The purpose of this study was to investigate the effect of curtain folds on both the amount of condensed water on the window glass surface and the airflow in the space between the curtain and window glass, focusing on the folds, which are the unique shapes of curtains. In this study, two experiments were conducted: one to measure the amount of condensation water and the other to visualize the airflow. In the condensation measurement experiment, the amount of condensed water on the window glass surface was measured while the indoor and outdoor temperatures and humidity were kept constant. The results showed that the amount of condensed water was lower with curtains than without curtains, and that lower with curtains without folds than with folds. The Lewis number of the space between the curtain and the window glass ranged from 1.7 to 2.1. In the airflow visualization experiment, the airflows in the space between the curtain and window glass and inside the folds of the curtain were measured. It was confirmed that the airflow in the space between the curtain and window glass flowed from top to bottom in large swirls, whereas the airflow inside the folds was uniform from top to bottom.