Practical guide
How to read lambda, R, and U
The three numbers describe different parts of the same thermal story. Lambda belongs to the material, R belongs to the layer, and U belongs to the whole assembly.
Start with the material
Check the lambda value first. Lower lambda usually means a better insulating material, but the final result still depends on thickness and placement in the build-up.
Then look at the layer
Thermal resistance increases when thickness goes up and lambda goes down. That is why the same material can behave very differently in two assemblies.
Finally check the whole assembly
The U-value is the final number you use for the full wall, roof, or floor. It includes every layer plus the surface resistances at both sides.
The two equations behind the calculator
Layer resistance
R = d / lambda
Thickness is converted to meters, then divided by lambda to get the resistance of one layer.
Assembly U-value
U = 1 / (Rsi + sum(R layers) + Rse)
Surface resistances and all layer resistances are combined to produce the final U-value.
A simple wall example
| Layer | Thickness | Lambda | Resistance |
|---|---|---|---|
| Mineral wool | 150 mm | 0.035 W/mK | 4.29 m2K/W |
| Surface resistance | - | - | Included in U |
The exact result changes with the rest of the wall build-up, but this is the same logic used by the calculator for every layer stack.
The layer order matches the real construction.
Every material has a realistic lambda value.
Junctions, edges, and openings are considered in the detail design.
The target U-value is realistic for the project and climate.
The exported result includes the right project name and units.
Calculator
Go back to the main calculator and build a layered assembly.
Wall calculator
Read the wall-specific guide and examples.
Roof calculator
Check roof build-ups and roof insulation guidance.
Floor calculator
Review floor assemblies and slab edge details.
R-value vs U-value
See how layer resistance differs from full-assembly performance.
Insulation thickness guide
Work through thickness selection and build-up limits.