Windows are one of the most high-tech products in residential construction. Since the early 1980s, the energy performance of typical windows has increased by more than 50 percent, the result of both improvements in glazingWhen referring to windows or doors, the transparent or translucent layer that transmits light. High-performance glazing may include multiple layers of glass or plastic, low-e coatings, and low-conductivity gas fill. and in frame construction.
The National Fenestration Rating Council (NFRC) publishes the energy performance of certified window products, and in some states manufacturers are required to label their windows with the NFRC's rating. These ratings are like the EPA mileage rating for cars—they may not provide actual energy consumption in a particular application, but are useful for comparison.
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-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor.
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-eLow-emissivity coating. Very thin metallic coating on glass or plastic window glazing that permits most of the sun’s short-wave (light) radiation to enter, while blocking up to 90% of the long-wave (heat) radiation. Low-e coatings boost a window’s R-value and reduce its U-factor. 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 argonInert (chemically stable) gas, which, because of its low thermal conductivity, is often used as gas fill between the panes of energy-efficient windows.
gas fill. Some super-energy-efficient windows have kryptonA colorless, odorless inert gas, often used with argon in fluorescent lighting and sometimes used as gas fill in high-performance glazing. 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 vinylCommon term for polyvinyl chloride (PVC). In chemistry, vinyl refers to a carbon-and-hydrogen group (H2C=CH–) that attaches to another functional group, such as chlorine (vinyl chloride) or acetate (vinyl acetate). 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, and many have weather-sealing problems over the life of the window due to the expansion and contraction of the plastic. They're better suited to sliders and double-hung windows than casements, because those styles are prone to warping and sagging. There are also concerns about the PVC resin itself and various compounds that are added to it to provide UV stability, flexibility, and flame resistance.
Wood windows are still the standard for energy efficiency. Vinyl or aluminum claddingMaterials used on the roof and walls to enclose a house, providing protection against weather.
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.
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