In moderate climates, adventitious ventilation helps in keeping the water vapor balance in a building under control. This does not hold in hot and humid climates, where the outside air is a moisture source. Adventitious ventilation should be avoided in such climates and intended ventilation flows must be dried before entering the space. Anyhow, could adventitious ventilation also generate moisture problems in moderate climates? To get an answer, a reference case was analyzed with the air leakage distributed over facades and roof. The calculated results show adventitious ventilation may cause severe interstitial condensation in the envelope, if too air-leaky. Especially the roof is critical. Part of the vapor released inside could condense in the roofs section where it induces mold on laths and battens and may result in unpleasant dripping of. A confirmation was found in a practice case, where severe winter condensation was noted in the cathedralized ceilings of the sleeping rooms.