A house in Gaithersburg, partly funded with a $12,000 federal solar design grant and recently sold for $193,750, will be open for public inspection tomorrow. It has an atrium living room with a two-and-one-half story ceiling.

On the first floor the atrium is surrounded by family and dining rooms, a kitchen and study. Four bedrooms overlook the atrium area on the second floor. The master bedrooms is 36 feet long.

The passive solar energy aspect of the 3,200-square-foot house includes a concrete slab covered by ceramic tile as the floor of the atrium.

An energy-efficient fireplace rises to the roof under a ceiling containing three skylights. The south-facing atrium wall is 90 percent glass, according to Alan Trellis, a partner in Trellis & Watkins Inc., the developer of this house built on a four-acre lot in the Meades of Muncaster subdivision.

Trellis said the passive solar atrium obtains heat from the winter sun and stores it in the ceramic-tile-and-concrete floor. At night that stored heat is radiated through the interior of the house by convection currents. About 40 percent of the heat is expected to be supplied by the solar system. An electric heat pump system provides back-up heating and air conditioning.

Other energy-saving features include triple-glazed windows, with scant fenestration on the north side. Windows on the west side are recessed to provide protection from prevailing winds, according to partner William Watkins.

Exterior walls are built with 2-by-6 studs and have R-19 insulation. Ceilings have 10 inches of insulation for an R-30 energy-saving rating. An airlock on the north side entrance isolates hot or cold outside air from the main house. Also, the garage and clothes closets are strategically situated within north walls to act as buffer zones.

A 48-foot, second-story balcony blocks the summer sun from the first floor.

To help satisfy requirements of the federal grant program, the house will be open from 1 to 5 p.m. tomorrow. It is reached by going east on Shady Grove Road from Rte. 270 and right on Muncaster Mill Road; left on Muncaster Road about three miles to Granby Road and left there, following signs to the site.