Windows 2000.

Saving Money With Energy-Efficient Fenestration 

Most of us think we're being energy-conscious when we remember to put up the storm windows in the fall. But just because there are two panes of glass between you and the outside world doesn't mean you're protected against significant energy loss. Window technology has made a lot of progress in the last 20 years.

Don't take your windows for granted. Drafty panes can account for 10 to 25 percent of a home's heat loss in winter months. If you have single-paned windows, you really are heating the great outdoors--they can lose 13 times as much heat as an equivalent area of wall space. Triple-paned windows are the best at retaining heat, but they'll pay for themselves only in the colder states.

The energy efficiency of windows is no longer a matter of speculation and weighing manufacturers' hyperbolic claims. The National Fenestration Rating Council (NFRC) created some order in the world of windows by developing national standards, which have since been adopted as the basis for the federal government's Energy Star sticker program. According to Energy Star's Sharon Spencer, if you see the sticker, it means the window has met NFRC criteria for one of three U.S. climactic zones. (A window destined for northern Minnesota, for instance, has to have a lower "U-value" meaning it will lose heat less rapidly than a window going to southern Florida.)

Susan Douglas, NFRC's administrator, says that "windows have become much more efficient in recent years. Now they're pieces of engineering, not just a pane of glass between four sticks." She cites technical breakthroughs as a factor in an accelerating pace of window replacement. With super-insulated windows, cost recovery occurs quickly.

How quickly? According to the Efficient Windows Collaborative, a typical 2,000-square-foot house in Boston with single-parted aluminum windows can expect to pay nearly $1,000 a year in heating costs. Take that same house and install "super-windows" and the cost drops 39 percent, to around $600 a year. If that house were in Phoenix instead of Boston, the savings are less dramatic, about $300 or a 32 percent reduction.

The "superwindow" has special powers but it isn't made of kryptonite. Its positive properties are numerous. Here are a few of the high-tech innovations:

Low-e. These very thin metal oxide or semiconductor window films, also known as sputtered coatings, cut down on what's called solar gain, or the amount of incoming heat from the sun. Like tinted glass in car windows, they can make a room more comfortable without reducing the view when applied between two panes of glass. Low-e films also have the added benefit of reducing heat loss in winter. Another type of Low-e coating is high solar gain, meaning that it increases the passive solar effect, making it useful for greenhouses and reducing energy costs.

Vacuums and Argon. What's between the panes of double-glazed windows? Air, right? Wrong! In the superwindows, argon gas, which is inexpensive, nontoxic, clear and odorless, fills up the cavity to increase thermal performance. You can also use, believe it or not, krypton (maybe there is some Superman connection here), which is even more efficient but also more expensive. Susan Douglas says the latest thing in window design is a cavity vacuum between the panes.

Filtering Light. Right now, you can buy a super-efficient window that combines multiple Low-e coatings, low-conductance gas fills, interpane barriers (to reduce convective circulation of the gas), and insulating frames and spacers. But likely to be on the market within two to five years are "smart" windows with glazings that can adapt their light filtering properties ill reaction to frequently changing ambient temperature. A second "active" type now under development uses a small electric current to alter its transmission of light.

If you're seriously thinking of replacing your windows, the South Carolina Energy Office has some advice. Look for the highest R-value, of course, but also look at window type. Casement windows are the most efficient as a general rule, with awning and sliding windows next. Double-hung windows have the highest leakage rates, but still might be the most practical for you.

Closely examine the window fit to make sure there are no leak-prone gaps. You can tell if air is leaking where the window frame meets the house by holding a lighted candle or lighter at the sash joint. If the flame flickers, there's a leak. Fixing it may be a do-it-yourself job, if all that's required is removing old caulk from around the window frame and applying a new layer. You can also apply weatherstripping between the sash and window frame--as long as it doesn't interfere with the window's operation.

The neighbors may not notice your well-insulated windows, but you will--when the heating bill arrives. CONTACT: Energy Efficient Building Association, 490 Concordia Avenue, St. Paul, MN 55103/[phone]; National Fenestration Rating Council, 1300 Spring Street, Suite 500, Silver Spring, MD 20910/[phone].

JIM MOTAVALLI is editor of E.