The whole-window value
Uw describes the thermal transmittance of a complete window. It is expressed in W/(m²·K). A lower value means less heat passes through a given area for the same temperature difference.
More than glass
Uw brings together the glass value, Ug, the frame value, Uf, and heat transfer at the glass edge. The proportions of glass and frame matter, so dimensions belong alongside the quoted value.
Compare like for like
Ask for values covering the same window size and configuration. Record the glazing and spacer specification with the figure, keeping a clear connection between the proposed opening and its supporting technical information.
