U-values describe heat transfer through complete building elements, whereas declared thermal conductivity, λD, describes the insulation material. Designers also need the thermal resistance of each layer and the construction details. Understanding these three quantities helps you compare insulation without confusing material properties with whole-wall or whole-roof performance.
U-values and lambda λD — what are we comparing?
λD is the declared thermal conductivity of a material, expressed in W/(m·K). A lower λD allows a layer of the same thickness to provide greater thermal resistance. This is a product property determined under the conditions specified in the relevant standard and documentation, rather than a measure of the energy performance of a complete roof or wall.
How is the thermal resistance of a layer calculated?
For a homogeneous layer, the calculation is R = d / λ, where d is the material thickness in metres. For example, 200 mm of insulation with λD = 0.035 W/(m·K) gives a declared thermal resistance of approximately 5.71 m²K/W. This result applies only to that layer and does not include rafters, fixings or other materials.
Why is the U-value of the complete element different?
Thermal transmittance, or U-value, describes the complete assembly: finishes, structure, insulation, surface thermal resistances and the effect of repeating thermal bridges. Timber rafters and metal studs conduct heat differently from mineral wool, so simply taking the reciprocal of one layer’s thermal resistance does not provide a reliable U-value for a roof or wall.
What should you compare before buying?
- λD and the thickness of the specific product variant,
- the declared thermal resistance RD stated in the documentation,
- pack dimensions and coverage,
- the intended use and installation method,
- the design U-value of the complete building element.
You can compare the properties of available Eurowool variants on the product pages. The final insulation thickness should be confirmed in the building element design. Compare products
Technical reference
Requirements for building elements should be checked against the current Polish Technical Conditions for buildings, while the declared performance of mineral wool products is covered by the harmonised specification EN 13162.
