1. A STRUCTURE COMPRISING: MEANS DEFINING A ROOM PROVIDED WITH AT LEAST ONE LIGHT TRANSMITTING WINDOW, SAID MEANS COMPRISING PARTS DEFINING WALLS, A FLOOR, AND CEILING; ALL OF SAID PARTS WHICH ARE IN POSITION TO RECEIVE RADIANT ENERGY ENTERING SAID WINDOW BEING OF SUCH MATERIAL THAT THEY HAVE A LARGE HEAT STORING CAPACITY AND GOOD THERMAL CONDUCTIVITY, ALL SAID HEAT STORING PARTS BEING CONNECTED IN GOOD HEAT CONDUCTING RELATION TO EACH OTHER, AND MEANS THERMALLY INSULATING ALL SAID PARTS FROM THE EXTERIOR SURROUNDINGS; SAID WINDOW COMPRISING SPACED INNER AND OUTER TRANSPARENT RIGID PANES, AND A PLURALITY OF SPACED TRANSPARENT PLASTIC FOILS IN THE SPACE BETWEEN SAID PANES AND EXTENDING ACROSS THE AREA OF SAID WINDOW.