Abstract: The storm window of the invention is adapted to be mounted in a window frame of a house interiorly of the existing window closure. It comprises a hollow structure made up of a plurality of components of thin flexible material, such as plastic sheet, in the form of a panel of substantial thickness with a periphery corresponding to the periphery of the window frame and inner and outer generally planar faces enclosing an insulating dead air space between them. At least part of the structure is inflatable to yieldingly enlarge the periphery and cause the panel to resiliently grip the window frame and retain the panel in position. The major portion of the planform area is substantially transparent to facilitate vision and light transmission through the panel. The structure may be deflated and folded into small compass for storage.