This study investigates numerically the occurrence and duration of higher relative humidities in a cold attic space, which are a consequence of excessive moisture supply from ventilating the attic and from air infiltration from inside the dwelling. Hygrothermal states of the attic air zone and the adjacent construction elements are calculated by a whole building heat, air and moisture simulation tool. Airflows to the attic are determined by taking into account the total distribution of pressure around and inside a building. The role of attic ventilation is analyzed by comparing naturally ventilated and unventilated attics in an open landscape or a city zone. For air infiltration from inside the dwelling only scenarios with the airflow directed mostly to the attic are analyzed. The simulations show that, if indoor air infiltrates through the attic floor, it is necessary to ventilate the attic. In that case, a building situated in a sheltered position is more susceptible to moisture problems in the cold attic than the same building in an unsheltered position. However, in the absence of air infiltration from inside the dwelling, the attic should not be ventilated. Simulations are performed for real climatic conditions.