By the year 2000, at least 98 percent of all the houses that now exist in the state of Maine, including its 19th-century farmsteads, will still be occupied.

Thus the Maine Audubon Society has created an unusual program called ''Farmhouse Retrofit,'' to demonstrate how to increase the energy efficiency of an existing house without destroying its architectural heritage.

The farmhouse renovations program serves to demystify a design field that the general public may have perceived as having been available only to energy specialists and architects. People have frequently been put off by the appearance of some retrofitted houses, which may ''end up looking like Martian spaceships,'' in the words of William Ginn, executive director of the Maine Audubon Society.

In 1975 the Audubon Society moved from downtown Portland to Gilsland Farm, a 78-acre wildlife sanctuary on the Presumpscot River in Falmouth. The society built a new headquarters building, based on a traditional saltbox design, but heated by an active solar system and wood - a stunning and efficient showcase for energy-saving techniques then in practice.

When the original farmhouse - an 1800 center-chimney Cape Cod house - came on the market in 1981, the society bought it as office space for its expanded energy department. It then incorporated in its renovation the latest energy technology now available for retrofitting existing structures without altering their historical character.

As a result, it has succeeded in retaining the stark beauty of the original dune-gray clapboard house with its wooden-shingle roof - and the additions, both interior and exterior, prove that the shapes and appearances of the new technologies have come of age.

Since ''Farmhouse Retrofit'' opened in October, thousands of people have walked through the showcase areas to familiarize themselves with the actual look and feel of living with the newfangled energy devices that have originated in Maine and elsewhere in New England.

The products and systems included had to be commercially available, technically viable, and cost-effective for homeowners.

Each installation is labeled with its ''payback period,'' the duration of time for the energy saving to equal the purchase cost; where applicable, its R-value, or resistance to heat loss, is also given.

''We hope to encourage homeowners to adopt one or more ideas for their own homes,'' says Christine Donovan, director of the energy department.

To achieve its dual goals of performance and aesthetics, the society first commissioned a computerized energy audit from the Cornerstones School for Energy-Efficient Housing in Brunswick.

The auditor evaluated the potential for solar-energy use,and recommended the best options for retrofitting the house.

Then the advisory service of Greater Portland Landmarks Inc. studied the structure and identified the original architectural elements worth protecting, as well as additions that could be altered.

The doorway of the five-bay facade with its delicate fan was considered the most important feature deserving restoration. The solid sliding shutters of the front rooms, which conserved heat in 1800, would continue to do the job now, if provided with new leather pulls rather than the old knobs, which allowed drafts.

The ell, the appendage of every Maine farmhouse, was pronounced a 1951 addition of no intrinsic value architecturally or historically. Therefore, architect Richard Renner of George Terrien Architects made it the focus of the technical innovations.

The front wall was removed and a 200-square-foot passive solar greenhouse was added. The greenhouse kit, three interlocking 4-by-10-foot units, was designed by Sunplace Corporation of Vermont.

In itself, the concept of a solar greenhouse is not unusual. This one is, however, because of its graceful appearance, the rounded contours of the roof, and the roomy interior that doubles the space of the ell.

Landscaping plays an important role in the appearance of the greenhouse. Because of an old stand of lilac trees left as a buffer between the house and the greenhouse, from certain angles the house may still appear to stand alone, protected from the north by tall cedars.

Inside, the view is spectacular. Three 8-foot-high, 1-foot-diameter, transparent water-storage columns, which absorb heat, serve to divide the space and thereby direct visitors along a circular route. Massive as these columns are , their transparency keeps them from being overwhelming.

In contrast to this strong vertical design are four, almost transparent, horizontal tubes suspended from the ceiling and filled with an innovative organic phase-change material that stores four times as much heat as an equal volume of water. The material becomes liquid at 70 degrees F. and absorbs more heat without an additional rise in temperature.

The quarry-tile floor over gravel on a concrete slab provides another form of heat storage. Proper nighttime insulation of the lights keeps heat from escaping. With all systems ''go'' on a sunny day, the greenhouse provides its own heat as well as that for the adjacent Energy Education Center.

Set into the roof of the ell and continuing to the greenhouse is another heating system: four pairs of 3-foot-square, plastic-sheathed Hansolar collectors, named after their inventor, John Hanson of Newcastle, Maine.

Two of the six window treatments are outstanding for appearance alone. The Window Quilt, a taut five-layered shade, raises or lowers on tracks installed on either side of the window. Magnapane, a clear plastic shield that snaps onto a magnetic strip framing the window, being practically invisible, is an excellent seal for such architectural features as sidelights.

Many of the designs are not limited in application to country homes, and may even be of use in city apartments.

One may order from the Maine Audubon Society an energy-information package containing two brochures, ''Retrofitting'' and ''Farmhouse Retrofit: Self-Guiding Tour,'' with illustrations by C. Michael Lewis and a text that describes the special innovations as well as the general work of insulating, weatherstripping, and caulking ($4 postpaid to Maine Audubon Society, Gilsland Farm, 118 Old Route 1, Falmouth, Maine 04105).

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