In warm temperate and Mediterranean climates, reducing cooling energy demand in buildings is a pressing necessity, requiring innovative solutions to lower electricity consumption during summer. In Europe, the increasing conversion of unheated attic volumes into liveable spaces has raised attention on the thermal performance of roof envelope components. In low-rise buildings, the roof is the largest surface exposed to solar radiation, making it a significant contributor to summer heat gains. Infrared reflective underlays offer a cost-effective solution to reduce summer heat gains, minimizing attic overheating. Despite extensive experimental evidence of their effectiveness, there is a limited literature focusing on the long-term performance of reflective membranes under real-world conditions. Aging factors such as dust accumulation, prolonged exposure to moisture and humidity, and physical wear can degrade reflectivity and thermal efficiency over time. This study examines the long-term performance of a roof reflective underlay membrane installed in a roof test room. The thermal performance of the test room was monitored and compared with a reference room without reflective underlay. Aging effects were evaluated comparing the reduction in roof heat gains after one year of installation. Additionally, thermal emissivity measurements were conducted on pristine and aged samples to assess material properties degradation. Preliminary results are encouraging, indicating slight in-situ performance reductions after one year. However, extended monitoring is planned to analyse natural aging effects and their implications on the long-term performance.