The building sector strongly affects the energy required for development, especially for heating and cooling processes. The need to reduce the energy demand and environmental impact must contend with the ever-increasing energy demand within buildings and obsolete building stock. The design and construction of new energy-efficient buildings are inadequate to address the issue, which can only be effectively tackled by improving existing buildings. Given that a significant number of these existing buildings are individually protected or located within protected contexts, interventions can be challenging due to several limitations that reduce the number of possible strategies that can be adopted. Therefore, strategies for the energy refurbishment of historical buildings – particularly those integrated within the building envelope – are crucial to achieving the ambitious aim of reducing the environmental impact of the building sector. The effectiveness of phase change materials (PCMs) was investigated when integrated within lime-based plasters for application on the outermost layer of walls. Experimental tests under real outdoor conditions were carried out initially to estimate whether, and to what extent, the addition of PCMs affected the thermal behavior of walls and the building’s energy demand for cooling. Plasters with different PCMs were developed and arranged in a customized setup at the TekneHub Laboratory, University of Ferrara, and subsequently tested over several months. The results demonstrated good performance in attenuating daily temperature fluctuations and reducing energy consumption.