There are many ways to retrofit a window. Most strategies involve replacing the glass, frame, and sash with double-paned low-E glass, and a new wood or vinyl frame and sash. Major manufacturers offer sash replacement kits so you can fit new glass and sash into an existing frame.

One retrofit method, however, is unique in that it involves replacing only the glass and tracks--both the original frame and sash are retained. There are several advantages to upgrading a window this way: it costs less, the look of the old windows is retained, and you don't have to throw away a lot of good lumber.

A window upgrade method designed by contractors Jim and Maisy Conachen of Canton, Massachusetts, uses a patented router guide system that they are commercializing with the help of the Department of Energy (DOE). The Conachens train other contractors to use the system, which they claim costs less than replacing the entire window.

To get an idea of how it works, take the example of an old wood-framed window with six divided lights (individual small panes of glass divided by wooden grillework, or muntins). The window technician first removes the sashes and all movable parts, and then installs tilt jamb-liners containing a new mechanical balance system. (These tilt tracks make the window open at an angle that allows for easier cleaning.)

The carpenter cuts and profiles the sides of the sash to fit the new tilt jamb-liners. The sash is then put in a router guide system (patented by the Conachens), to cut back the exterior side of the muntins and remove the glass. (The interior side of the muntins is preserved.) The router is used to widen the gap where the glass fits into the sash, and the new double-paned glass is glazed in (set into siliconized latex sealant). An exterior wood stop is fastened, and the muntins, now purely decorative, are attached. The interior muntin is from the original grillework; new matching grillework is installed on the exterior. The muntins are strengthened at the joints and can be made removable. Finally, the window is weatherstripped and reinstalled into the existing window frame.

Variations of this technique are used for other types of windows. Also, the homeowner may decide to do without the exterior grillework, or to have the contractor glaze the glass flush with the outside of the sash to reduce exterior maintenance. One application particularly well-suited for the Bi-Glass system is in renovations of historical buildings, since the original look and feel of the window is retained.

According to Jim Conachen, a contractor can retrofit six to eight windows per day. The technician brings the router system and other tools in a trailer or van and does all of the work on the window and sash at the site.

After using their Bi-Glass system for eight years, the Conachens received a grant in 1990 from DOE's Energy-Related Inventions Program for their router guide system. DOE gave them assistance and expertise on how to structure a commercialization effort. As a result, Bi-Glass offers a five-year technical license to companies who wish to use the system for an initial fee of about $19,000 and an ongoing license fee of $500 per month. Licenses are for existing companies who wish to add window retrofitting to other related services.

More than 4,000 windows have been converted in more than 350 homes in the New England area since 1986. Any type of glazing can be used, but so far about 90% of the retrofitted glass has been argon-filled and low-E coated.

Licensees are chosen by the Conachens, who will train only one contractor in a given region. The goal is to cover all areas of the country, says Jim Conachen, who doesn't assign territories, but tries to avoid overlap in areas served by licensees. Window upgraders already are operating on the East Coast from New England to Atlanta, Georgia, and west to Michigan and Texas. The Massachusetts-based team trains one to two licensees per week. The Conachens have also licensed the technology to a Canadian company, who will handle commercialization efforts in Canada.

According to the Conachens, a houseful of windows can be upgraded for about $125 per casement window or $200 per double-hung. Any style window with a wood frame can be upgraded.

While there is more labor involved in upgrading a window than in installing a new one, savings in material costs more than make up for it. Increases in wood prices may make window upgrades even more economical for those who want wood windows.

• -- Jeanne Byrne