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Decoding the Window Rating Label

If you need to replace rather than restore a window, an energy-efficient model can help keep you comfortable and keep your heating bills under control during cold weather. With so many window choices, how can someone compare their pluses and minuses? Windows (as well as doors and skylights) are rated for energy efficiency by the National Fenestration Rating Council (NFRC); the results appear on the window’s energy performance label.

Graphic courtesy of NFRC

Graphic courtesy of NFRC

Like a food label that lists calories and fat grams, the NFRC label lists a window’s heat retention, air leakage, and other properties in a standardized way that allows for comparison. But not every food with a nutrition label is healthy, and not every window with an NFRC label has the highest energy savings. Only the most efficient windows receive the Energy Star label. Here are some helpful tips on what to look for:

U-factor measures the rate at which heat escapes through the materials that make up the window itself. Look for a low number; this means that little heat is lost by conduction through the glass, sash, and frame. U-values generally fall between 0.20 and 1.20.

Solar heat gain coefficient measures how well a product blocks heat caused by sunlight, expressed as a number between 0 and 1. Residents of highly air-conditioned climate zones should look for a lower this number (less solar heat transmitted into the house) and those in colder climates can choose a higher number (more solar heat transmitted).

Visible transmittance measures how much visible light comes through, also expressed as a number between 0 and 1. The higher the visible transmittance, the more light comes through. Low-e (“low emissivity”) coatings on the glass, which are often made of metal or metallic oxides, can affect this rating.

Air leakage indicates how many cubic feet of air per minute pass through a square foot of window area (cfm/sq ft). In contrast to the U-factor (heat loss by conduction through the window itself), air leakage measures heat loss by infiltration through the cracks. The lower the number, the less air will pass through cracks in the window assembly.

Condensation resistance measures the window’s ability to resist the formation of condensation on the interior surface. This is measured on a scale of 0 to 100, with a higher number meaning that the window is better at resisting condensation.

The quality of the window installation is just as important as the factors measured on the label. For example, if the frame is spread when the window is installed, the sash can become loose and the infiltration will be compromised.

It’s also important to choose a window that will stand the test of time. BBR recommends making sure the window has a good quality block-and-tackle balance (the mechanism that holds the window sash up when you open it). It should be easy to take the sash in and out, and vinyl windows should be heat welded, not assembled with screws.

Purchasing windows with the highest energy efficiency can get expensive quickly. Is there a point where price becomes prohibitive for extra energy saved? Doing a heat-loss calculation for your house can help you make those decisions. A heat-loss calculation is based on a house’s square footage of walls and windows, and best guesses about existing insulation. Using a computer spreadsheet, you can compare how much energy you would save from buying the most efficient windows compared to spending the same amount of money on other improvements, such as insulating, caulking cracks, or replacing a door. (An online heat loss calculating spreadsheet is available at http://www.pprbd.org/plancheck/heat_loss.html.)

Energy Star windows available from BBR’s co-op include the Harvey Majesty (wood with aluminum clad exterior), Tribute (vinyl), Classic (vinyl), and Slimline (vinyl). Velux skylights also earn the Energy Star rating.

Adapted from an article that originally appeared in the Spring 2007 issue of our HandsOn newsletter.