Abstract: A storm unit for installation in an existing window system to provide for greater insulation and weather protection. The unit includes a pair of side rails for replacing the existing inside stops of the window. The rails have a pair of parallel, vertically-extended grooves formed therein into which are buckled deformable, transparent panels whereby the panels can be raised and lowered. The top and bottom edge of each panel has a horizontally-extended cap secured thereto. When the panels are closed against the window frame, the bottom cap of the upper panel is horizontally aligned with the upper cap of the lower panel. A weathertight seal is provided that is capable of sealing the region between the adjacent faces of the two aligned caps when the panels are placed in a closed position.