Heat transfer coefficient = U-value (formerly k-value): central unit of measurement when determining the heat loss of a building component. It indicates the amount of heat that passes through 1 m² per unit of time at a temperature difference of 1 Kelvin between the adjacent room and outside air. The smaller the U-value, the greater the thermal insulation. The unit of measurement is W/(m²K).

The heat transfer coefficient, also called the U-value, describes the insulation properties of a building component. The lower the coefficient, the better the thermal insulation.

The so-called heat transfer coefficient, also called U-value (U = unit of heat transfer) or thermal insulation value, is used to calculate the heat loss of a house. As soon as there is a difference in temperature between two sides of a building element, a heat exchange takes place. The U-value indicates the heat transfer through a layer of material (e.g. a window or a wall) from one fluid (e.g. a gas or a liquid) into a second fluid. Simplified: The U-value is a measure of how much heat is released to the outside through a building component and thus provides information about the thermal insulation of a window. Strictly speaking, it measures the amount of energy per area that flows through a solid body. The smaller the value, the better the insulation and the lower the heat loss. The U-value is particularly low, for example, with building materials such as softwoods (0.13 W/m2K), hardwoods (0.18 W/m2K) or bricks (0.23 W/m2K) – these materials are therefore well suited for thermal insulation. In contrast, materials such as acrylic glass (5.3 W/m2K) or iron (60 W/m2K) have a high U-value. The unit of the thermal insulation value W/m2K means “Watt per square metre and Kelvin”.