Windows are much more efficient than they were during the 1970s twin energy crises that catalyzed the long- running string of window improvements. You have a lot more choices than in those days, and the National Fenestration Rating Council sticker helps you make them. Some window manufacturers still confuse everyone by highlighting one strong feature — say, R-8 center-of-glass R-value.
NFRC ratings quantify the differences between windows and level the playing field. They “normalize” information, putting it in a common language and consistent format. They also allow remodelers to “tune” their windows for optimum year-round performance. Here’s a quick summary of the key window properties that the NFRC rates.
U-value is the key factor. It sums up the heat transfer rate of the entire window product: glazings, air-space thickness, type of edge spacer and frame, plus low-E coatings and insulating gas fillings. U-value is the inverse of R-value: the lower the number, the slower the heat loss during winter. The impacts of incremental changes to one sample (real) window are listed in the table on page 62.
Generally, a primed wood window performs about the same as a vinyl unit with comparable glazing and edge spacer. But adding a cladding outside wood frames increases (harms) the U-values of most low-E, gas-filled wood windows by 10% to 25%. Furthermore, depending on how they are detailed, metal claddings can exacerbate a window’s tendency to cause condensation around the perimeter of the glass.
Footnote for remodelers in mountain areas: Unfortunately, you can’t ship high-performance windows to higher altitudes. This limits their performance to about U-0.25.
Solar heat gain coefficient
This factor addresses the question: How much of the solar radiation that hits the outside of a window — both glass and frame area — gets transmitted into the home? This goes to the heart of a practical concern: comfort.
The solar heat gain coefficient (SHGC) is a dimensionless number from 0 (no solar heat gain) to 1.0 (100% solar heat gain). Given the blockage of the sun by window frames and various kinds of glass, the normal range for windows is 0.25 to 0.65 (65% solar gains). Most vinyl double-glazed windows are rated at about 0.65; adding low-E coatings decreases solar heat gain, as does shifting to a wood frame.
In a perfect world, all remodelers and builders would “tune their windows.” East- and west-facing windows would have low SHGCs (0.25 to 0.35) to cut down on air-conditioning loads, while south-facing windows would transmit more sun (0.50 to 0.65) to increase wintertime solar heating.
Air leakage around window sash and frames in a new home or addition is generally a small fraction of the total air leakage — typically 5% or less. Unless a home is located at a very windy site, window U-value and SHGC are much more important to energy efficiency and comfort than window air leakage is. If tightness is important, casements, awnings and fixed units are nearly always tighter than sliders, single-hungs and double-hungs. And remember that noise — whistling sounds — can be a real drag in windy sites, so pick accordingly for such homes.
While the window sash of vinyl and wood windows is relatively tight, home energy raters generally find that wood frames are a bit leakier than vinyl because drywall returns are easier to seal to vinyl frames than wood trim is to wood frames. (Note: Fiberglass chinking is not sufficient to stop air leakage around vinyl or wood windows. Low-expansive foam or foam backer rods are recommended for this task.)
Visible light transmittance
Controlling the amount of visible light entering a window is important when you want to avoid glare. This factor parallels the solar heat gain number. It’s a fraction of the light transmitted through glass compared with the total light striking the outside of the window glass and frame. It, too, is expressed as a number from 0 to 1.0. Again, selection of glass and type of frame are the key factors.
First, ignore any center-of-glass claims by manufacturers. Second, don’t buy a window that hasn’t been rated by the NFRC. Third, set a performance target and shop for the best product and service that support that performance. A reasonable target for remodelers is an insulation value of U-0.35. Finally, savings on ductwork and air-conditioning sizing can more than pay for the cost of upgrading to higher-performance windows.
Remember to sell the benefits of your high-performance NFRC-rated windows: better year-round comfort, less condensation on glass, much less fabric fading, smaller heating and cooling equipment required, lower energy bills and, eventually, higher resale value.
Steve Andrews is a Denver-based energy consultant and freelance writer and a board member of the Energy & Environmental Building Association.