Build up a wall, roof or floor layer by layer and get its U-value and total R-value.
Method per BS EN ISO 6946:2017; compliance check against Approved Document L.
Indicative material λ values are generic (CIBSE Guide A / BRE BR 443). Use manufacturer or BBA-certified data for reporting. For design guidance only. Always verify with a qualified engineer.
About this U-value / R-value builder
This free U-value calculator lets you build up a wall, roof or floor layer by layer and read off both its total thermal resistance (R-value) and its thermal transmittance (U-value) in W/m²K. It is aimed at building-services engineers, architects and self-builders who need a quick fabric check during design, before confirming the figure with a full condensation-risk and thermal-bridging assessment. Each layer is entered as either a material (thickness and conductivity) or a fixed air-gap resistance, and the surface resistances and Part L limit follow your chosen element type. Everything runs in your browser. Nothing is uploaded.
How the U-value is calculated
The total thermal resistance is the sum of the surface resistances, every material layer and every air gap: Rtotal = Rsi + Σ(d/λ) + ΣRairgap + Rse, where d is the layer thickness in metres (mm ÷ 1000) and λ is its thermal conductivity in W/m·K. The U-value is then simply U = 1 / Rtotal. This is the one-dimensional method of BS EN ISO 6946:2017 (Clause 6) for elements built from homogeneous layers. Internal surface resistances are taken from BS EN ISO 6946 Table 7 by heat-flow direction: 0.13 m²K/W for a wall (horizontal), 0.10 for a roof (upward) and 0.17 for a floor (downward), with an external Rse of 0.04 in all directions. The result is compared against the limiting U-value for new dwellings in Approved Document L 2021 (England), Volume 1, Table 4.1 (0.26 wall, 0.16 roof, 0.18 floor), or against a target value you enter.
Frequently asked questions
What is the difference between U-value and R-value?
R-value is thermal resistance: how well a layer or build-up resists heat flow (higher is better). U-value is the reciprocal of the total resistance, the rate of heat loss per square metre per degree (lower is better). This builder reports both: it sums the R-values of every layer and surface, then takes U = 1 / Rtotal.
Does this account for thermal bridging and repeating studs?
No. This is the simple one-dimensional ISO 6946 method for homogeneous layers, ideal for a quick check or a cavity wall. It does not apply the combined (upper/lower limit) method for bridged layers such as timber or steel studs, nor ΔU corrections for fixings or air gaps, so treat the figure as indicative for framed or bridged constructions.
Which lambda values should I use?
The built-in material presets are generic indicative conductivities from CIBSE Guide A and BRE BR 443. For a compliance submission or BBA-backed reporting, always enter the manufacturer's declared or certified λ value for the specific product, as these can differ from the generic figures.