Abstract: Spaced apart overlying inner and outer transparent sheets extend across a window opening in an outside wall of a room, to define an air space having no communication with the outside but which space is communicable at its top and bottom with the room. The outwardly facing side of the inner sheet has dull infrared ray absorbing areas to abstract heat from sunlight entering the window and the inwardly facing side of the inner sheet has infrared ray reflecting areas. Thermostatically controlled valves at the top and bottom of said space, when opened in response to a temperature rise in said space, enable room air to circulate through the space and abstract heat therefrom.