Maximizing thermal performance
The emergence of energy-efficient windows is a key part of a breakthrough in the overall design of houses and light-commercial buildings. Glazed surfaces no longer have to lose a lot of heat or feel cold in winter, so heating systems can be much smaller and less expensive. For example, heating elements are no longer required beneath windows to compensate for the drafts and cold surfaces that windows used to generate. In a well-designed, highly energy-efficient house, central heating may no longer be necessary at all—though air distribution systems are still important to ensure good indoor air quality.
Low-e glass coatings, which increase the R-value of standard double-glazing from 2 to about 3, are gaining in market share each year. The premium of 10 to 20 percent for low-e easily pays for itself in a few years in most applications. The added benefit is a warmer window surface that’s more comfortable to be near both in cold weather and in very hot weather. Double low-e and HeatMirror™ coatings on suspended films are available in premium windows, and can increase the center-of-glass insulating value up to R-9.
By careful selection of low-e coatings, windows can be “tuned” to optimize the performance of a structure—balancing heat loss, solar gain, and visible light transmission through the glass. In hot climates, coatings that transmit less solar gain should generally be preferred. In cold climates, where solar gain can be beneficial in winter, glazing that transmits more solar energy is preferable on the south side of a building. On the east and west, less solar gain is preferable even in cold climates, because solar gain is greatest on these orientations during the summer, when air conditioning is likely to be used.
Use of an inert, low-conductivity gas in the space between layers of glazing is another way to improve thermal performance. Most low-e windows have argon gas fill. Some super-energy-efficient windows have krypton or a mix of argon and krypton between the glazing layers.
About window frame materials
Although standard for many years, aluminum windows are disappearing from most cold-climate markets. If aluminum frames are used, they should be constructed with a thermal break between the inner and outer surfaces to improve energy performance. Aluminum windows are rapidly being replaced by vinyl frames.
Vinyl frames are much better than aluminum in terms of thermal performance, but there are some environmental concerns associated with the production and eventual disposal of PVC (polyvinyl chloride). Vinyl windows vary greatly in quality. A particular weakness of vinyl windows is that vinyl expands considerably (more than wood or fiberglass) when heated by sunlight, and many consumers complain about weather-sealing problems over the life of the window due in part to this issue.
Wood windows are still the standard for energy efficiency. Vinyl or aluminum cladding adds value because of its low-maintenance qualities. Wood-window manufacturers are facing increasing difficulty in finding affordable, knot-free material from which to manufacture their product, and some are using finger-jointed material with an interior coating and exterior cladding.
Other energy-efficient frame materials include fiberglass, with or without foam insulation in the hollow channels, and composites such as a combination of recycled vinyl and wood fibers.
With any window materials, durability of the edge seals and spacers that separate the layers of glass is extremely important, as failure of this seal will cause condensation inside the window (fogging), and the loss of any low-conductivity gas fill.
BIBLIOGRAPHY ITEMS
Window Systems for High-Performance Buildings
by John Carmody et al.
(2004)
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