An insulated glass unit (IGU) is a window that consists of two or more panes of glass with a sealed air gap between the panes. The air gap acts as insulation and can either be vacuum sealed or filled with an inert gas such as argon or krypton. The inert gas filling reportedly helps increase the insulating properties.
Most insulated glass units are double-pane, but some areas of the country require further insulation using triple-pane units or even quadruple pane units. Insulated glass units are also known as insulating glass units.