Insulated window glazing refers to windows with two or more panes of glass. They are also called double-glazed, triple-glazed, and—sometimes more generally—storm windows.
To insulate the window, the glass panes are spaced apart and hermetically sealed to form a single-glazed unit with an air space between each pane of glass. The glass layers and the air spaces resist heat flow. As a result, insulated window glazing primarily lowers the U-factor, but it also lowers the solar heat gain coefficient.
Some window manufacturers use spacers—which separate two panes of glass—that conduct heat less readily than others. These spacers can further lower a window’s U-factor.
Other technologies window manufacturers use to improve the energy performance of insulated glazing include these:
* Gas fills
* Low-emissivity coatings.
Source: EERE, U.S. Department of Energy