Insulating glass (IG), more commonly known as double glazing (or double-pane, and increasingly triple and quadrupleglazing/pane), consists of two or more glass window panes separated by a vacuum or gas filled space to reduce heat transferacross a part of the building envelope.

Insulating glass units (IGUs) are manufactured with glass in range of thickness from 3 to 10 mm (1/8″ to 3/8″) or more in special applications. Laminated or tempered glass may also be used as part of the construction. Most units are produced with the same thickness of glass used on both panes but special applications such as acoustic attenuation or security may require wide ranges of thicknesses to be incorporated in the same unit.