Depending on the climate region, the local comfort standard and the efficiency of existing building services, the share of energy for cooling, heating and artificial lightning of buildings is between 30% and 50% of the overall final energy consumption of the country. The required energy for cooling of buildings can be significantly reduced by night ventilation. In this case cool air is ventilated through the building to discharge heat energy stored in walls, floor slabs, and furniture. The efficiency of this method depends mainly on the air exchange rate between the outdoor environment and the indoor air volume. Driving forces for the air flow through façade openings are the outside air temperature drop during night, wind pressure distributions acting on the façade due to local wind and the availability of cross wind flow through the building. Resistances for the air flow are given by the size and the geometry of facade openings which can be quantified by discharge coefficients. In the facade design phase the relation between window size and effective opening area must be considered. With predicted discharge coefficients the overall energy efficiency and indoor temperature of buildings with natural ventilation can be analyzed in transient multi zone models. Exemplary buildings in Campinas, Brazil and Hamburg, Germany are analyzed concerning their potential of this method to save energy.