The thermal performance of an insulation product can be expressed in different units. Below is a brief overview:
| Unit | |
| Lambda | W/m.K |
| Rd-value | (m².K)/W |
| Rc-value | (m².K)/W |
| U-value | W/(m².K) |
| Meaning |
| The λ-value expresses how much energy passes through a surface of 1 m² with a thickness of 1 m per second, per Kelvin temperature difference between both sides of the surface. |
| Thermal resistance of a material (R declared) R = d / λ D = Thickness of the material in meters λ = Thermal conductivity coefficient of the material |
| Thermal resistance of a construction (R construction) Rc = Sum of the Rd-values of the different layers in the construction+Rsi+Rse |
| Thermal transmittance coefficient |
| Explanation |
| The lower the thermal conductivity (λ-value), the better the thermal performance of an insulation product. A more efficient insulation layer is characterized by a lower λ-value. |
| The higher the thermal resistance (R-value) of a construction, the better its thermal performance. A more efficient insulation layer results in a higher R-value. |
| The higher the thermal resistance (R-value) of a construction, the better its thermal performance. A more efficient insulation layer results in a higher R-value. |
| The higher the thermal resistance (R-value) of a construction, the better its thermal performance. A more efficient insulation layer results in a higher R-value. |