Insulated glazing, more commonly known as double glazing is double glass window panes separated by a vacuum or other gas filled space to reduce heat transfer across a part of the building envelope.

Insulated glass units are manufactured with glass in range of thickness from 3 mm 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 manufactured with the same thickness of glass used on both panes[citation needed] but special applications such as acoustic attenuation or security may require wide ranges of thicknesses to be incorporated in the same unit.