Every window worth its salt comes with a label from the National Fenestration Rating Council (NFRC) that provides a simple standard for window energy performance (see example at left). Four key numbers on this label describe the impact of the entire window unit, not just the glass, on the heating and cooling load of the building. Of these values, two — the U-factor and the solar heat gain coefficient (SHGC) — bear close scrutiny.
U-factor is a measure of heat flow (and the inverse of R-value). The lower the U-factor, the less heat will move through the entire window. Mark LaLiberte, co-founder and president of Construction Instruction, a national training and education consultancy that works with many of the nation's top home builders, recommends a U-factor of 0.35 or less, regardless of the climate in which you build. LaLiberte, a team adviser for the Department of Energy's Building America program, acknowledges that this recommendation is more conservative than the Energy Star program, which allows U-factors up to 0.60 in hot climates where radiant solar heat gain is of much greater concern than conductive heat flow (The Energy Star program divides the country into four climate regions and sets prescriptive and performance values for maximum U-value and SHGC. These values can be found at www.energystar.gov, then navigating to Products, then to the Residential Windows, Doors and Skylights energy-efficiency guidelines.)
Solar heat gain coefficient (SHGC) measures the amount of solar heat gain that passes through a window. An SHGC of 0.35 means that 35% of the radiant heat striking the window will pass through the glass, while 65% will be reflected back outdoors. The lower the SHGC, the lower the cooling load on a home will be. Janet McIlvaine, a research analyst at the Florida Solar Energy Center, urges that a "low solar heat gain" window of at least SHGC 0.35 is acceptable for hot climates, though lower is advisable. In cooler climates, designers sometimes spec windows with a higher SGHC, wishing to capitalize on solar heat gains. The most common strategy is to design the home with a 2-foot overhang (the widest overhang usually allowed in high-wind regions) to shade the windows from the high summer sun, and select a window with a SHGC above 0.50 to capture solar gains in winter when the sun is low. But LaLiberte and McIlvaine caution that this strategy should be employed only when the orientation and shading of windows can assure that the windows won't "see" the hot summer sun. Anywhere air conditioning is used during the summer, a low-SHGC window is a safer bet.
Visible-light transmittance (VT) refers to the amount of daylight passing through the entire window area. The heavier the frame and the more divided lights a window has, the lower the VT will be. This number is far more important in commercial buildings, which often use heavily tinted or mirrored glazing that can reduce daylighting possibilities and increase electric lighting costs. For residential windows, McIlvaine advises that the VT rating should be higher than the window's SHGC. That is, there should be more light than heat coming through the glass. But otherwise, it's not a number worth sweating over.
Air leakage (AL) ratings are expressed as an equivalent cubic feet of air passing through a square foot of window area. The lower the AL, the less air will leak through the window assembly. However, this rating is optional, and rarely included on residential windows, even though more energy is usually lost from air infiltration and exfiltration than by conduction or radiation. The justification put forward by some manufacturers for not including this rating is that air leakage between the rough opening and the window unit has the potential to carry away far more than heat than would leak through the unit. The bottom line: Be sure to properly seal between the window unit and the rough opening.
For more on window performance, see "Selecting Windows for Coastal Homes."
