The windows U-factor measures heat loss, that is, the resistance of the window glass to the heat flow, and it is the most important information about windows in cold climates. It is expressed in units of Btu/hr-ft2-°F or W/m2-°C.
The lower the U-factor the better. The best windows have a U-factor of 0,3 or less. To attain such measures, the window should include features like double or triple-plane glass, low-e coating on the panes, argon gas filling, insulated frames and warm edge bridges.