Abstract: In a window construction of the type incorporating particularly operating sash or sashes, and in certain embodiments an operating sash in conjunction with a fixed sash, a perimeter frame surrounds the composite of sashes, and supports an intermediate frame that is arranged between each pair of adjacent sashes. The perimeter frame is constructed of semi-tubular like structure, and that portion of the perimeter frame surrounding the exteriorly opening operating sash includes a formed shoulder that cooperates to form a sealing contact with face flanges integrally connecting to the framework of the operating sash. Where a pair of operating sashes are arranged adjacently within the perimeter frame and intermediate frame, the formed shoulders contact the sash flanges when said sashes are closed, while the back of the sashes exposes a surface that retains a sealing contact with the proximate rear flanges of the perimeter frame when said sashes are maintained in closed position.