YOU MIGHT say the Baers hibernate in the winter, in a cross between a cave and a giant sand pile.

Or think about Karen and Ferdinand Baer as bottles of beer, insulated for the picnic in an ice chest.

Yet the south wall of their house is 90 percent glass, so that if science fails, Baer, a University of Maryland meterologist, can look outside to see if it's raining. In his case, he looks to see if his furnace, the sun, is working.

A heat pump makes up the difference when the sun doesn't shine. The Baers, hearty types ("I don't believe in waste," he said), keep the house at a cool 65. Which explains why their heating bill has been less than $100 a month, not bad at all for a 3,400-square-foot detached house.

Architect Roger Lewis explained the other day how passive solar heat, the ice chest and berm insulation combine to help keep the Baers warm during the winter.

The back of the three-level house is half sunk into the north hillside. At least 50 percent of the house is actually underground, thanks to the berm (an artificial dirt hill) pushed against the house.

Three of the walls -- north, east and west -- according to a suggestion by another architect (energy specialist David Ward) are built on the ice-chest theory. These walls are 12 inches thick -- concrete block filled with 250 tons of sand. On the inside, the wallboard is glued to the block. On the outside, a Fiberglas mesh is attached to the block. A layer of Styrofoam covers the mesh. Then a stucco called Dryvit, developed in Germany, is troweled on top of the Styrofoam.

"The block acts as thermal heat storage. It collects the heat from the sun and then radiates it back when it's needed," said Lewis. He collaborated on the house with David Lord. Both are also University of Maryland professors.

The living room and kitchen are covered with quarry tile, which helps absorb the heat from the glass. In the summer the south wall of sliding and fixed glass is protected by a 2 1/2-foot overhang. The fireplaces in the living room, between the kitchen and dining room, and in the master bedroom are recirculating Majestic models. Karen Baer says Ferdie Baer is a pyromaniac.

In the summer, the house is cooled off by opening the sliding glass.

The Silver Spring site, to begin with, is perfect for a solar house. The hill slopes steeply to the south. Tall deciduous trees brazenly drop all their leaves in the winter to bare their trunks to the sun. In the summer, they deck themselves out in green leaf finery to protect against the heat.

From the street, the house returns a rather blank stare, though there are those who think the three small windows -- one up high and two together lower -- make it look like a cyclops. This is the west side of the house and the windows are sort of peepholes. The "mouth" is glass set between the top and bottom cabinets. We walked up a steep stair to the entry portico. At this point you can see clearly how the house is nestled into the hillside.

The Baers greeted us in the front hall, a rather dark narrow passageway that soon widens out into the great (25-by-30-foot-wide, 10-foot-high) room, with its south wall of glass and a deck that stretches along for three-quarters of the way.

An alcove formed by the stair and a closet stereo serves as a stage for all the musicmakers the Baers favor -- recorders, kruphorn, viola gamba, flute.

"The room is so alive, with all that glass and hard surfaces, that it sounds really good," Baer said. The dining and sitting area are separated by support columns and a wide wood beam, part of the exposed structure of the house.

The kitchen is obviously a place where everybody cooks. "You notice we have two paper-towel holders," said Baer. The overhead shelving is all open. hA counter with a bar sink in it separates the working side of the kitchen from the breakfast area and serves as a buffet for parties. "It never seems crowded," said Karen Baer.

The Baers plan to add a greenhouse at the kitchen end of the deck. Baer is a great grower of soybean sprouts.

At the other end of the house the deck is cut away to allow light into the ground floor studio where Karen Baer works at her weaving. She is an art therapist and artist-in-residence at the Montpelier Art Center. She shows her weaving at Potomac Craftsmen Gallery. She studied weaving in Sweden when the Baers lived there in 1974-75.

Baer's study is on the top floor, along with a guest bedroom, the master bedroom with its deck and the laundry room.Their bath has a sauna and a great built-in tub. Baer's children are grown, and Karen's children don't live with them.

The Baer's didn't have the easiest time in the world building the house. "We had to learn how to be our own contractors when the one who started with us went bankrupt," they said. With the help of Charlie Turner, "a great finish carpenter," they completed it for $200,000. The house has 3,400 compact feet.

The Baers, when they get the money, would like to install ceiling tiles made of eutectic salt, a substance that absorbs heat in the day and releases it at night; rolldown insulated shades over the windows; and solar collectors (part of the roof is angled for them) for domestic hot water.

Baer, with more confidence than most weathermen, says his prediction for his house is sunny days.