Abstract: A storm window unit incorporating extruded metal frame members assembled in the form of an inner frame including slide tracks for holding removable, slidably movable window panels, and an outer frame comprising a plurality of expander caps fitted about the inner frame and movable by jackscrews to expand the outer dimensions of the window unit. The expander caps extend preferably along three sides of the inner frame, and corner caps are provided to fill any gaps created between the ends of adjacent frame members by expansion of the outer frame, so that the window unit can be installed to fit snugly and sealingly within window openings which are slightly oversize or which are of distorted or otherwise irregular shape.