Thermal Energy Storage systems are acknowledged solutions frequently used for reducing indoor thermal fluctuations by behaving as thermal buffer. In this view, phase change materials (PCMs) are included into building envelope components. At the same time, cool roofs represent passive cooling techniques aimed at maximizing the solar radiation reflected by the roof without contributing to the indoor overheating, in hot summer conditions, in particular. In this panorama, the authors developed a new composite material made by a polyurethane liquid waterproof cool membrane with non-capsulated PCMs acting as shape stabilized thermal buffer additive. In this work, the behaviour of such composite material when exposed to accelerated weathering long-term tests (QUIT test) has been studied. Temperature, humidity and ultraviolet radiation are varied in order to test the membrane vulnerability to realistic environmental conditions. The purpose of such experimental analysis was to clarify if the PCM inclusion could help the membrane to better behave during the course of the time, since PCMs are supposed to be responsible for thermal stress reduction. Optic-energy, morphology, thermal and mechanical tests are carried out in order to analyse the membrane degradation imputable to the weathering stress. The inclusion of PCMs in different concentrations allowed identifying the best performing PCM-doping inclusion, i.e. 25 wt%. Such composition, in fact, was able to register a better spectral reflectance reflectance in the near infrared region of the solar spectrum up to 10% with the fewest modification during the course of the QUIT test, i.e. 7% of weight loss after 60 days of exposure, and to preserve the required flexibility of the membrane together with its superficial finishing characteristics.