Office buildings in Chile show higher cooling than heating energy demand. The climate of the country show important differences between cities by the ocean and those of interior regions, located between the coastal and the Andes range. Main cities of Central Chile, where more than 40% of buildings are constructed every year are Santiago and Valparaíso, both located at around 33° S. Santiago presents a Mediterranean climate, with a high temperature oscillation between day and night during cooling period. Valparaíso, by the coast, shows lower temperature fluctuation compared with Santiago during identical period. In order to define design strategies for energy efficiency of office buildings in mentioned cities, a sensitivity study has been made, considering variables like size of windows (window to wall ratio), type of windows (clear and selective glazing, including low e, single and double glazing), use and type of solar protection and use of nocturnal or diurnal ventilation. In opaque facades (walls and roofs), thermal insulation is considered. In case of walls, in order to increase thermal inertia external insulation is assumed. The sensitivity analysis is developed considering a square building containing office rooms on all four orientations. This10 story building has been specially proposed and designed for this analysis. Methodology considers an evaluation of heating and cooling demand of the building in both cities. For this purpose, a simulation software under dynamic conditions has been used (TAS of Environmental Design Solutions Limited). The lowest cooling energy demand is reached when using the lowest window to wall ratio (20%), with solar protection in east, west and north oriented glazed areas. In fact, fully glazed facades in both cities are not recommended. Nocturnal ventilation was highly effective for decreasing cooling demand in both cities. In the case of Valparaíso, due to relatively low temperature during cooling period (maximum lower than 26°C), diurnal ventilation for cooling purposes is also effective.