U-value

U-value measures how well a building material or assembly, like a wall or window, insulates by indicating the rate of heat transfer through it. The lower the U-value, the better the material is at preventing heat from escaping in winter or entering in summer, helping to keep indoor temperatures stable. It is expressed in units of watts per square meter kelvin (W/m²·K). A building with low U-values is more energy-efficient, as it requires less heating or cooling to maintain comfort. This measurement helps compare and select materials that improve a building’s thermal performance.