Phase-change materials (PCM) in buildings are considered a promising option to prevent overheating in warm seasons. Numerous studies have shown a noticeable improvement in thermal comfort through PCM, but in real applications they have often underperformed. User behaviour is often neglected as an important factor in determining PCM performance and might be a limiting factor. Another factor could be time-dependent degradation, which has also been scarcely researched so far. We used simulations within two case studies to investigate whether the PCM applications, each of which had been in operation for more than ten years, were still functioning from a technical perspective and what influence user behaviour had on their performance. We found that the PCM applications still had a positive influence on the thermal performance of the rooms, although the effect due to behavioural optimizations was significantly greater. The PCM was able to reduce the time of discomfort by 9–45% in the baseline scenario with real documented user behaviour in both rooms. Improved user practices increased the reduction in discomfort to 33–52%. For future studies evaluating PCM and its use, we recommend considering realistic user habits, as implementing optimal behaviour could lead to an overestimation of PCM performance and dissatisfaction with the technology.