The building sector strongly affects the energy required, especially for heating and cooling. The need to limit the energy demand and the impact on the environment has to deal, however, with an ever-increasing energy demand inside buildings and with an obsolete building stock. The design and construction of new and energy efficient buildings is not enough to tackle the problem, which can be really addressed only by working on the improvement of existing buildings. Considering that a consistent number of these existing buildings are individually protected or are inserted in wider protected contexts, interventions may be challenging due to several limitations that reduce the number of possible strategies that can be adopted. Therefore, possible strategies for the energy refurbishment of historical buildings to be integrated within the building envelope are crucial to reach the ambitious aim of reducing the impact of the building sector. The efficacy of phase change materials (PCMs) was investigated in case of integration within lime-based plasters for the application on walls’ outermost layer. Experimental tests under real outdoor conditions were carried out initially to estimate whether and how much the addition of PCM affected the thermal behaviour of walls and the building energy demand for cooling. Plasters with different PCMs were realized and arranged in a customized set up realized at the TekneHub Laboratory of the University of Ferrara and were then tested for several months. Results showed good performance in terms of attenuation of the daily temperature fluctuations and reduction of the energy required.