UwWerte

Uw-Wert is the thermal transmittance of the entire building envelope. It expresses how much heat, in watts, is lost per square meter of exterior surface for each degree of temperature difference between indoors and outdoors. In practice, a lower Uw value indicates better insulation and reduced heating demand.

Calculation and components

The Uw value is typically calculated as a weighted average of the thermal transmittances of individual components (Ui) and their exposed areas (Ai) across the building envelope: Uw = sum(Ui * Ai) / A_total. Ai represents the exterior surface area of walls, roofs, floors, windows and doors, and A_total is the total exterior envelope area to which heat loss is attributed. The calculation may incorporate corrections for thermal bridges and, in more detailed methods, separate components such as the window unit (Uf for the frame and Ug for glazing) and their areas.

Applications and significance

Uw is a key metric in energy performance calculations and is used in energy certificates (Energieausweis) and building regulations in Germany and the broader European context. It provides a single figure to compare the overall insulation quality of different buildings or designs, guiding renovations and new construction toward lower energy consumption.

Typical values and influence

Uw values depend on climate, building type, and construction quality. Modern, well-insulated buildings often target Uw values around 0.15–0.25 W/(m²·K) or lower, while older or poorly insulated buildings may exceed 0.40 W/(m²·K). Windows usually have higher U-values than walls, so improving glazing can significantly reduce the overall Uw of a building envelope.