R describes thermal resistance; U describes heat transfer per unit area. Check units and assembly boundaries before comparing values.
R and U
In SI units R is measured in m²·K/W and U in W/(m²·K). Higher R and lower U mean lower steady-state transfer under equivalent conditions.
Layers and surfaces
For a homogeneous layer R=d/λ. A simplified plane-element model gives U=1/(Rsi+ΣR+Rse); surface resistances must match the conditions, not a desired answer.
Non-uniform assemblies
Framing, fixings and mortar joints change performance compared with continuous insulation. The insulation resistance alone is not a whole-wall rating.
Which conductivity to use
Tie declared conductivity to the specific product and application. Design conductivity may reflect different moisture and temperature conditions from the laboratory declaration.
Example and calculator
An assumed 0.20 m layer with λ=0.040 W/(m·K) has R=5.0 m²·K/W. This is an arithmetic example, not a product claim; the calculator lets you vary assumptions.
Method limits
Layer arithmetic does not assess bridges, moisture, fire safety or structural behaviour. ISO 6946 has a defined scope; windows and ground heat transfer should not automatically be treated as ordinary walls.
| Item | Parameter | Check / result |
|---|---|---|
| R | m²·K/W | Higher means more resistance |
| U | W/(m²·K) | Lower means less transfer |
| λ | W/(m·K) | Material property |
What this means for your project
Retain thicknesses, conductivities, surface resistances and excluded bridges with the result. Otherwise the U-value cannot be reproduced.