What Is a Double-Paned (or Double-Glazed) Window?

Glazed window

 

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What Is a Double-Paned Window?

A double-paned window has two panes of glass set into a frame to create a sandwich of glass with an air pocket to better insulate a room. It is sometimes referred to as a double-glazed window or an insulating glass unit (IGU).

A single-paned window with one sheet of glass is an older style that has since been replaced by the more popular and energy-efficient double-paned window. Though double-paned windows were introduced commercially in the 1950s, they became common in homes around the 1970s. Double-paned windows are now standard for both new construction and replacement windows. Triple-paned windows are sometimes recommended in environments with harsh weather for maximum insulation.

Advantages of Double-Paned Windows

Though glass itself isn't much of a thermal insulator, it can seal and maintain a buffer from the outside. Double-paned windows offer a significant advantage when it comes to the energy efficiency of a home, providing a better barrier against outside temperatures than single-paned windows.

The gap between glass in a double-paned window is commonly filled with an inert (safe and non-reactive) gas, such as argon, krypton, or xenon, all of which increase the window's resistance to energy transfer. Though gas-filled windows have a higher price tag than air-filled windows, the gas is denser than air, which makes your home significantly more comfortable. There are differences between the three types of gas that window manufacturers prefer:

  • Argon is a common and most affordable type of gas.
  • Krypton is typically used in triple-paned windows because it performs best within extremely thin gaps.
  • Xenon is a cutting-edge insulating gas that costs the most and is not as commonly used for residential applications.

Tip

The simple way to see if you have single- or double-paned windows is to hold up an item (like a pen or pencil) to the glass until you see a reflection. If you see one reflection of the item, you have a single-paned window. Two reflections of the item indicate a double-paned window.

Understanding R-Values

R-values are given to different products to help consumers understand the expected thermal resistance of the material. Though the insulating value of a window can be measured in a number of different ways, the most common is the R-value system. The R-value measures the material's resistance to energy transfer. The higher the R-value number, the greater the resistance and the higher the insulating value of the window.

Some windows have a special low-E (low emissivity) insulating film on the glass that bumps up the R-value. The film adds another way to reflect heat energy into or out of the home. The thin transparent coatings on the window are comprised of a metal oxide or silver applied to one or more of the glass surfaces to further reduce the energy transmission.

Pane Air/Gas Coating R-Value
Single None None 0.9
Double Filled with 1/2" of air None 2.084
Triple Filled with 1/2" of air None 3.226
Double Filled with argon Low-E (1 coating) 3.846
Triple Filled with argon Low-E (1 coating) 5.433

Tip

A low-E glaze coating on a window reflects ultraviolet (UV) rays to better protect furniture and artwork from fading.


Wall vs. Window R-Values

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, which is considered to be on the lower end compared to other types of materials and sidings. Window manufacturers continue to develop technologies that will allow windows to approach the higher R-value of the walls themselves.

Tip

Don't confuse R-values with U-values. R-values rate the window's effectiveness of heat resistance. U-values measure the loss or gain of heat transfer, but it is not a rating. R-values are more common. Look for high R-value and low U-value windows for best energy efficiency.

Tips for Improving Window Efficiency

No matter how well-designed they might be, double- and triple-paned windows can always be helped along to eliminate energy loss. Here are tips to help improve the efficiency of your windows:

  • Use thermal curtains: Thick thermal curtains drawn across the windows at night significantly raise the window's overall R-value.
  • Add window insulating film: You can apply your own thin clear layer of plastic film to the window trim with adhesive. Application of heat from a hairdryer will tighten the film.
  • Weatherproofing: Older windows may have hairline cracks or they are beginning to open up around the framing. Those problems let cold air enter the home. Using an exterior-grade silicone caulk can close up these leaks.
  • Replace foggy windows: Windows that are foggy between the two panes of glass have lost their seals and the gas has leaked out. It is usually best to replace the entire window to regain the energy efficiency in your room.