Designing energy efficient building it is important to achieve low level of energy consumption and to provide high parameters of the thermal comfort. This research is devoted to ventilation which has effect on both the comfort of the building and its energy efficiency. The IDA-ICE 4.7 software was used as a tool providing a detailed study of the building’s indoor climate, energy balance and energy efficiency. The thermal properties, the hourly loads for equipment, occupancy and lighting were set for the two-storey residential building situated in Baltic sea costal. Natural and mechanic ventilation was simulated and analyzed. The comparison of internal air overheating and energy consumption depending on the ventilation strategy was made. The results show that use of natural ventilation with window’s opening is effective in case of large window area but in case of north oriented room with 50% window to wall ratio mechanic system is more efficient.