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ISO 6946:2017Homogeneous layersFree

U-Value
Calculator

Calculate total thermal resistance and U-value for homogeneous wall layers in series. Supply project-appropriate thicknesses and design conductivities; the result does not determine code or certification compliance.

Who it's for: architects and energy designers screening a homogeneous wall build-up. Enter each layer's thickness and conductivity; the tool returns total R-value and U-value without selecting a regulatory target, typically at schematic design.

1
Add wall layers
From exterior to interior: cladding, insulation, structure, finish
2
Enter thickness + conductivity
λ (lambda) values in W/m·K — common materials pre-loaded below
3
Review the bounded result
Compare the calculated U-value with a separately verified project requirement
✓ ISO 6946 homogeneous-layer series method✓ Visible surface-resistance assumptions✓ No automatic code or certification verdict
Input boundary

Use design conductivity values from the applicable product data or project method. Do not use this series calculation for repeating bridges, cavities, ground contact, or junction losses.

Wall Assembly — U-Value
Layer-by-layer · ISO 6946

How to use this calculator

1
Add your wall layers
From exterior to interior: cladding, insulation, structure, and internal finish. Use one row per material.
2
Enter each layer thickness
In millimetres, measured through the assembly (the "Thickness mm" column).
3
Enter each conductivity λ
In W/m·K from the material datasheet. Common values are listed on the page: PIR 0.024, mineral wool 0.038, dense concrete 2.0, brick 0.77.
4
Confirm the surface resistances
The page starts with Rsi 0.13 and Rse 0.04 m²K/W for horizontal heat flow through a vertical wall. Change them only when the applicable method supports another condition.
5
Calculate
The tool sums each layer R = thickness ÷ λ, adds the entered surface resistances, then inverts the total: U = 1 ÷ ΣR.
6
Review the calculation
Use the layer breakdown to verify the inputs. Determine any code, certification, or project target separately for the applicable jurisdiction and assembly.

The formula

Each wall layer resists heat flow in proportion to its thickness and in inverse proportion to how conductive it is. Sum every layer resistance plus the air-film resistances at each face, then invert the total to get the U-value.

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

d — layer thickness in metres (enter in mm; the tool divides by 1 000)
λ — thermal conductivity of the material (W/m·K), from the product datasheet or reference table
Rsi — interior surface resistance: 0.13 m²K/W (ISO 6946, horizontal heat flow)
Rse — exterior surface resistance: 0.04 m²K/W (ISO 6946, horizontal heat flow)
ΣR — sum of all individual layer resistances

Worked example

Example
A wall stacks a 200 mm concrete block (λ = 0.51), 150 mm insulation (λ = 0.024), and 12.5 mm plasterboard (λ = 0.25). Each layer R = thickness ÷ λ: 0.392, 6.250, and 0.050 m²K/W. Adding Rsi 0.13 and Rse 0.04 gives total R = 6.862 m²K/W. U = 1 ÷ 6.862 = 0.146 W/m²K. This is a method result, not a compliance verdict.

When this estimate will be off

  • Uses the homogeneous-layer series method. Repeating or point thermal bridges, fixings, studs, cavities, air movement, ground contact, and junction losses are not modelled.
  • The starting surface resistances are for horizontal heat flow through a vertical wall. The user must select values applicable to the actual direction and boundary condition.
  • Conductivity values must be project-appropriate design values from the applicable product data or method; the calculator does not supply or validate them.
  • Layer thicknesses are assumed uniform across the wall. Tapered insulation, drained cavities, or elements that vary in section each require a separate calculation for that zone.

Frequently asked questions

A common mistake: Uses the homogeneous-layer series method. Repeating or point thermal bridges, fixings, studs, cavities, air movement, ground contact, and junction losses are not modelled.

A U-value is the rate of heat loss through a building element, in watts per square metre per kelvin (W/m²K). It states how much heat passes through one square metre of wall, roof, or window for each degree of temperature difference inside to outside. Lower is better: a 0.15 W/m²K wall loses half the heat of a 0.30 wall.

They are reciprocals. R-value measures thermal resistance (how well a layer resists heat flow); U-value measures conductance (how readily heat passes). U = 1 ÷ ΣR, where ΣR sums every layer resistance plus the surface resistances. Higher R means lower U. A wall with total R of 6.86 m²K/W has U = 0.146 W/m²K.

No. The applicable target depends on the adopted code, project type, climate, certification method, assembly definition, and local amendments. This calculator returns only the bounded thermal-transmittance calculation.

Rsi and Rse are the thin air films on each face of a wall that add thermal resistance. ISO 6946 gives a horizontal-flow interior surface resistance Rsi of 0.13 m²K/W and an exterior Rse of 0.04 m²K/W. Both are added to the layer resistances before inverting, so total R = Rsi + ΣR(layers) + Rse.

Sources

  • ISO 6946:2017 — Building components and building elements — Thermal resistance and thermal transmittance — Calculation methods. This page implements only homogeneous layers in series with user-visible surface resistances.

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, target selection, and compliance are excluded.

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