What Is a Double-Glazed Window?

Glazed window


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Glass has long come between us and the outside world, protecting us from rain and snow and keeping us warm, all while letting in light. Glass is a simple invention that has worked efficiently for hundreds of years. So what could be better than this miraculous sheet of glass? How about two sheets of glass? That's where double-glazed windows come in.

What Is a Double-Glazed Window?

A double-glazed window is a window with two sheets or panes of glass.

The term glazing derives from the Middle English word for glass. People who repair window glass are called glaziers. If you hear the term glazing, it will usually be when some window manufacturers refer to their window designs as being single-glazed or double-glazed.

A single-glazed window has one sheet of glass. A double-glazed window has two parallel and separate sheets of glass. In extreme climates, triple-glazed or triple-paned windows are sometimes recommended.

The term glazing is infrequently used by major window manufacturers anymore to describe their windows. The company might use the terms glass, panes, or sheets. Often with double-glazed windows, the number of sheets of glass is not even mentioned in the product literature since double-glazing is standard.

In the window industry, the double-glass assembly is sometimes called an IGU, meaning insulated glazing unit or insulated glass unit.

Advantages of Double-Glazed Windows

Double-glazed insulated panel windows with two panes of parallel glass offer a significant advantage when it comes to the energy efficiency of the window. Double-glazed windows are now standard for both new-construction windows and replacement windows.

The idea behind multiple glazing is that the two panes of glass, including the intermediate buffer zone, provide a better barrier against outside temperatures than single-paned windows. In fact, the glass itself isn't much of a thermal insulator. Its value is that it can seal in and maintain a buffer.

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 that space to be filled with 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. Other measures can also help increase the energy efficiency, such as applying thin coatings of special materials to one or more faces of the glass. These low-E windows (the term stands for "low emissivity") have thin transparent coatings of a metal oxide or silver applied to one or more of the glass surfaces to further reduce the energy transmission.

Double-Glazed Windows' Energy Efficiency

The insulating value of a window can be measured in a number of different ways. Common is the R-value system, a measurement of material's resistance to energy transfer. 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 two-by-four 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.

Improving Double-Glazed Windows' Efficiency

No matter how well-designed they might be, windows are always a major point of energy loss in a home, when compared to other building elements. There is simply no comparison between the most energy-efficient double-glazed window and even an ordinary wall system. To help improve windows' efficiency:

  • Use Thermal Curtains: Thick thermal curtains drawn across the windows at night significantly raise R-values.
  • Add Window Insulating Film: A thin clear layer of plastic film can be applied to the window trim with adhesive. Application of heat from a hairdryer will tighten the film. Insulating film adds another dead-air barrier that can help slow thermal loss.
  • Weatherproofing: Older windows may be cracking or opening up around the framing, letting cold air enter the home. Use exterior-grade silicone caulk to stop up these cracks.
  • Replace Foggy Windows: Windows that are foggy between the two panes of glass have lost their seals and gas has leaked out. It is usually best to replace the entire window or the window sash.