How do you calculate R and U-value?
Each layer has a resistance R = thickness ÷ λ (lambda). The construction’s R-value is the sum of all layers. The U-value is U = 1 / (Rsi + R + Rse), where Rsi and Rse are the inside and outside surface resistances (together 0.17 for a wall, 0.14 for a roof). Higher R and lower U mean better insulation.
Frequently asked questions
What is the difference between R and U?
R (m²K/W) is the thermal resistance: higher = better. The U-value (W/m²K) is the inverse including surface resistances: lower = better.
What is a good U-value?
New-build external walls are often required at U ≤ 0.24 W/m²K, roofs stricter. Check the applicable regulation.
Which lambda do I use?
Use the product’s certified λD value. The presets here are typical guide values; the real figure is on the datasheet.
Does an air cavity count?
A still air cavity gives a fixed resistance (about 0.17 m²K/W at 25–50 mm). This calculator uses material layers; add a cavity separately if needed.