Abstract Building retrofitting is one of actions promoted by the European Union in order to reduce the energy dependence, the consumption of fossil fuels and the CO 2 emissions. In climates with high thermal variability, thermal mass can increase the hours of thermal comfort and reduce the need for mechanical conditioning systems, helping to reduce the energy consumption in both new and existing buildings. However, it is not always feasible to use traditional materials to increase thermal energy storage in building retrofitting. The use of Phase Change Materials (PCM) can be an alternative to provide high thermal storage capacity to rehabilitated buildings as their applications have relatively low weight and need little or no additional space. Different ways in which PCM can be used in building rehabilitation have been analyzed, and the influence of PCM drywall panels and fenestration was evaluated in different Spanish cities. The results reflect the importance of the Window to Wall ratio and the Shading Factor in the thermal behavior of buildings. Also, they show that, with proper selection of these parameters, the use of PCM drywall can contribute to increase the thermal comfort, reducing the peaks and temperature fluctuations in existing buildings, particularly under overheating conditions.