UwWert

UwWert, often written as Uw value, is the thermal transmittance of a window unit. It expresses how much heat flows through one square meter of the element for a one-degree temperature difference between indoors and outdoors. The unit is W/(m²·K). The Uw value applies to the complete window, including glazing, frame and edge effects.

Calculation and measurement: Uw can be determined by standardized testing in a guarded hot box or by calculation according to EN ISO 10077-1/2 and related standards. The result depends on glazing (Ug), frame (Uf), edge properties, and the relative area of glass and frame. Installed windows may differ from the tested value due to thermal bridges and installation quality.

Typical values: Modern double-glazed windows commonly yield Uw values around 1.0–2.0 W/(m²·K), while triple-glazed units often range from about 0.8–1.3. Older or poorly insulated windows may exceed 2.5. Choices such as warm-edge spacers, low-emissivity coatings, and gas fills can reduce Uw; frame material (PVC, wood, aluminium) also influences the result.

Use: Uw is used in energy performance certificates and building regulations to compare window performance and estimate heat losses and heating demand. It is distinct from the glazing’s Ug value and the frame’s Uf value, and lower Uw values contribute to higher energy efficiency in buildings.