Double glazing is the glazing process in which a window is formed by two panes of Glass with a space between the panes. The space between the Glass is usually several millimeters thick. Air is trapped between the panes of Glass and forms a layer of insulation. Before the unit is sealed, a drying agent is added to ensure that no moisture is present inside the finished Glass unit.

The double glazing unit must be airtight. If condensation appears between the panes, it indicates that the window is not airtight. Double glazing windows cannot be repaired; if condensation appears, the unit must be replaced.