U-Value Calculator — Calculate Thermal Transmittance for Walls, Roofs & Floors

A U-value (thermal transmittance) measures how effective a building element is as an insulator. It is expressed in W/m²K — the lower the U-value, the better the insulation. UK Building Regulations Approved Document L 2021 Volume 1 Table 4.1 sets the limiting (maximum) U-values for new dwellings: external walls 0.26 W/m²K, all roof types 0.16 W/m²K, ground floor 0.18 W/m²K and party wall 0.20 W/m²K. When upgrading an existing element (renovation), the looser Table 4.3(b) limits apply: internal/external-insulated wall 0.30, cavity-insulated wall 0.55, roof 0.16 and floor 0.25 W/m²K.

Under Approved Document L 2021 Volume 1, the limiting U-values for new dwellings (Table 4.1) are: external walls 0.26 W/m²K, all roof types 0.16 W/m²K (there is no separate flat-roof figure — pitched and flat roofs share the 0.16 limit), ground floor 0.18 W/m²K and party wall 0.20 W/m²K. For new elements in an existing dwelling, Table 4.2 is stricter: roof 0.15, wall 0.18, floor 0.18. For upgrading a retained existing element, Table 4.3(b) is looser: internal/external-insulated wall 0.30, cavity-insulated wall 0.55, roof 0.16, floor 0.25. These are maximum values — lower is better. Use the Compliance Basis selector to pick new build or renovation.

Rsi is the internal surface thermal resistance and Rse (0.04 m²K/W) is the external surface resistance. They account for the thin layers of still air at the surface of an element. Per BR 443 (the UK convention to BS EN ISO 6946) Rsi depends on heat-flow direction: wall (horizontal) 0.13, roof (upward) 0.10 and floor (downward) 0.17 m²K/W; Rse is 0.04 in every case. This calculator sets the correct Rsi automatically when you choose the element type.

An unventilated air cavity of 25mm or more has a thermal resistance of approximately 0.18 m²K/W. On its own this provides limited insulation. Filling the cavity with insulation (mineral wool, blown bead or foam) dramatically improves thermal performance.

PIR (polyisocyanurate) boards such as Celotex or Kingspan have the lowest thermal conductivity at around 0.022 W/mK, meaning you need less thickness to achieve a given U-value. Phenolic foam (0.020 W/mK) is even better. Mineral wool (0.035 W/mK) is cheaper but requires greater thickness for the same U-value. Use our Insulation Thickness Calculator to compare materials.