Insulation Thickness Calculator — How Much Insulation Do I Need?

This calculator determines the insulation thickness required to achieve a target U-value for your building element. It uses the standard thermal resistance calculation method from BS EN ISO 6946 and references UK Building Regulations Approved Document L.

The formula

Required insulation R = (1 / target U-value) - Rsi - Rse - existing construction R

Insulation thickness = R × thermal conductivity × 1000 (mm)

For example, to achieve 0.30 W/m²K on a cavity wall (base R = 0.45) with mineral wool (0.035 W/mK): Required R = (1/0.30) - 0.13 - 0.04 - 0.45 = 2.71 m²K/W. Thickness = 2.71 × 0.035 × 1000 = 95mm.

Building Regulations Part L targets (AD L Vol 1, 2021 ed. inc. 2023 amendments)

• External walls: 0.26 W/m²K (new-build limiting, Table 4.1); 0.18 W/m²K (new element in an existing dwelling or extension, Table 4.2); renovation of a retained wall (Table 4.3) — 0.55 by cavity insulation, 0.30 by internal/external insulation
• Roof (all types): 0.16 W/m²K (new-build limiting, Table 4.1 — a single "all roof types" value with no pitched/flat split); 0.15 W/m²K (new element in an existing dwelling or extension, Table 4.2); 0.16 W/m²K (renovation of a retained roof, Table 4.3)
• Ground floor: 0.18 W/m²K (new-build, Table 4.1; new element/extension, Table 4.2); 0.25 W/m²K (renovation of a retained floor, Table 4.3)

The thickness is rounded up to the nearest 5mm to match standard product sizes. Always check manufacturer datasheets for exact lambda values, as these can vary slightly between products.