when a window is described as being single-glazed, double-glazed, or triple-glazed, it means the window has either one pane of glass, or two or three parallel panes of glass. Some manufacturers now use more modern terms of "single-paned," "double-paned," or triple-paned to identify these different designs. Windows with double or triple panes are often said to have "thermal glass" or "insulated glass."
Double-glazed windows can be engineered so there is simply dead air space in the gap between the window panes, but today it is more common for windows to be designed so that the space between the glass panes is filled with an inert gas, such as argon or xenon, which increases the window's resistance to energy transfer. Triple-glazing (three panes) is used in very harsh climates to further improve the insulating value of a window. There are also other measures that can also help increase the energy efficiency, such as applying thin coatings of special materials to one or more faces of the glass. So-called "low-E windows" (the term stands for "low emissivity") have very thin transparent coatings of a metal oxide or silver applied to one or more of the glass surfaces to further reduce the energy transmission.
The insulating value of a window can be measured in a number of different ways. Most commonly is the R-value system, a measurement of material's resistance to energy transfer. In this system, the higher the R-value, the greater the resistance and the higher the insulating value.