In accordance with the present invention, a rectangular inner sub-sash frame 50 is enclosed within the outer sash frame 12 and is formed by elongated sub-sash frame members 52 (FIG. 2) and 54 (FIG. 3) which are rigidly connected at the corners of the sub-sash frame, for example, by mitered cuts and welding or by mechanical fasteners such as screws.