By David Welch
The buzzwords in the window manufacturing industry these days are energy efficiency. The consumer, and the Federal Government, want to lighten energy usage and the latest breakthroughs in window design make energy efficient windows one way to achieve that goal.
"The trend for energy efficient windows is sweeping the country," said John McFee of the National Window & Door Association. "That trend bodes well for the manufacturers of wood windows. Wood is already the choice material of the consumer. And because wood lends itself to energy efficiency, the material is the first choice of designers and builders."
"In our market, customers demand the warmth, beauty and tradition of wood windows," says Russell E. Arkin, president of Arkin Homes. "With the good performance promised by modern wood windows, we can meet that demand and still provide outstanding energy value."
Today's Busy Consumers Want Natural Light
"Society is more high-paced than ever," said Gay Fly, owner of Gay Fly Designer Kitchens & Baths in Houston, Texas. "Whole families are either at work or in school during the day. Despite this trend, natural light will be more important than ever in kitchens and baths. When at home, consumers want to feel comfortable, safe and secure. Natural light helps convey that feeling. Wood windows are true purveyors of natural light.
"Advanced glass and glazing systems lessen heat loss, so this is less of a consideration than in the past. In very cold climates, for example, windows framed in wood with double and triple-glazed glass act as insulators against the cold, yet let the homeowner still enjoy a picturesque view. In contrast, in warmer areas, we usually see woods used with lighter, cooler tones. Soaring windows are used to open and heighten spaces. In both types of climates, cold and hot, the types of glazing systems make a big difference in the transfer of heat or cold."
Breakthroughs in Design Improve Energy Performance
The total performance of windows and doors in new homes depends on a variety of factors, according to a report by Robert Sullivan and Stephen Selkowitz, group leaders of the Windows and Daylighting Group at the Lawrence Berkeley Laboratory, Berkeley, Calif. The type of glazing coatings and spacers used in doors and windows particularly impact that performance. With more options available today than ever before, manufacturers need to develop a thorough understanding of product features and how they impact the thermal performance of a home.
Today's glazing products provide a way to increase performance and save energy. These options cover a wide gamut of solutions to energy loss problems. They range from tinted glass and reflective coatings and films to gas-filled insulating glass units and improved edge spacers designed to cut heat transfer at the most vulnerable point.
There are three basic methods employed to increase glazing energy performance, according to McFee. These include changing the glazing material itself, applying coatings to the material surfaces and assembling units using gas-filled glazing and improved edge spacers.
"The first of these three approaches carries some benefits to energy performance," he said. "Using tinted glass, for example, not only reduces solar heat gain, but also softens glare. Tinted glass has found its biggest usage in commercial buildings, but is gaining favor in residential applications in warmer climates.
"The second solution involves the application of reflective coatings and films. Again, these options grew primarily out of the commercial market, but are finding more and more usage in residential construction. Low-emissivity coatings, or low-e, have given designers the option of placing larger expanses of glass in all areas of the home, including applications where an afternoon sun would normally present a severe solar gain problem. In addition to increasing energy performance, low-e windows substantially cut glare.
"The third technique involves the actual construction of double-glazed units. Double and triple glazing substantially increases thermal resistance by creating a void between glass panes. Adding an inert gas, which provides further resistance to thermal conductivity, can also enhance this resistance. The most commonly used gas is argon, although krypton provides even better insulating value. However, krypton is also more expensive. As a compromise, some manufacturers combine the two gasses in filling the space between glass panes."
Manufacturers also have substantially improved the edge spacers that separate the layers of glass. Thermal breaks (spacers that combine traditional aluminum or stainless steel with less conductive metals), foam spacers and various combinations of these techniques have reduced the problem of heat transfer at the traditionally cold edges of window and patio door glazing.
According to the Efficient Windows Collaborative (EFC), high-performance windows not only provide reduced annual heating and cooling bills, but also reduce the peak heating and cooling loads as well. This has benefits for the homeowner, in that the size of the heating or cooling system may be reduced, and it also benefits the electrical utilities, in that load factors are reduced during the peak times in summer. The peak load for a building is the maximum requirement for heating or cooling at one time. These loads determine the size of the furnace, heat pump, air conditioner and fans that must be installed.
Argon or Krypton Gas Fills
Traditionally, the space between glazings was filled with air, according to the Alecia Ward, program director for the EWC. "Manufacturers have introduced the use of argon and krypton gas fills to improve thermal performance. Argon is inexpensive, nontoxic, nonreactive, clear and odorless. The optimal spacing for an argon-filled unit is the same as for air, about 1/2". Krypton has better thermal performance, but is more expensive to produce. Krypton is particularly useful when the space between glazings must be thinner than normally desired, for example, 1/4" (6 mm). The optimum gap width for krypton is 3/8". A mixture of krypton and argon gases is also used as a compromise between thermal performance and cost.
Low Conductance Spacers
The layers of glazing in an insulating unit must be held apart at the appropriate distance by spacers, according to Ward. "Because of its excellent structural properties, window manufacturers started using aluminum spacers in the 1960s and 1970s. Unfortunately, aluminum is an excellent conductor of heat and the aluminum spacer used in most standard edge systems represented a significant thermal 'short circuit' at the edge of the insulating glass unit (IGU), which reduces the benefits of improved glazings. In addition to the increased heat loss, the colder edge is more prone to condensation.
"To address this problem, window manufacturers have developed a series of innovative edge systems to address these problems, including solutions that depend on material substitutions as well as radically new designs. One approach to reducing heat loss has been to replace the aluminum spacer with a metal that is less conductive, e.g. stainless steel, and change the cross-sectional shape of the spacer. These designs are widely used in windows today.
"Another approach is to replace the metal with a design that uses materials that are better insulating. The most commonly used design incorporates spacer, sealer and desiccant in a thermoplastic compound that contains a blend of desiccant materials and incorporates a thin, fluted metal shim of aluminum or stainless steel. Another approach uses an insulating silicone foam spacer that incorporates a desiccant and has a high-strength adhesive at its edges to bond to glass. The foam is backed with a secondary sealant. Both extruded vinyl and fiberglass spacers have also been used in place of metal designs."
Low-E Coatings
According to the EWC, low-emittance (Low-E) coatings are microscopically thin, virtually invisible, metal or metallic oxide layers deposited on a window or skylight glazing surface primarily to reduce the U-factor by suppressing radiative heat flow. Low-E coatings are transparent to visible light. Different types of Low-E coatings have been designed to allow for high solar gain, moderate solar gain, or low solar gain.
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