UWerten

UWerten is the plural form of U-Wert, a term used in German-language texts to describe the thermal transmittance of a building element. It expresses how much heat passes through a component per unit area for each degree of temperature difference between indoors and outdoors. Lower UWerten indicate better insulation and reduced heat loss.

UWerten are measured in watts per square meter per kelvin (W/(m²K)). The standard concept is that the heat transfer rate through an element equals the reciprocal of its total thermal resistance. For a simple plane wall, U = 1 / (Rsi + ΣRi + Rse), where Rsi and Rse are internal and external surface resistances and Ri are the conductive resistances of material layers (Ri = di/ki).

In practice, UWerten are calculated according to international and regional standards, such as ISO 6946 and EN ISO 10077, with specific procedures for windows and building assemblies. They are widely used in energy performance assessments, building codes, and certification schemes to compare designs and ensure compliance with regulatory requirements.

Typical value ranges vary by element and climate. Exterior walls often fall in the 0.15–0.30 W/(m²K) range, roofs around 0.12–0.25, and windows commonly between 0.8–2.5. Modern highly insulated systems and triple-glazed windows approach the lower end, depending on construction details.

UWerten are a fundamental but partial measure of building energy performance. They do not capture dynamic effects such as thermal mass or complex thermal bridges, nor do they reflect air leakage. For a full assessment, UWerten are used alongside other metrics like g-value, airtightness, and overall energy modeling. The term reflects the German rooting of heat-transfer calculations, with U as the heat transfer coefficient.