The energy saving performance of a window is measured by the Window Energy Rating scale. The most energy efficient windows will achieve an A rating on the WER scale but this depends on the combination of the window frame used and all the components within the double glazing window. Improving the Window Energy Rating is dependant on minimising heat lost from the room and maximising use of solar heat energy.
To reduce heat loss, double glazing window components need to be made of materials that are low thermal conductors. Double glazing window with the highest performing energy ratings use a combination of Low E glass, warm edge spacer bars and an argon gas-filled cavity, all within a hermetically sealed double glazed unit.