Night ventilation has been applied successfully to many passively-cooled or low-energy office buildings. This paper analyses the thermal comfort achievable according to European standard EN 15251:2007 by applying this strategy in office buildings in Spain. Specifically, the comfort level is evaluated using the Degree Hours (DH) criteria and the maximum indoor temperature. For the DH criteria, four base temperatures are considered: 25ºC and the three Categories for acceptable ranges of operative temperature around the adaptive comfort temperature established in the standard for free running buildings. Considering the interest of architects and engineers in the prediction 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 are obtained where an optimal air change per hour (ACH) that ranges from 1 to 50 ACH is defined. As an example of the obtained results, in Almeria, a city in the south of Spain characterized by hot summers with average daily temperature of 26ºC, in the case of a Linear Typology with 30% glazed façade, the best comfort result is achieved by a night ventilation flow of 20 ACH for Limit of Category II. Further increases in night ventilation flow produces marginal improves. In this case, the Mean Peak Temperature is reduced in 0.5ºC. Furthermore, in Soria, a city in the north of Spain characterized by mild summers with average daily temperature of 20ºC, in the case of a Linear Typology with 30% glazed façade, the best comfort result is achieved by a night ventilation flow of 6 ACH for Limit of Category II In this case Peak Temperature is reduced 0.6ºC. The research shows that passive night ventilation should be considered as an effective strategy to reduce cooling demand in buildings with high daily internal gains (i.e. offices buildings), improving comfort conditions and flattening peak temperatures.