It is popular in US and Canada because the buildings already have a ventilation system for heating and cooling. Two main types of air panels are glazed and unglazed. Glazed solar collectors are designed primarily for space heating and they re-circulate building air through a solar air panel where the air is heated and then directed back into the building. Those solar space heating systems require at least two penetrations into the building and only perform when air in the solar collector is warmer than the building room temperature. Most glazed collectors are used in the residential sector. Unglazed solar collectors are used to pre-heat make-up ventilation air in commercial,institutional and industrial buildings with a high ventilation load. They turn building walls or sections of walls into low cost, high performance, unglazed solar collectors. They employ a painted, perforated metal solar heat absorber that also serves as the exterior wall surface of the building. Heat conducts from the absorber surface to the thermal boundary layer of the air 1mm thick on the outside of the absorber and to air that passes behind the absorber. The boundary layer of air is drawn into a nearby perforation before the heat can escape by convection to the outside air. The heated air is then drawn from behind the absorber plate into the building’s ventilation system.