Calculators · Consulting

U-value calculator

BS EN ISO 6946 · combined method for framing

Build up your construction layer by layer and get the U-value to BS EN ISO 6946, with surface resistances set automatically and the combined method for timber and steel framing. Exact maths; use real product λ-values for a real spec.

ISO 6946 the recognised standard method

Exact not an estimate

✓ checked against PHPP & our certifiers

Build-up

Element type

Rsi 0.13 · Rse 0.04

Layers · outside → insidethickness · resistance

Framing fraction (studs / joists)

U-value · BS EN ISO 6946

0.15W/m²K

Total resistance R_T = 6.72 m²K/W

Meets the Passivhaus rule of thumb (≤ 0.15)

Passivhaus (opaque, cool climate)≈ 0.15

Ireland Part L, wall backstop0.18

EnerPHit component≈ 0.35

Get this detail modelled

The calculation is exact to ISO 6946. The result is only as good as your λ-values, use certified product data for a real specification.

How it’s calculated

U = 1 / R_T, where R_T = Rsi + ΣR + Rse and each layer R = thickness ÷ λ.

Surface resistances are set by element type per ISO 6946: wall 0.13 / 0.04, roof 0.10 / 0.04, floor 0.17 / 0.04 m²K/W.
Unventilated air cavities ≥ 25 mm use 0.18 m²K/W.
Mark a layer as framed and it uses the ISO 6946 combined method, the average of the upper and lower resistance limits, so timber/steel studs are accounted for, not ignored.
λ-values shown are typical design values; override any with your product’s certified figure.

Why trust it: it’s the same method PHPP and SAP use, and every figure we issue is checked by one of our 7 accredited Passivhaus certifiers.

Take it further

Need this detail modelled, or the junctions checked?

A U-value is the easy part, the heat usually escapes at the junctions it never sees. Send us the build-up and we’ll model the thermal bridges and confirm it for certification.