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ISO 6946Layer build-upFree

Wall U-Value
Calculator

Build a wall assembly layer by layer and get its thermal resistance (R) and transmittance (U) per ISO 6946. Enter each layer's thickness and conductivity; the tool adds the interior and exterior surface resistances.

Who it's for: architects and energy consultants specifying external walls — enter each layer's thickness and conductivity at construction documents to get the assembly R-value and U-value per ISO 6946.

U = 1 / (Rsi + ΣR + Rse)
Each layer R = thickness (m) / λ (W/m·K). Default surface resistances: Rsi = 0.13, Rse = 0.04 m²K/W (horizontal heat flow). U-value in W/m²K — lower is better.
Assembly
R · U

How to use this calculator

1
Enter a layer name
Type a short label (e.g. "mineral wool") so the breakdown is readable — the name is optional but helps when you have many layers.
2
Enter thickness in mm and conductivity λ in W/mK
Thickness comes from your drawings or product data; λ (thermal conductivity) comes from the manufacturer datasheet or EN 12524. Mineral wool is roughly 0.032–0.040, brick 0.6–0.9, dense concrete 1.0–1.5 W/mK.
3
Add or remove layers with the + add layer button
Build the assembly from the outer leaf inward. The page pre-loads a typical cavity wall; replace, add, or remove layers to match your specification.
4
Adjust surface resistances if needed
Rsi (0.13 m²K/W) and Rse (0.04 m²K/W) are the ISO 6946 defaults for horizontal heat flow through a vertical wall. Change them only if a different standard or orientation requires different values.
5
Read the total R and U-value
A lower U-value means less heat loss per square metre. Compare the result against your local code minimum or passive-house target (typically ≤0.10–0.15 W/m²K).

The formula

Each wall layer contributes a thermal resistance R; the U-value is the inverse of the total resistance including both surface films.

Layer R = d / λ
U = 1 / (Rsi + ΣR + Rse)

d — layer thickness in metres (mm ÷ 1000)
λ — thermal conductivity in W/mK (from product datasheet)
Rsi — interior surface resistance, default 0.13 m²K/W (ISO 6946, horizontal heat flow)
Rse — exterior surface resistance, default 0.04 m²K/W (ISO 6946, horizontal heat flow)
ΣR — sum of all individual layer resistances
U — thermal transmittance in W/m²K

Worked example

Example
Default 4-layer cavity wall: 102 mm brick outer leaf (λ 0.77 W/mK), 100 mm mineral wool (λ 0.035 W/mK), 100 mm concrete block (λ 1.13 W/mK), and 13 mm plaster (λ 0.57 W/mK), with ISO 6946 surface resistances (Rsi 0.13, Rse 0.04 m²K/W).

Layer resistances: brick 0.132, mineral wool 2.857, block 0.088, plaster 0.023 m²K/W. Total: R = 3.27 m²K/W. Result: U = 0.306 W/m²K — a typical mid-spec cavity wall that meets many European national code thresholds.

When this estimate will be off

  • Assumes heat flows through all layers in series (the ISO 6946 simple method). Walls with repeating thermal bridges — metal-stud frames, timber studs, concrete columns — need the ISO 6946 combined or fractional-area method instead.
  • λ values from product datasheets are dry-lab declared values. In-service thermal conductivity can be 10–20% higher for some materials due to moisture uptake.
  • Unventilated air cavities must be entered as a fixed resistance (typically 0.18 m²K/W under ISO 6946), not as a thickness and λ — the air gap is not a solid layer.
  • Corner effects, junctions, and linear thermal bridges are not captured. A full facade U-value calculation per ISO 14683 is needed to account for those.

Frequently asked questions

A common mistake: Assumes heat flows through all layers in series (the ISO 6946 simple method). Walls with repeating thermal bridges — metal-stud frames, timber studs, concrete columns — need the ISO 6946 combined or fractional-area method instead.

For typical new construction, ≤0.25–0.30 W/m²K meets many European codes. UK Part L 2022 targets ≤0.18 W/m²K for new dwellings; passive house requires ≤0.10–0.15 W/m²K. The lower the U-value, the better the thermal performance.

λ is thermal conductivity — how readily a material conducts heat per metre of thickness, in W/mK. Find it on the product datasheet or in EN 12524 / national annexes. Mineral wool is roughly 0.032–0.040; brick 0.6–0.9; dense concrete 1.3–1.5 W/mK.

Rsi and Rse represent the still-air film on each wall face. ISO 6946 specifies 0.13 m²K/W interior and 0.04 m²K/W exterior for horizontal heat flow through a vertical wall. They are mandatory in a code-compliant U-value calculation, though their effect is minor for highly insulated assemblies.

Avoid this tool for walls with repeating thermal bridges (metal or timber stud frames, embedded columns), unventilated air cavities (use the fixed ISO 6946 cavity resistance instead), and roofs (which use different surface resistance values). ISO 6946 section 6.9 provides the combined method for bridged assemblies.

Sources

  • ISO 6946:2017 — Building components and building elements — Thermal resistance and thermal transmittance — Calculation methods (series method for homogeneous layers)

Sources and review record

Sources and method basis

Limitations

  • User supplies design conductivities and surface resistances; repeating/bridged layers, cavities, corrections, ground contact, air permeation, junctions, and compliance are excluded.

This free Wall Assembly U-Value Calculator is built and maintained by DataDrivenAEC, using the relevant codes and standards. It does not substitute for professional judgment.