Window Shopping The Latest in Energy-Efficient Windows

It’s a perfect time to shop for windows, since the new federal stimulus bill includes $1,500 in tax credits for homeowners who install them—provided they’re energy efficient. The National Fenestration Rating Council is a good first stop, since it rates windows on factors like air leakage and offers a U-factor measurement of how well the window guards against heat loss. There are a couple terms to know: “Solar heat gain coefficient” (SHGC) tells you how much solar energy (and heat) gets through; “visible transmittance” gauges how much light is lost in the balance. While it will take some time to recoup the cost, Energy Star reports that replacing your windows will save you between $126 and $465 a year and save the environment the equivalent of 980 to 4,545 pounds of carbon dioxide.

There are two approaches to window replacement: inserts and full-frame replacements. Inserts fit inside the old window frames, but installing them is only an option if the frame is in good shape, without rot, and square. Replacement inserts are normally custom built to fit your openings. The small gap between the old window frame and the new insert must be caulked and hidden with molding.

Full-frame replacements typically require the removal of the entire existing window, including the casings, frame and exterior trim. This method can be used to correct situations where the old window frame has deteriorated, is out of square or when you want to upgrade your style. While full-frame replacements involve more labor, cost and disruption, they will allow you to ensure that the area around the window frame, a common location of energy leakage, is well insulated.

Doubling, or Tripling, Up

Efficient windows typically have two layers of glass and are called “double glazed.” The small gap between the glass layers creates a barrier to heat flow by conduction. That barrier may be enhanced with an additional layer of glazing, in which case the window is called “triple glazed.” Some windows have two layers of suspended plastic film and three gaps or spaces. The more spaces, the better the barrier will be at stopping heat flow. The gaps between layers of glazing are usually filled with a gas like argon or krypton that further reduces heat flow.

In addition to glazing, you will have to decide about reflective films, glazing tints and low-emittance (low-E) coatings. Reflective films block much of the radiant energy striking a window, and most of the visible light, too. In addition to looking like poor-quality mirrors, they may cause occupants to use more electric lighting to compensate for the loss of daylighting. Bronze- and gray-tinted glazings reflect radiant energy and reduce cooling loads without reducing as much of the visible light entering the home. Spectrally selective tints (usually light blue or light green) are even better, allowing less solar gain but more visible light in.

Low-E coatings are more versatile than either tints or reflective films and are virtually invisible. These microscopic layers of metal or metallic oxides suppress radiant heat flow out the window and can be formulated to allow varying degrees of solar radiation in, depending on your climate and window orientation (South- and West-facing windows get the most sun). The material with which the window frame is built will alter its efficiency, too. Insulated vinyl and fiberglass perform better than wood, wood clad and uninsulated vinyl. Aluminum, even with a thermal break, performs worse than all of the above. Experts suggest minimizing the frames altogether by opting for windows with large unbroken areas of glass.

Prices vary widely. According to Tim Skeer, owner of the installation company Pelican Glass, Inc. in Carlsbad, California, it would run about $6,000 to replace 10 windows and one sliding glass door in his community. If you opt for custom-made fiberglass instead, the price would double. If you prefer wood frames, the price would go up further.

The truth is, replacing old windows is rarely the most cost-effective, energy-saving strategy. Think about it: You’re upgrading a relatively small area of your exterior from something close to R-2 to R-4 or R-5 at best. To put a real dent in the utility bills, you’d be wiser to invest in insulation, weatherstripping, caulking or even insulating curtains first.