Here's what the Energy Star website says about fenestration products and federal tax credits available under the American Recovery and Reinvestment Act of 2009 (ARRA): "To qualify for the tax credit, windows, doors and skylights placed in service after February 17, 2009 must have a U-Factor and Solar Heat Gain Co-efficient less than or equal to 0.30."

Pretty straightforward ? unless you make, sell, or install skylights. Because while many window and door manufacturers make products that meet those criteria ? and have the National Fenestration Rating Council (NFRC) stickers on the glass or wood to prove it ? there are few skylights that would qualify for a tax credit under the current rules, which became effective June 1. And those skylights that would qualify are custom triple-glazed units, such as, for instance, several manufactured by CrystaLite, in Everett, Wash.

Gas Guzzlers?

Is that because skylights, which have an estimated 2% share of the market for fenestration products, are the industry's equivalent to gas-guzzlers?

That might be the wrong way to think about it. Roger LeBrun, product certification engineer for manufacturer Velux, says that skylights don't qualify because the rules determining which products save enough energy to qualify and which don't unfairly exclude them. The ARRA language "deviated from an Energy Star­-rated product to a simpler, single-number qualification," LeBrun says. "And skylights cannot meet those numbers."

To be eligible for tax credits under the ARRA rules, as of June 1, a window (or skylight) must have a U-factor of 0.30 or less and a Solar Heat Gain Coefficient of 0.30 or less. U-factor measures heat loss through the window, SHGC measures unwanted heat admitted. Any window tested, rated, and certified to show that it meets the 0.30/0.30 criteria is eligible for tax credits.

But skylights, at least the units on the market today, have almost no chance of meeting those criteria. Before the ARRA laid down the 0.30/0.30 rule in February, skylights with a U-factor of 0.55 to 0.50, depending on climate zone, and an SHGC of 0.30 to "any," again depending on climate zone, were considered efficient according to 2009 Energy Star standards.

"Take the same design for a window sufficient to produce a 0.30 U-factor rating and apply it to a skylight and it wouldn't be nearly sufficient," says Nils Petermann, project manager of the Efficient Windows Collaborative in Washington, D.C. "It's not that the skylight is any less efficient than the window, it's that it faces harsher conditions. The skylight faces the cold sky."

Typically, more heat escapes through a skylight, which is essentially a window on the roof, and more still escapes if it is a raised skylight. "You have more molecules in contact with the glass more frequently, so it increases the rates of heat flow," LeBrun explains. That alone, he says, could account for as much as a 25% increase in heat loss through the windowpane. In short, he says, "Windows and skylights can't be compared."

Light and Heat

Skylights are windows placed on the roof. Some are flat, some are domed. Some open, some don't. Most use double-paned glass and some use insulating gasses and glass coatings such as low-E. Their purpose is to let natural light into the house ? a lot of it.

A window, LeBrun says, may provide natural light sufficient to read for a distance of, say, 5 feet from the opening. Skylights, on the other hand, typically admit far more light. In doing so, they save energy that would otherwise be consumed burning electricity and light bulbs. That aspect, says LeBrun, was not a factor in the ARRA tax credit calculations, which took into account only the amount of heat escaping and the amount of unwanted heat admitted. And determining efficiency based only on U-factor and SHGC means it's unlikely that skylights will qualify, at least for tax credits under the ARRA. "We don't have any ability to make a product that qualifies [for tax credits] without throwing out every design we have," LeBrun says.

Petermann points out that to make skylights qualify would mean that manufacturers would have to completely redesign their products, then re-tool. "There are ways to do it ? [additional glass layers, plus coatings, plus changes to the frame] ? and it is possible, but they all cost money," he says. "But it would have to happen before the end of the year." And that's unlikely.