Uwaarde

Uwaarde, commonly written as U-value, is the measure of thermal transmittance of a building element such as a wall, roof, door, or window. It expresses the rate at which heat is lost through the element per unit area for a given temperature difference between indoors and outdoors. The unit is watts per square meter per kelvin (W/m^2K). A lower U-value indicates better insulation and less heat loss.

For a multilayer element, the overall U-value is the inverse of the sum of thermal resistances: U = 1 / (Rsi + ΣRn + Rso). Rsi and Rso are interior and exterior surface resistances, and Rn are the resistances of each material layer. The calculation yields the rate of heat loss per square meter at a given ΔT and is used in energy assessments and product declarations.

U-values are used in energy performance calculations, building codes, and product labeling. They help compare insulation performance of walls, roofs, windows, and doors. Typical modern constructions aim for U-values around 0.15–0.30 W/m^2K for walls and roofs and around 0.8–1.5 W/m^2K for upgraded windows, with targets varying by climate, building type, and design.

Measurement and standards: U-values are determined by standardized tests in laboratories or through calculation methods that account for thickness, materials, installation conditions, and surface resistances. Results are published for individual components and used in energy performance certificates and product declarations.