Facts
The two elementary energy saving values
Thermal conductivity lambda value (λ-value)
The lambda-value denotes the material-specific thermal conductivity of a building material. It indicates the amount of heat in watts that passes through the material of 1m in an hour, with a temperature difference of one Kelvin (K = Kelvin= °C). The smaller the lambda value, the better the insulation. Its unit is watts per meter and Kelvin [W/mK].
Heat transfer coefficient U-value
The U-value (aka. Insulation value, formerly k-value) is a measure of the heat flow passing through a one-or multi-layered material, if there is a temperature differential on both sides. It indicates the amount of energy per unit time, which flows through an area of 1 m2, when the difference between the outside and inside air temperatures differ by a stationary 1 K. Units are watts per square meter and Kelvin [W/m2K]. The heat transfer coefficient is a specific characteristic of a component including all layers and materials. It is essentially determined by the thermal conductivity and thickness of the used materials, but also by the direction of the heat flow (e.g. horizontally in the outer wall).
