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. For a roof or a window, use the roof U-value calculatoror the window U-value calculator.
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.
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.
The three starting layers are a worked example, not verified product properties. Replace their thicknesses and design conductivities for your assembly. Every listed layer must be valid; remove an unused row explicitly.
The interactive calculator needs JavaScript. The formula and worked example below remain readable.
Homogeneous-layer series result using your entered values. Repeating bridges, cavities, corrections, junction losses and compliance are outside this calculation. Display values are rounded; intermediate calculations are not.
How to use this calculator
The formula
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
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
Treating the example conductivities as verified properties for your product, or leaving an incomplete layer in the assembly. Supply applicable design values for every layer; remove any unused row explicitly. This tool does not model bridges or cavities.
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.
It depends on the jurisdiction and the compliance route. For new dwellings in England, Approved Document L Volume 1 (2021 edition incorporating 2023 amendments) sets a notional-dwelling wall of 0.18 W/m²K for target setting (Table 1.1) and a limiting wall value of 0.26 W/m²K (Table 4.1); Scotland, Wales and Northern Ireland have their own documents. We hold no cited wall targets for the US energy code (IECC prescribes R-values and U-factors by climate zone), the EU, or Passive House, whose criteria are climate-specific, so none is listed here: compare the result with the verified target for your project yourself.
Use the declared design value on the product’s datasheet or the design values tabulated in EN ISO 10456 for the material and its moisture condition. We do not list typical conductivities here because they vary by product and density and we hold no cited table; the example values in the tool are placeholders to replace.
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
- ISO 6946:2017 homogeneous-layer series resistance method
- https://www.iso.org/standards.html
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.