Not too long ago, the solar heat transmission properties of glazings were similar enough that they didn't figure into window energy ratings. But today's windows can have selective glazings that control heat loss in winter, or block solar heat gain in summer (see October 1994). In response to this, the National Fenestration Rating Council has developed a new window-rating system that considers solar heat gain in addition to R-value and air leakage.

Starting later this year, NFRC ratings will include two numbers between 0 and 70 that reflect annual heating and cooling performance. The numbers, which represent the Fenestration Heating Ratio (FHR) and the Fenestration Cooling Ratio (FCR), indicate the percentage of annual household heating or cooling energy the window will save compared to a worst-case window with single glazing and an aluminum frame. The higher the number in either category, the greater the savings. For example, an FHR 20 window adds 20 percent less to the home's heating bill than a worst-case window; an FCR 20 window adds 20 percent less to the cooling bill. FHR and FCR can't be used to do energy-load calculations. They're meant only as a tool for comparing windows.

According to Jim Larsen of Cardinal IG development group, who helped develop the ratings, a typical double-pane window might weigh in at around FHR 20 and FCR 10, while a very efficient vinyl window with low-e, argon fill and non-conductive spacers might have an FHR of 35 and an FCR between 20 and 35, depending on the coating.

Whether you choose high heating or cooling numbers will depend on your climate. "It's like looking at one car that gets 16 MPG in the city and 30 on the highway, and another that gets 25 city and 32 highway," says Larsen. "If gas mileage is a priority, you'll ask yourself 'Do I drive more in the city or on the highway?" Similarly, the separate heating and cooling ratings let you pick the right window for your climate. If most of your energy costs go toward heating, then you'll look for a high FHR. If most of your costs toward cooling, you'll look for a high FCR. If you live in a mixed climate where heating and cooling costs are about the same, you'll prefer roughly equal numbers.

One possible consequence of the new ratings, predicts Larsen, may be on energy codes in the South. In cooling climates, windows can account for 60 percent of a home's cooling load, even though they only take up 15 percent of the wall area. But the R-values most windows carry isn't what you need to pay attention to in cooling climates-the important number is solar heat gain. The lack of an accurate rating system has hindered residential codes from including solar heat gain requirements, but Larsen thinks the new NFRC numbers could change that.