Abstract: A building having window structures in exterior walls of the building. Each window structure has an exterior and an interior panel spaced from one another to define a plenum therebetween, with radiation absorbing blinds positioned in the plenum. The blinds absorb solar energy, and air from within the building is circulated through the plenum to absorb energy from the blinds. The heated air can be circulated through a heat exchange heat storage structure, and selectively discharged to the outside, circulated to another building portion, or recirculated into the same building portion. Control means are provided to combine outside air with the inside air being circulated to maintain proper temperature in the building within predetermined upper and lower temperature limits.