An IGU has two or more panes of glass with a factory sealed air (or gas) gap between, and is often known as Double Glazing (DG).
A single pane of glass has very poor thermal resistance. The addition of an airgap increases the insulation value significantly by eliminating heat transfer since air is a very good insulator. An IGU works to insulate in both winter and summer conditions and reduce the cost of heating and cooling in buildings. The measurement of thermal performance is known as the U-Value, and this can range from 3 down to 1 W/m2K depending on the air space and glass type.
Common gases used in the sealed gap are air, argon and krypton. Air is the most common as it is cheaper and more widely available. Argon and krypton have better insulating properties than air.
The glass layers can include performance low E glass, which reduces the U value and increases thermal performance.
IGUs provide many advantages to a building's envelope: reduced heat loss, heat gain, condensation, reduced noise and potentially reduced mechanical plant - and above all, increased occupant comfort.