Night ventilation has been applied successfully to many passively-cooled or low-energy office buildings. This paper analyses the thermal comfort achievable in office buildings in Spain according to European standard EN 15251:2007. Furthermore, the comfort level is evaluated using the Degree Hours (DH) criteria and the maximum indoor temperature. Considering the high interest of architects and engineers in the evaluation of optimal comfort condition as a function of building typology (8 typologies), glazing ratio (30% and 60%) and climate (12 different Climate Zones), a total of 192 different study cases were simulated. For every case, an optimal air change per hour (ac/h) that can range from 1 to 50 ac/h was then determined. The research shows that passive night ventilation should be considered as an effective strategy for all Spanish Climates to reduce cooling demand in buildings with high daily internal gains (i.e. office buildings), improving comfort conditions and flattening peak temperatures.