Basic theory

Lambda value/ thermal conductivity (λ)

The lambda (λ) value, also referred to as thermal conductivity, is a value indication how well a material conducts heat. It indicates the quantity of heat (W), which is conducted through 1 m² wall, in a thickness of 1 m, when the difference in temperature between the opposite surfaces of this wall equals 1 K (or 1 ºC). In practice λ is a numerical value expressed in terms of W/(mK). The lower the λ value, the better the insulation property of the material.

Examples at 10 ºC
Steel: λ = 50 W/mK
Concrete: λ = 1.6 W/mK
Glass: λ = 1.1 W/mK
Wood: λ = 0.12 W/mK
ROCKWOOL: λ = 0.033 W/mK

Most materials will increase in λ at rising temperatures


The transport of thermal energy through a structure is expressed by a coefficient, U (Thermal transmitance coefficient). It represents the flow of heat (in W) through 1 m² of a structure, when the difference between the two surrounding temperatures is 1 K (or 1 ºC). The thermal transmittance coefficient is expressed in W/(m²K). The lower the coefficient, the better the structure insulates.

R value

Thermal resistance, R, is a measure used in a construction. The R value is the reciprocal U-value. Increasing the thickness of an insulating layer increases the R-value.