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The U.S. Department of Energy estimates that 25%-30% of energy wasted in the home is due to inefficient windows. So clearly, when buying a new home or upgrading your current home, selecting the ideal set of windows is an important part of the decision.
But what’s the right window for your home? “It depends,” says Paul DeJesus, a LEED-accredited architect and principal at Paul Chris Design, a design-build firm working in the San Francisco area. “What works best for your needs will vary depending on several factors like the local climate, the orientation of the house, and of course your budget,” he says.
Also consider that windows must serve multiple functions – controlling heat transfer between the inside and outside of the home, blocking harmful aspects of the sun’s rays while letting in daylight, reducing exterior noise, and allowing air ventilation.
Make sure to “balance” all these factors when evaluating windows, advises DeJesus. And because windows are highly technical products these days, it’s a good idea to become familiar with some of the basics of new window construction.
High performance windows will have multiple panes of glass, two or even three for especially cold climates or noisy outside environments. The panes are separated by a vacuum or gas filled space using argon or krypton gas. The multiple panes provide more insulation to reduce heat transfer, so are “leaps and bounds more energy efficient over a single pane,” says DeJesus.
When choosing windows, often it’s the things you can't see that are most important. This is the case with windows manufactured with low emissivity (low-e) window coatings, which are microscopic layers of metallic oxides.
Low-e coating reflects heat back to its source, so it helps your home stay cooler in the summer and warmer in the winter. The coating also protects your home from unwanted ultraviolet rays which can fade your carpets and furniture. And because these coatings are transparent, they allow as much natural light into the house as possible.
The most common types of window frames are made from wood, vinyl, fiberglass or aluminum. These all have cost/performance trade-offs. For example, vinyl is cheaper but less durable and not as good an insulator as the others. Plus, some people don’t like the look of vinyl frames, which can affect the resale value of the home.
Wood frames are more aesthetically pleasing, are good insulators, but are expensive. And like any wood product exposed to the elements, will require regular maintenance. Aluminum frames are durable, but not a great insulator. Fiberglass is durable and an excellent insulator but can be pricey.
Regardless of the type of material, the best window designs will also offer a “thermal break” between the panes, says DeJesus. This is a resin or plastic material installed in the window frame that separates the interior part of the window from the exterior part to provide insulation against heat and cold conduction.
Besides the various types of windows, it’s important that the windows are installed properly, says DeJesus. This means proper caulk or foam has been used to seal the space between the window frame and the adjacent wall framing to prevent heated air from escaping the home.
Installation won’t be an issue for a new home, but if you are replacing the windows in an existing home, “Make sure your installer is certified by the window manufacturer because this an area that sometimes gets overlooked,” says DeJesus.
A lot of the decision-making can be simplified by consulting the EPA’s ENERGY STAR ratings for windows. The EPA has divided the country into four climate regions – Northern, North Central, South Central, and Southern – and within each zone prescribed a set of performance measures for windows, such as U-factors and solar heat gain coefficients.
Don’t worry, you won’t need your calculator for this one, because the National Fenestration Rating Council has done all the work for you. The NFRC, an independent non-profit organization, tests, certifies and labels windows from all major manufacturers.
Their rating system scores windows on U-factor (how well a window resists heat loss from a room), Solar Heat Gain Coefficient (how well a window resists heat gain), Visible Transmittance (how well a window allows daylight into the room) and Air Leakage (how much air enters a room through the window).
A performance label with these scores is affixed to the window pane. Again, don’t get bogged down in the science. The NFRC website makes it easy by offering a Shopping Guide and Certified Product Directory.
Many city and state governments are setting energy efficiency requirements, like the California Title 24 code, for new and remodeled residential and non-residential buildings.
New home construction will be required to meet these standards. For home remodels, you may fail the building inspection if improvements, like window replacement, don’t meet the local codes. So, make sure you are familiar with the local green building codes.
If replacing inefficient windows is not possible, products are available to upgrade your existing windows. For example, low-e coatings can be applied as a film to windows already installed in the home. The NFRC rates the energy performance of window films and the International Window Film Association (IWFA) website offers a directory of qualified installers.
Window shades can also help, especially those with air pockets known as insulated cellular shades. These provide insulation much like double pane windows. The problem with shades is they cut down on daylight entering the room. However, choosing shades with lighter colors will minimize this effect, advises DeJesus.
Windows are just one part of the LEED ratings envelope but an important and challenging one, says DeJesus. So, consider carefully.
Image credit: Adobe Stock/gmcgill
Jim Witkin is a writer based in Silicon Valley and London focused on business, technology and the environment. His work has been featured in the New York Times and Guardian newspapers. He holds an MBA in Sustainable Management from the Presidio Graduate School.