In response to the changing climate, effective building design is crucial for regulating indoor conditions while minimizing the energy consumption. This study addresses the impact of translucent fabric layers (indoor curtains) as solar protection devices, considering user-driven factors in their energy performance. We measure the indoor radiation in a residential case study in two phases: (i) a supervised phase over a day, with curtains rolled up and down by the investigators, and (ii) an unsupervised long-term phase (1.5 years) where curtains were controlled by the users. In the supervised phase, the curtains demonstrate an average filtering coefficient of 51.5%, which varies throughout the day: a minimum filtering coefficient of 40% occurs around solar noon, while raises up to 70% in the evening. In the unsupervised phase, curtains remain down for over 30% of the year, with users' choices possibly influenced by factors beyond thermal considerations. These results enhance our understanding of user interactions with movable passive systems and offer valuable insights for the design of energy-efficient buildings.