Some modern windows have a thermal break, usually Bakelite, between the interior and exterior part of the window to prevent condensation during the winter months. The sash includes the glass, its supporting framework, locks and lifts. Most insulated glass units are double or triple-pane windows that are sealed with an epoxy to create an air gap between each plate of glass. The window frame is perforated on the inside and filled with a desiccant material that absorbs the moisture vapors from between the glass plates. The wider the dry air space, the greater the insulating value of the unit.
Manufacturers also use a coating over the glass for greater energy efficiency.
The coating is often called low-emissivity or low E glass. Low E glass reflects radiant heat in the summer and retains interior heat in the winter. Very few manufacturers hermetically seal or create a dry air gap between the panes and then use an air tight seal. Anderson Windows switched from hermetically sealing the window to using the epoxy and desiccant method. A double-paned window is different from an insulated glass unit, because the glass panes are not sealed to create an insulated air space.