Double glazing
Double glazing is a window or door unit made of two panes of glass separated by a sealed gap, usually filled with air or an inert gas. The gap reduces the transfer of heat, which keeps rooms warmer and quieter, and cuts the risk of condensation on the inside pane.
The panes are held apart by a spacer bar and the edges are sealed to keep out moisture, so the glass inside the unit cannot be touched or cleaned. Decorative glass that is built into a double glazed unit must therefore be encapsulated inside, between the two panes. This applies to leaded lights and stained glass.
Overlay panels work differently. The bevels, lead and colour are applied to the outside face of the unit, which means they can be touched and are available on single, double or triple glazed units.