''It is called draperies by the bolt,'' explains New York designer Charles E. Pavarini III. He is referring to an unusual window treatment shown in the handsome room done by Pavarini/Cole Ltd. for the Design Showhouse here of the International Society of Interior Designers.
Mr. Pavarini, who has also done theater costume design, says he did here what the couturiers do. He simply draped by hand the yards of iridescent silk taffeta over three brass rods placed at different levels. He intertwined the moss green/rust taffeta with the same cream-colored silk that is on the walls, cascading the fabrics over the rods at different intervals. Once the draperies were arranged in place, the ends were hemmed by hand.
''We felt this draped 'couture' treatment framed the windows nicely and that no scrim curtain was necessary behind it,'' says Mr. Pavarini. Although the windows look out on a busy city street, exterior wrought-iron trim helps diffuse the view. And inside, to further stop the eye, the designers placed high 19 th-century antique painted wood pedestals in front of each window.
The partners lighted the window area to bring out the shimmery gleam of the iridescent taffeta, and the lights and shades of the cascades and folds.
The walls in the room, upholstered in cream-colored silk, were padded with Du Pont's new Quallofil cushioning and stitched in the identical block pattern of the brick exterior of the 1904 Georgian house. The room also features a circular fireplace cut into a new wall of cream-colored fossil marble. The ceiling is covered with silver Chinese tea paper, which has a gentle sheen.
Many of the furnishings are English Regency antiques. The candelabrum and the pedestals are in Empire style.
The room, designed by Pavarini/Cole for ''relaxation, renewal, and entertainment'' is all set to welcome guests for an after-theater buffet supper.
This room is one of many exhibited in the design showhouse now at the landmark Sara Delano Roosevelt House, 47 East 65th Street in New York. The house was also the home of Franklin and Eleanor Roosevelt from 1908 to 1928.
The 21 settings done by members of the New York chapter of the International Society of Interior Designers will be open to the public through Oct. 30. Hours are 11 a.m. to 6 p.m. Tuesday, Wednesday, and Friday; until 8 p.m. Thursday; and from 12 noon to 5 p.m. Saturday and Sunday. Admission is $8.