Thermal resistance calculator
R resistance, U-value and indicative class of a multi-layer wall.
Result
Indicative calculation, run in your browser. No data is sent anywhere.
The thermal resistance of a wall is the sum of its layer resistances, each being its thickness divided by its conductivity. To that you add the inner and outer surface resistances, which represent the thin layer of still air on each side.
The U-value is the inverse of that total resistance: the lower it is, the less heat the wall lets through. Comparing two build-ups amounts to comparing two U-values, which this calculator lets you do by changing one thickness.
You can also let the plan supply these figures for you.
External insulationFrequently asked questions
Where do I find a material's lambda?
On the product data sheet, expressed in W/(m·K). The picker offers common values to rough things out, but a specific insulation can depart noticeably from its family average: enter the manufacturer's value if you have it.
Is the displayed class an energy rating?
No. It is an indicative mapping between a wall's U-value and a letter, to place an order of magnitude. An energy assessment covers the whole building, its systems and its use, not a single wall.
Are thermal bridges taken into account?
No, the calculation covers the plain part of the wall. Framing, lintels, floor junctions and window reveals degrade the real performance, sometimes markedly: that is the job of a full thermal study.