Abstract: A window insulator comprising an insulator core of lightweight rigid insulating materials and/or flexible foam materials, the flexible foam materials being employed at least around the periphery of the core, enclosed by a cover for placement in a window for blocking the entry of sunlight into the interior portion of the structure, for reducing the level of sound energy passing through the window, and for substantially preventing or reducing the transfer of heat through the window. The insulator core may comprise a one-piece body of lightweight, relatively rigid but slightly hand-compressible insulating material. Alternatively, the core may comprise flexible foam material. Alternatively, the core may comprise an interior body of lightweight rigid insulating materials, and flexible, resilient foam members affixed to the peripheral edges of the interior body.