In recent years, outdoor air humidity has significantly increased, especially during the hot and humid summer season in Japan. In this regard, if a room is overcooled for a long period of time, then there is the risk of condensation in areas where humid outdoor air enters interior rooms through gaps. Thus, the purpose of this research is to understand the effects of airtightness and ventilation on the humidity environment of non-residential spaces (e.g., inside partition walls) in detached houses. Based on the findings, in airtight non-residential spaces, no effects of high humidity were observed, due to outdoor air infiltration. However, if there were small holes at the top/bottom of the partition walls, then the humidity in the non-residential spaces increased. In the case of exhaust ventilation, the humidity in the non-residential spaces increased, which promoted the entry of outdoor air into such spaces. This is because the inside of the building had negative pressure. As for the case of balanced ventilation, the effect was smaller. Overall, these results provide important information for considering effective methods for suppressing summer condensation.