Insulated glazing is a piece of glazing consisting of two or more layers of glazing separated by a spacer along the edge and sealed to create a dead air space between the layers. When it is made of glass it is called insulated glass. It is mounted in a sash or frame as if it were a very thick piece of glass. The dead air space may be filled with air but an inert gas like argon provides better insulating performance. Typically the spacer is filled with dessicant to prevent condensation and improve insulating performance.

Insulated glass may not be cut to size in the field like plate glass but must be manufactured to the proper size in a shop equipped with special equipment.