The term glazing is a specialized window term that derives from the Middle English word for glass. Even today, people who repair window glass are called glaziers, and you are likely to hear the term glazing used in a technical manner when window manufacturers refer to their window designs as being "single-glazed" or "double-glazed."
Very simply, when a window is described as being single-glazed or double-glazed, it means the window has one pane of glass or two parallel panes of glass. Some manufacturers now use more modern terms of "single-paned" or "double-paned" to identify these different styles. Windows with double panes are often said to have "thermal glass" or "insulated glass." It is also possible for windows to be triple-glazed, with three panes of parallel glass.
Especially in more extreme climates, insulated panel windows with two panes of parallel glass offer a significant advantage when it comes to the energy efficiency of the window. The idea behind double glazing is that the double panes of glass, plus the intermediate buffer zone, provide a better barrier against outside temperatures than single-paned windows. Double-glazed windows are now standard for both new construction and replacement windows. They can be engineered so there is simply dead air space in the gap between the window panes, but today it is more common that windows are designed so the space between the panes is filled with an inert gas, such as argon or xenon, which increases the resistance to energy transfer through the glass panel. Triple-glazing (three panes) is used in very harsh climates to further improve the insulating value of a window, and there are 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.
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.
The insulating advantage of double-glazing becomes obvious from the tested R-value of various window designs. By comparison, a standard 2 x 4 stud wall with batt insulation and wallboard and wood siding as an R-value of R-12 to R-15.
- Single pane: R-value 0.9
- Double pane with .5" air space: R-value 2.04
- Triple pane with .5" air space: R-value 3.22
- Double-pane with argon and low-E coating: R-value 3.846
- Triple-pane with argon and low-E coating: R-value 5.433
Today's glazing options can create windows that have almost the same insulating value as walls themselves. The production window with the highest R-value currently available is from Alpen Windows, and is described as a "heat mirror insulating glass window with three heat mirror films suspended inside of an insulating glass unit, creating four air spaces, with xenon gas filling." The R-value of this astounding window is R-20. It's better than a standard 2 x 4 wall filled with fiberglass batt insulation.