Insulated glazing (IG) also known as double glazing are double or triple glass window panes separated by an air or other gas filled space to reduce heat transfer across a part of the building envelope.

Glass in windows is used to provide light and allow a view from either side to the other side. While the composition and manufacturing of glass is covered elsewhere, for the purposes of this article, its importance to the construction is its dimensional stability over a wide temperature range.

Insulated Glass Units (IGUs) are manufactured with glass in range of thickness from 3 mm to 10 mm 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.

To reduce shear effects on the sealed unit (a major cause of premature failure), manufacturers use a rule of thumb that permits a difference of 1 mm between the thickness of the panes of glass used in the unit and still maintain the warranty for the unit.^{[citation needed]} For example, a unit may be ordered with a 4 mm pane on the exterior and a 3 mm pane on the interior. These variations are allowed for architectural and cost reasons. Other combinations can be specified and produced but the manufacturer may reserve the right to limit the term of the warranty or refuse to warranty the unit altogether.