 (limiting value, same for renovations). Flat roofs have a limiting value of 0.18 W/m²K. The notional design target for pitched roofs at ceiling level is 0.11 W/m²K.

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).