What is the U-value?

The U-value (thermal transmittance) indicates how much heat flows per second through 1 m² of a building element when there is a temperature difference of 1 K between inside and outside. Unit: W/(m²·K). A lower U-value means better thermal performance.

Calculation (EN ISO 6946)

For plane building elements with homogeneous layers:

U = 1 / R_total where R_total = Rsi + Σ (d / λ) + Rse

• d — layer thickness in m
• λ — thermal conductivity of the layer in W/(m·K)
• Rsi, Rse — internal/external surface resistance in m²·K/W (default values depending on heat-flow direction per EN ISO 6946)

The calculator on the home page implements this formula. It provides a simplified estimate: thermal bridges and inhomogeneous layers (EN ISO 6946, section 6) are not taken into account.

Placeholder scaffold. For formal proofs, use verified λ-values (e.g. from the declaration of performance under the CPR or from the EPD) and, where necessary, a full calculation per EN ISO 6946.