Abstract: An insulative window assembly for use in portable shelter units such as mobile homes, motor homes and trailers. The assembly is attached to the inside wall of the portable shelter unit surrounding the existing window and window molding, but may be easily removed in warm seasons to regain access to the existing window. The frame of the insulating window assembly is of sufficient depth to allow room for the stem of the crank to which the crank handle is attached for opening and closing a jalousie window assembly. The frame of the insulating window assembly contains insulating, sag resisting chambers which contribute to the heat insulative properties of the insulating window. The frame also contains an inner flange and an outer flange which, when bolted to the inside wall of the portable shelter unit, lends stability to the unit and helps prevent the sag and broken seals that frequently result from the vibration and weather forces to which portable shelter units are particularly vulnerable.