Abstract: The present invention provides a window insulator and a frame therefor. The window insulator and frame comprise (a) one or more panels of an insulating material; (b) guides for the panels, including channels in which the panels are free to move; and (c) pressure-contact means for holding the guides in the casing of an existing window. One or more of the panels may consist of a thick layer of expanded polystyrene disposed between two films or sheetings of a substantially transparent (poly)methyl methacrylate resin, the polystyrene layer partially cut away to provide transparency for the window.