Office buildings in Santiago of Chile show a high cooling energy demand during summer. Most of these buildings are based on design patterns of developed countries and they do not consider the local climate and elemental design strategies for improving their thermal behaviour in cooling periods of the year. Due to this, office buildings show overheating problems and/or a high cooling energy demand. For two buildings of Santiago, preliminary studies through simulations made with TRNSYS 2000, allowed to conclude that some free cooling design strategies -such as nocturnal natural ventilation- are highly effective. Through TRNSYS 2000, it is not possible to study the effect of wind on natural ventilation of buildings, nor can either quantify the effect caused by opening of windows in ventilation. In this work we have used TAS, software that allows the indicated studies. This paper shows the feasibility of reaching the necessary ventilation through windows, for an efficient cooling of two buildings already constructed in Santiago. It is observed that using this strategy, energy saving is possible to be obtained. More than 50% of energy saving respecting to the two studied buildings of Santiago, may be reached when using this effective design strategy with solar protection and efficient artificial lighting systems.