When comparing windows, three values consistently appear in technical datasheets: Uw, Ug and Uf. These coefficients express the thermal resistance of each component of a window. The lower the value, the better the insulation.
Ug — the glazing coefficient
Ug measures heat loss through the glazing alone, expressed in W/(m²·K). A standard double glazing unit achieves Ug ≈ 1.1. High-end triple glazing drops to Ug ≈ 0.5. Swiss standard SIA 380/1 recommends Ug ≤ 0.7 for ambitious renovation projects.
Uf — the frame coefficient
Uf measures thermal losses through the frame and sash. A 76 mm PVC profile typically achieves Uf ≈ 1.2 to 1.4 W/(m²·K). The Kömmerling 88 MD reaches Uf ≈ 0.98. Aluminium profiles with thermal break from Aluprof and Aliplast achieve Uf ≈ 1.0 to 1.2.
Uw — the overall window coefficient
Uw is the most important value: it reflects the real-world performance of the installed window, accounting for glazing, frame and spacer bar. It is calculated to EN ISO 10077. For renovation projects in Switzerland, targeting Uw ≤ 1.0 is a good benchmark. For passive houses (Minergie-P), Uw ≤ 0.8 is required.
What these values mean for your project
A window with Uw = 1.4 lets through up to twice as much heat as one with Uw = 0.7. Across 20 windows in a home, this difference translates to hundreds of francs saved in heating costs each year. BO Fenster helps you select the right Kömmerling, Aluprof and Aliplast systems for the best balance of performance, budget and aesthetics.
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