Edge Spacers

The element that holds the panes of glass apart and provides the airtight seal in an insulated glass window is known as an edge spacer. Edge spacers are designed to interrupt the transfer of heat between the two panes of glass. Aluminum, a material with high thermal conductivity, was the traditional material used for edge spacers. But as more effective glass coatings became available, aluminum edge spacers were found to offer fewer benefits than those made of other materials. Stainless-steel edge spacers, for example, are preferable to aluminum because of their lower conductivity. This new generation of edge spacers helps maintain higher temperatures at the edge of the window unit, improving insulation and reducing condensation. Aluminum with thermal breaks, silicone foam and butyl rubber are other new technology options for edge spacers.

Frames

Window frames may be composed entirely of aluminum, wood, vinyl, and fiberglass, or they may be a combination of materials such as wood-clad vinyl or aluminum-clad wood. Because the frame occupies about one-quarter of the total window area, frame materials should be thermally non-conductive. Aluminum frames tend to have low interior surface temperatures even during the heating season and for that reason may not be suitable for all climates.

Wood frames have lower U-values, are not affected by temperature extremes, and usually do not promote condensation. Vinyl frames, too, have low U-values, and offer the benefit of reduced maintenance and competitive pricing. Generally speaking, wood, vinyl and fiberglass provide better insulating value.

Low-E Coatings

More than any other single improvement, the development of low-emissivity (low-E) coatings in the 1980s revolutionized window technology. Low-E glass is coated with microscopically-thin, optically transparent layers of silver sandwiched between layers of antireflective metal-oxide coatings. According to the U.S. Department of Energy, which has made substantial investments in a series of energy-efficiency research and development projects over the years, low-E glass coatings have saved the nation more than $8 billion in energy costs. According to industry estimates, over 50 percent of windows now sold have low-E glass.

In order to understand the benefits of low-E coatings, it is important to note the components of sunlight. Among other things, sunlight contains visible light, ultraviolet (UV) light, and infrared (IR) light. Visible light enables us to see things and is welcome in a building interior. Ultraviolet light damages skin, wood, and fabrics and causes colors to fade; it is also associated with premature aging and skin cancer. The infrared portion of the spectrum consists of varying wavelengths. Short-wave IR light is absorbed by objects both inside and outside the building and is transformed into long-wave energy or heat. Infrared energy is desirable when the goal is to heat a room's interior by natural means; however, it is undesirable whenever building interiors become too hot, and excessive demands are placed on air-conditioning and ventilation systems.

The thin, transparent low-E coatings allow visible light to pass through, but they effectively reflect infrared heat radiation back into the room, which keeps interiors warmer and reduces heating costs. This reduces heat loss through the windows in the winter. In the summer, low-E glass windows admit visible sunlight while blocking infrared and ultraviolet solar energy that drives up cooling costs and damages window treatments, carpeting and furnishings. According to the DOE's Energy Efficiency and Renewable Energy Clearinghouse, windows manufactured with low-E films typically cost about 10 percent to 15 percent more than regular windows, but they reduce energy losses by as much as 30 percent to 50 percent. Further, advanced glass with spectrally selective coatings can reduce the cooling requirements of new homes in hot climates by more than 40 percent.

Not all low-E coatings are the same. Placement of the coatings and the types of light wavelengths that they block affect their overall performance. A variety of low-E windows are now available for various climate zones and different applications in any particular location.

 

Spectrally selective coatings distinguish between visible light, unwelcome UV and near Infrared rays. The result is improved solar heat control, fading protection, and visibility.