How to choose the best Uw coefficient and what is it?

The value informing us about the amount of heat that escapes through the entire window is the Uw coefficient, i.e. the heat transfer coefficient. The lower its value, the better the thermal properties of the window. Windows with high energy efficiency have a Uw coefficient not exceeding 1.3 W/m2K, and windows that meet passive building standards have a Uw value not higher than 0.8 W/m2K. Leading manufacturers offer windows with even better thermal insulation, below 1.0 W/m2K or even 0.8 W/m2K, which are characterized by high thermal efficiency, energy savings and passive operation.