The lambda value (λ), or thermal conductivity, quantifies how readily an insulation material transfers heat. It is expressed in W/(m·K) and represents the heat flow through one metre of material thickness for a temperature difference of one Kelvin. The lower the λ value, the better the material's insulating performance.
Declared vs. Design Lambda
Two related but distinct figures appear on product documentation. The declared thermal conductivity (λD) is determined under standardised laboratory conditions and statistically evaluated so that a defined percentage of production conforms to the stated value. The design lambda (λU), by contrast, accounts for in-service conditions such as moisture uptake, aging, and installation effects, and is typically used by planners for actual building physics calculations. Relying on λD alone for design purposes can lead to underestimated U-values and energy performance gaps.
Typical Lambda Values by Material
Thermal conductivity varies considerably across insulation materials:
Expanded polystyrene (EPS): approx. 0.030–0.040 W/(m·K)
Extruded polystyrene (XPS): approx. 0.030–0.035 W/(m·K)
Mineral wool (glass or stone wool): approx. 0.032–0.040 W/(m·K)
Polyurethane (PUR/PIR) rigid foam: approx. 0.022–0.028 W/(m·K)
Vacuum insulation panels (VIP): approx. 0.005–0.008 W/(m·K)
Wood fibre insulation: approx. 0.038–0.045 W/(m·K)
These ranges depend on density, manufacturing process, and board thickness, and should always be verified against the specific product's technical data sheet rather than assumed from the material category alone.
Relevance for U-Value Calculations
The λ value is a direct input into the calculation of the U-value (thermal transmittance) of a building component. For a homogeneous layer, thermal resistance R is obtained by dividing the layer thickness (in metres) by λ. The sum of resistances of all layers, plus surface resistances, gives the total R-value, from which the U-value is derived as its reciprocal. Specifiers use this relationship to determine the insulation thickness required to meet a target U-value for walls, roofs, or floors.
Factors Influencing the Declared Value
Several variables affect the lambda value stated for a product: material density, cell or fibre structure, moisture content, mean operating temperature, and aging behaviour of the insulating medium (relevant particularly for foams with blowing agents). Manufacturers determine λD through testing under defined laboratory conditions, which may not fully reflect long-term in-situ performance, making the design lambda the more conservative and reliable figure for compliance calculations.
Coverage on This Platform
The distinction between declared and design lambda, and its practical consequences for insulation specification, is examined in How to Choose the Right Lambda Value for Your Insulation Project, which explains how overlooking this difference can create performance gaps relative to energy targets.