Today's advanced materials, better science, and new manufacturing processes make for high-performance window units. Those building new or wanting to replace inefficient windows with improved units have a lot of information to digest.
A window's performance is determined by three parts: the glazing, the sash and the spacer system. Measuring performance means looking at such factors as how the window conducts heat and transmits light. Follow a few simple steps to increase your windows' energy efficiency.
1. Look for an Energy Star label
Energy Star is a joint program of the U.S. Environmental Protection Agency and Department of Energy. Its label confirms a window's energy efficiency based on its impact on heat gain and loss in cold weather and heat gain in warm weather for a given climate zone. The labels pertain to four climate zones: Northern,mostly heating; North/Central, heating and cooling; South/Central, cooling and heating; and Southern, mostly cooling.
2. Review the label from the National Fenestration Ratings Council (NFRC)
The NFRC is an independent organization that rates and certifies energy performance in all windows. Its label lists several performance rankings. Understanding the ratings is key to making the right choice. While some rating scores are clearly beneficial, others have to be interpreted to your specific needs.
- U-factor. This measures how a window conducts non-solar heat flow. Ratings usually fall between 0.20 and 1.20. The lower the value, the greater the energy efficiency.
- Solar Heat Gain Coefficient or SHGC. This measures how much solar heat gain is admitted through a window and then released as heat into a home. The lower the SHGC, the less solar heat it transmits.
- Visible Transmittance or VT. This measures how much visible light comes through a window. Expressed as a number between 0 and 1, the higher the VT, the more light transmitted.
- Light-to-solar gain or LSG. This rating, not always provided, is the ratio between the SHGC and VT. It gauges the relative efficiency of different glass or glazing types in transmitting daylight while blocking heat gains. The higher the number, the more light is transmitted without adding excessive heat.
- Air Leakage. This rating is optional so you may not find it on some products. The lower the number, the tighter the window and the less air that will pass through cracks in the assembly.
- Condensation Resistance. Another optional rating, it measures the ability of a product to resist the formation of condensation on its interior surface. Shown as a number between 0 and 100, the higher the CR, the better the product at resisting condensation formation.
NFRC's chief executive officer, Jim Benney, says, “It is important for consumers to understand the impact that windows have on their energy bills. By choosing the right windows for their climate, homeowners can save more than 30 percent on their annual energy costs.”
“The two most important measurements are the U-factor, which indicates the rate of heat lost through a window, and solar heat gain coefficient, which measures how well a product blocks heat caused by sunlight,” Benney says. For example, he says dual-paned windows with low-e coatings are typically 20 to 25 percent better than uncoated dual-pane windows. Using gas-filling and warm edge spacers (low-conductance spacers to reduce heat transfer near the edge of insulated glazing) along with the top-of-the-line low-e coatings can reduce U-factors to 0.30, a 40 percent improvement.
Benney says that today's window frame materials that are more energy-efficient and low-maintenance include fiberglass, wood/fiberglass composite, other wood composites, vinyl windows, and vinyl and aluminum cladding, as well as thermally broken aluminum.
3. Visit the Efficient Windows Collaborative (EWC) website
Check out the EWC web site, developed by DOE's Windows and Glazing Program at the Lawrence Berkeley National Laboratory and the University of Minnesota's Center for Sustainable Building Research. The EWC is a coalition of manufacturers, research organizations, government agencies, and others interested in expanding the market for high-efficiency fenestration products.
Fact sheets provide details about different classes of windows for hundreds of locations across the country. A Window Selection Tool allows you to compare costs for a typical house by window type, city and more.
4. Download a simulation program
Get more information about window efficiency by downloading a simulation tool from the Berkeley Lab. While it does operate with some presumptions, you can personalize it with such factors as housing type, location, orientation, utility costs and characteristics of the windows you are considering. The software calculates approximate energy use and costs to help you compare windows.
5. Check with the American Architectural Manufacturers Association (AAMA) and Window and Door Manufacturers Association (WDMA)
While Energy Star and NFRC labels look at energy efficiency, two other labels look at window performance. The American Architectural Manufacturers Association (AAMA) and the Window and Door Manufacturers Association (WDMA) each offer a window product line certification process. Their programs are based on their harmonized standards and those of the Canadian Standards Association.
The WDMA's Hallmark Certification Program and the AAMA Gold Label Certification Program certify that a sample of that product met performance standards for air and water infiltration at the specified pressures, structural integrity and resistance to forced entry. Directories of certified products are available on their web sites.
WDMA Vice President of Technical Services Jeff Lowinski says that at times energy efficiency and strength are trade-offs. “The more air tightness in a window, the harder they might be to open,” he says. “Increasing the strength of a framing unit tends to make them more energy conductive.” He says consumers need to realize that whatever windows they choose, they must still comply with state and local codes.
Placement and size figure into how much benefit you will get from your windows. It's important to properly position them to let in enough daylight to reduce the need for artificial lighting, yet it's vital to recognize the needs of the various rooms and the local climate. Energy Star, for example, suggests the cardinal directions-north, south, east and west-be considered in locating windows rather than how their placement will affect your home's street-side appearance.
A U.S. Green Building Council spokeswoman, Jennifer Easton, says glazing techniques should be balanced with size and layout. “The tendency to 'over-glaze' with large windows, especially in kitchens, may increase a room's energy load,” she says. “Kitchen windows have different protocols than bathroom and bedroom windows, for example. Kitchen windows tend to be expansive and with little curtain cover, so energy performance is a priority.”
“Bathroom windows should seal out moisture-a challenging task considering the moisture generated from daily shower regimes,” she adds. “Windows in wet bathroom areas, such as the tub or shower surrounds, are problem areas. Sills in these windows should be pan-flashed/sealed as if they were exterior sills. Two people taking one shower each day for 8 minutes is equivalent to approximately 1,000 inches of driving rain per year!”
The USGBC's REGREEN handbook, she says, notes that “if different glazings are used on different orientations, as many green designers recommend, the optimal solar heat gain coefficient may be higher for south-facing windows, particularly in more northern climates. Many green designers also recommend selecting even higher-performance windows with triple glazing and two low-emissivity coatings in northern climates, in which case the U-factor could be as low as 0.20. Visual transmittance (VT) numbers should always be as high as possible.”