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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.
| | | |  | |  | | | | | The NFRC label can carry a lot of information. 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 lets you gauge the relative efficiency of different glass or glazing types in transmitting daylight while blocking heat gains. The higher the number, the more light 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.
| | | | | | | | | | | Proper Design
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.
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. 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.”
Read other Green Homes Special Series articles here.
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Text by Maureen Blaney Flietner
© 2008 BobVila.com
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