Lämpöläpäisykerroin

Lämpöläpäisykerroin, often denoted by the symbol U, is a measure of how well a building element, such as a wall, window, or roof, conducts heat. It quantifies the rate of heat transfer through a unit area of the element for a unit temperature difference between the environments on either side. The SI unit for the U-value is watts per square meter per kelvin (W/(m²·K)). A lower U-value indicates better thermal insulation, meaning less heat is lost from the building in winter and less heat enters in summer. Conversely, a higher U-value signifies poorer insulation and greater heat transfer.

The calculation of the U-value involves considering the thermal resistance of each layer within the building element. Thermal resistance (R) is the reciprocal of thermal conductivity, which measures how well a material resists heat flow. By summing the thermal resistances of all component layers, including air gaps and surface resistances, the total thermal resistance of the element is determined. The U-value is then the reciprocal of this total thermal resistance. Factors such as material properties, thickness of layers, and the presence of thermal bridges (areas with higher heat transfer) significantly influence the U-value. Building regulations often specify maximum U-values for different building elements to ensure energy efficiency and a comfortable indoor environment.