Abstract: A temperature modifying system for a building having windows, some of which are exposed to the direct rays of the sun and some of which are not. The system includes a plurality of double paned windows, a heat exchange unit, an air ducting system interconnecting the windows and heat exchange unit, and an air handling unit for forcing air through the ducting system. The air moves sequentially through the heat exchange unit, the air handling unit, the ducting to the windows, through the double paned windows and back through ducting to the heat exchange unit. Each window includes parallel transparent panels spaced apart in a conventional manner, but including apertures for the entrance and exit of air conducted through the system. Light and heat reflecting slat means are mounted between the parallel panels to reflect outwardly the radiant heat from the sun. A temperature sensing device is mounted to measure the temperature of the air exiting from each active window unit.