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Your Guide to Energy-Efficient Window TechnologyApril 18th, 2014 by
If your windows aren’t energy efficient, you’re probably familiar with the woes of too much heat loss in the winter and too much heat gain in the summer—perhaps seen most notably in your utility bills. Fortunately, energy-efficient window technology can minimize that energy waste and improve the performance of heating and cooling systems, leading to savings worth the investment.
Components of Energy-Efficient Windows
Construction. The frame and operation styles of windows can contribute to their energy efficiency. EBSCO Research has provided a guide that outlines those options and also explains the measurements listed on a product’s National Fenestration Rating Council (NFRC) label. This label describes a window’s performance, which can help you choose the most energy-efficient windows or window films for your home and climate.
Glass. It’s important to carefully consider the glass used for your windows. Insulated Glass Units (IGUs) are today’s standard for energy efficiency; these windows have at least two panes of glass spaced apart—called double glazed or double pane—and the space between the panes is filled with a gas to improve the window’s thermal performance. For more information about IGUs, read EBSCO Research’s blog on choosing the best types of glass for your windows.
Coating. Another type of energy-efficient window technology involves covering a window’s glass surface with either a coating or a film. Coatings—such as low-emissivity (Low-E) coatings—can be applied during the manufacturing process. These windows are more expensive than windows without coatings, but over time, they pay for themselves in energy savings. Alternatively, films—like solar control window films—can be retrofitted to windows; these are typically a less expensive option than replacing old windows.
A variety of coatings and films are available; this blog will focus on the benefits and types of this energy-efficient window technology.
Benefits of Energy-Efficient Window Coatings
Reduction in heating and cooling costs. The US Department of Energy states that window coatings can reduce energy loss by 30 to 50 percent. In summer, the coatings block heat from entering the home, reducing air conditioning costs; in winter, they keep heat within the home, decreasing heating expenses. The coatings also help to maintain more consistent room temperatures by reducing hot spots, which can enable heating and cooling systems to run more efficiently.
Protection from the sun. Glass will filter ultraviolet B (UVB) rays but not ultraviolet A (UVA) rays, so regular windows can’t protect you or your household furnishings from the harmful effects of UVA rays. Fortunately, window coatings and films can block out almost 100 percent of those damaging rays. The Skin Cancer Foundation states, “This helps prevent not only sunburn, but also the brief daily UV exposures that cumulatively accelerate skin aging and multiply the risk of skin cancer.” Limiting this sun exposure also helps prevent fading of furniture and artwork.
Reduction in glare. Window tints can help to minimize glare, which can be especially helpful when the sun is low on the horizon.
Privacy. Some window tints can make it more difficult to see inside the home, providing privacy and security without needing to use drapes or blinds.
Types of Energy-Efficient Window Coatings
Low-E coating. A low-E coating is a very thin, almost invisible metal layer applied to one or more glass panes. It is typically applied by the manufacturer, but after-market low-E films are also available. The low-E coating lowers the window’s U-factor—the rate of non-solar heat flow. A low U-factor means the window is more resistant to heat flow, which means better insulation—keeping the heat out during the summer and the heat in during the winter.
Low-E coatings are available in low, moderate, and high solar gains. Solar gains—measured by the Solar Heat Gain Coefficient (SHGC)—refer to the fraction of solar radiation transmitted through a window. A lower SHGC means the window admits less heat, keeping the home cooler; with a higher SHGC, the window transmits more heat to the home. Your climate, home’s orientation, and windows’ locations should all be considered when deciding on the appropriate level of solar gain.
A low-E coating can also reduce a window’s Visible Transmittance (VT), which is the amount of visible sunlight that comes through the window. A lower VT means that the window admits less visible light, while a higher VT means it transmits more light.
Spectrally selective coating. Spectrally selective coatings are beneficial for those who want the benefits of a low-E coating, but with a higher VT. These coatings reflect the infrared and UV radiation of sunlight but permit visible light, thus blocking unwanted solar heat and damaging rays while also allowing sunshine into the home.
Tinting. Tinting lessens the SHGC and VT as well as glare, but it does not lower the U-factor unless combined with a low-E coating. Gray and bronze are the most common tint colors; they reduce the transmission of both light and heat. Blue and green tints allow more visible light through but are not as effective at blocking heat. Tints are also available in a range of shades, so you can choose one as light or dark as desired. Lighter tints allow more visible light to come in, and darker tints provide more heat resistance and better glare reduction.
Reflective coating. As the name suggests, this metallic coating reflects sunlight, lowering the window’s SHGC and VT while reducing glare. It is available in colors such as silver, gold, and bronze. This coating is typically used more in hot climates. Because of its low VT value, more indoor lighting may be necessary; so in order to experience energy savings, this coating should be used selectively around the house. Take note that a reflective coating can appear mirror-like.
Whether purchasing new windows or retrofitting old windows with films, be sure to refer to each product’s NFRC label and consider your home’s location and climate when making decisions. A professional can also give guidance as to the most energy-efficient product for you and can ensure the windows are installed properly or the films are applied correctly.
Sources: Efficient Windows Collaborative; Florida Solar Energy Center; International Window Film Association; The Skin Cancer Foundation; US Department of Energy.
For more information on our sources, please contact us directly.