Low-E glazing is marketed under a variety of names: SunBlocker, Sun-Coat, SunGate, and ComfortGlaze to name a few. Although it's clear that these products have something to do with energy efficiency, the name doesn't really tell how one might perform compared to another.

“Low-E coatings are certainly not all the same,” says Steve Easley, a building-science consultant based in Danforth, Calif. “How manufacturers use low-E technology varies.” Standard low-E coatings do a good job of reducing heat loss by reflecting heat back into a room, Easley says. A window with this coating would be a good choice when there is little concern about summer air-conditioning costs. But wherever air conditioning is used for a substantial portion of the year, windows need spectrally selective low-E coatings.

Spectrally selective coatings let most visible light through but block the shorter wavelength infrared solar heat waves, which greatly improves summer performance. These windows also block a considerable amount of the ultraviolet light that can fade fabrics and other interior finishes.

Read the Numbers The only reliable way to choose a window based on energy performance is to pay close attention to the window sticker, Easley says. The National Fenestration Ratings Council (NFRC) has established clear standards for window performance that provide an objective measure of how well the entire window unit, not just the glass, will help reduce the energy load on the building. The two most important numbers on the label are the U-factor and solar heat gain coefficient. U-factor is a measure of heat flow and is the inverse of R-value. The lower the U-factor, the less heat will conduct through the entire window. In any climate, a window with a U-factor below 0.35 is a good baseline.

The solar heat gain coefficient (SHGC) measures the amount of solar heat gain that passes through a window by radiation. An SHGC of 0.33 (an optimal limit) means that 33% of the radiant heat striking the window is passing through it. The rest of the sun's heat is reflected back outdoors. The lower the SHGC, the lower the cooling load on a home will be.

Fading Factor The other number commonly found on the NFRC label, visible-light transmittance (VT), is more important in commercial windows. In most residential windows with low-E coatings, the VT rates are only slightly less than those for totally clear units. You should focus on the U-factor and the SHGC in the window style that fits the job, Easley says, and then, if you have a choice, pick the window with the highest VT. This number should not be confused with the window's ability to block natural light that causes fading of fabric or wood finishes.

Manufacturers often claim to protect against fading by citing the amount of UV light blocked by the glass coatings on their products. But, Easley argues, some fading still occurs due to visible light. “Total damage weighted” (Tdw) values describe how much fade-causing natural light (both UV and visible) actually passes through a window. Tdw values are not often publicized, but manufacturers will provide the information. A Tdw value of 33% means that a window is blocking 67% of the rays that cause fading.