Detached residential wooden houses are a common type of housing in Japan. Decay of wooden components within the walls caused by condensation or defective flushing, is sometimes an issue. To solve this problem, a double-skin system with a room-side air gap was developed. In this system, during summer, the airflow that is driven by ventilation fans moves through the room-side air gap in the wall, and removes heat load either from the inner surface of the insulation material, or from the surface adjacent to the rooms inside. The purpose of this study is to evaluate the flow rate in the ventilated wall, controlled by ventilation fans. The airflow in the ventilated wall removes heat from the surface of the wall and influences occupants’ thermal comfort. Therefore, it is important to develop an expression for flow rate in the wall. An airflow-energy simulation program was used to predict flow rates in the ventilated wall, and the performance of air-flows in several exterior walls of the house was investigated under various conditions. The results verify that the flow rate in the ventilated walls increased with the ventilation fan speed. By using forced ventilation, the cooling load was reduced by 10% to 20%.