What Is a Double-Glazed Window?

Modern Fixed Windows

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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, 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."

Advantages of Double or Triple Glazing

Especially in more extreme climates, insulated panel windows with two or three panes of parallel glass offer a significant advantage when it comes to the energy efficiency of the window. The idea behind multiple glazing is that the two or three parallel panes of glass, including 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 windows and replacement windows.

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.

The insulating advantage of double-glazing becomes obvious from the tested R-value of various window designs:

  • Single pane: R-value 0.9
  • Double pane with .5-inch air space: R-value 2.04
  • Triple pane with .5-inch 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

By comparison, a standard 2 x 4 stud wall with batt insulation and wallboard and wood siding has an R-value of R-12 to R-15. Window manufacturers are now developing technologies that allow windows to approach the R-value of the walls themselves.

Most Energy-Efficient Windows

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 actually better at blocking heat loss than a standard 2 x 4 wall filled with fiberglass batt insulation.