For state of the art buildings ventilation heat loss can exceed 50 percent of the total heat loss of the building. Balanced ventilation systems with heat recovery from the exhaust air lead to bjg costs for ducts. Alternatively, the incoming air of exhaust ventilation systems can be pre-heated by curtain wall facades. A newly developed simulation program for transient analysis of curtain wall systems is validated by measurements. Parameter studies for pre-heating air with curtain walls are presented. Location, orientation, construction, and geometry are considered.