Practical guide
R-value vs U-value
R belongs to one layer, while U belongs to the whole assembly. They are not competing numbers; they describe two levels of the same thermal story.
Use R for the layer, and U for the full wall, roof, or floor.
Lambda
Lambda describes the material itself. It tells you how well one material conducts heat before thickness is taken into account.
R-value
R-value describes a single layer or a part of the build-up. It grows when thickness grows and lambda drops.
U-value
U-value describes the complete assembly, including all layers and the surface resistances on both sides.
Simple interpretation
If two insulation boards have the same lambda, the thicker board has the higher R. But the final U still depends on the whole assembly, not on one board alone.
Use lambda to compare materials.
Use R to compare layer thickness and insulation effect.
Use U to check the final performance of the full construction.
Do not mix layer resistance and whole-assembly performance.
Continue with the calculator and guides
Calculateur
Build the layer stack and calculate the full U-value.
How to calculate U-value
Read the practical guide for lambda, R, and U.
Calculateur de mur
Check wall assemblies and insulation layers.
Calculateur de toiture
Review roof build-ups and roof insulation guidance.
Insulation thickness guide
See how thickness is chosen in practice.