Windows are the weak link in the building systems that keep a home warm and dry, according to the U.S. Department of Energy.
Average windows have an insulating performance that’s about 70 percent worse than a well-insulated wall. Installing windows that meet Energy Star requirements could save a home up to 15 percent on its energy bill, compared to non-qualified products.
If you’re looking for windows to remodel your home, or if you’re building a new home, look for products that bear the Energy Star seal and also the National Fenestration Rating Council (NFRC) certification. These products meet current standards for energy efficiency and durability.
Here’s a look at developments in some key aspects that you should consider when choosing windows for your green home.
Double & triple panes
Multiple panes of glass provide an insulation factor for windows. The most energy efficient windows are typically double or triple pane with low-e glazing (or glass) with an insulating gas between the panes.
For instance, Marvin Windows offers a double-pane window with low-E glass and argon gas (see below) that can reduce heating costs by 34 percent in cold climates and 38 percent in warm climates.
Windows with low emissivity or low-E glass block heat transfer from the warm side of the glass to the cool side of the glass. The ultra-thin metallic coatings typically come in low, medium and high solar gain. Which one is best depends on where you live. In warm areas, the objective is to reduce solar heat gain so a high-gain coating makes sense. In cooler climates, the sun can help warm a home, so a low-gain choice makes sense. The medium option may make the most sense in moderate climates. Blocking UV rays can also reduce damage to interior furnishings.
Glass can do more than keep the sun under control. Andersen offers Low-E4 glass that features an exterior coating that, when activated by sunlight, reduces water spots by up to 99 percent. It also helps reduce dirt build-up and promotes faster drying on the exterior glass.
For greater insulating performance, windows with multiple panes of glass may be filled with argon, krypton, nitrogen, or a mix of gases. These inert gases are better at managing heat than plain old air. But your pal Superman may not be able to visit.
The gas minimizes the convection currents in the space between the panes, reducing the transfer of heat between the inside and outside.
Argon gas is most common, often found in double-pane configurations. Krypton gas is more expensive and performs best in thinner spaces, so it is more common in triple-pane windows. Argon is often blended with krypton or nitrogen to improve performance and reduce costs.
R-value/U-value: R-value indicates the resistance to heat flow through a window, also used for wall insulation. Here, the higher number the better. The opposite of R-value is U-value or U-factor, which is a measure of a window's tendency to transfer heat. Current Energy Star requirements are for U-values of .35 or less.
A highly insulated wall in an energy-efficient home may have an R-value of 19. Highly efficient windows may have had an R-value of 3.3. Now many window manufacturers offer products with an R-value of 5 due to improved insulation performance. Future windows in development may have an R-value as high as 10.
Solar Heat Gain Coefficient (SHGC) - This measures how well a product blocks heat from the sun. The lower the number the better, especially in warmer climates.
Look for windows that offer indoor-air quality benefits such as no-added formaldehyde and low or zero-VOC content. Windows with these features contribute to a healthy indoor environment by reducing exposure to toxic chemicals inside the home. Products certified by Green Seal meet certain sustainability and performance standards.
Window frames and components come in many materials, from wood to a variety of materials using recycled content. Wood is a renewable resource, and most major manufacturers offer products made from certified wood programs, such as the Forest Stewardship Council and the Sustainable Forestry Initiative.
Products such as Pella Corp.’s Fibrex incorporate recycled wood and vinyl for a strong composite material. Other common materials include fiberglass, aluminum and composite products. Marvin, for example, developed Ultrex for windows and doors. It’s primarily made from silica sand, an unlimited natural resource.
(Photo courtesy of Andersen Corporation)