Holman and Roberta DAvis live in Topsham, Maine, where fall comes early, spring comes late, and winter, at times, seems interminable. Yet in a south-facing room they recently added to their home, they can gain a winter tan as effectively as on any subtropical beach. And while they bask in their "little bit of Florida," this same room is pumping warm air into the rest of the house at a rate that should cut 20 to 25 percent from their annual fuel bill.

The Davises' home was a typical 1 1/2- story Cape Cod until they added their sun room, greenhouse, or walk-in solar collector, as some people call it. They needed additional living space and were planning to add a conventional room when they read of the advantages of passive solar construction. Why not, they thought, and they have been glad they scorned convention ever since.

The 10-by-16-foot addition was designed by Charles Wing of Cornerstones Energy Group Inc. of Brunswick, Maine, and built by a local contractor for $20 a square foot. It consists of a conventional insulted roof and east and west walls. The south-facing wall, however, is all double-paned glass (actually patio doors, minus handles and locks), which slopes at an angle of 60 degrees to allow for maximum penetration of the winter sun.

On any day the sun shines, the sun room "heats the entire house to around 70 degrees F.," Mrs. Davis days. But she adds that "on really cold days the house quickly cools down once the sun goes down, and the central heating has to come on." On just heat savings, the payback on such construction could take anything from 10 to 15 years. But the Davises were looking primarily for additional living space, and in the process they found a system that will eventually pay for itself in the free heat it delivers.

Moreover, the new room is what the family describes as quality living space. "Delightful," Mrs. Davis calls it. "I love to come out here to read and sew, and it makes a perfect game room for the children."

The Davises expect to get the most advantage from their sun room during late January and February, when winter temperatures plunge to their lowest. Mr. Wing explains why: "By the end of January the sun has come a lot farther north. In other words, there is a good deal more sun around. At the same time there are fewer cloudy days in the depths of winter, so that the full strength of the sun gets through."

The Northeast Solar Energy Center lists these advantages to be gained from an attached greenhouse or sun room:

* More space: The sun room provides pleasant year-round living space that can be used as a playroom, breakfast nook, sewing and ironing room, greenhouse, etc. Some people even use it for sunbathing during winter.

* Reduced fuel bills: As fuel costs for the rest of the home continue to rise , the free heat from an attached greenhouse can save 15 percent and more of the annual bill, depending on the size of the home. The high heat of a greenhouse ( 80 to 90 degrees on even the coldest days) will vent naturally into the rest of the house through an open window or door. Better still, a small exhaust fan will speed up the process. The Davises spread the heat evenly though the house using a standard three-speed household fan.

* Added resale value: As home heating costs continue to rise, any addition that improves the energy efficiency of the home increases its value by more than just the cost of construction.

* Fresh low-cost vegetables year round: The United States Department of Agriculture predicts a 10 percent increase in the price of fresh produce at the marketplace this year. But a moderate-size greenhouse can readily produce $275 worth of home-grown produce in a year, depending on the skill of the homeowner.

* Low-cost construction: Construction costs of attached solar greenhouses should be no more, and generally less, than conventional construction costs. Materials for a moderate-size greenhouse (150 to 200 square feet) cost from $800 to $1,200 for the do-it- yourselfer. Double that price for an installed greenhouse.

An important component of the solar greenhouse is thermal mass -- any material (brick, stone, concrete, water) that absorbs heat during the day and then returns it to the air as the temperature begins to fall. A concrete floor, poured over foam insulation, provides excellent thermal mass. Another option is water-filled barrels or plastic bottles. "Two to three gallons of water per square foot of glazing will maintain a temperature difference of a about 30 degrees F. above outdoor lows overnight," according to the Northeast Solar Energy Center. Good thermal mass also prevents a greenhouse from overheating during the day.

A wide range of books and brochures is available, with plans for solar-greenhouse construction. Your local Federal Department of Energy office will be able to assist as well.

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