A double-glazed window is a type of window having two layers (panes or glazing) of glass separated by an air space. Each layer of glass and surrounding air space reradiates and traps some of the heat that passes through thereby increasing the window's resistance to heat loss (see R-value). In factory-made double glazing units, the air between the glass sheets is thoroughly dried and the space is sealed airtight, eliminating possible condensation and providing superior insulating properties.
The gap between the two glass panels should be a minimum of 9 millimeters and is usually filled with still air. The performance of double glazing windows can be increased by approximately 20% with a U-value of under 2.0 W/m2K by filling the space with gas instead of still air, such as argon or SS6 gas. Another way to reduce heat loss through glazing is to use low-emissivity glass. The glass has a special coating on the inside of the air space which reflects radiant heat back inside the building, hence the transmission of radiant heat from the warmer glass at the inside from the house to the colder glass at the outside gets reduced.