Thermally Activated Building Systems (TABS) are building elements that transfer and sometimes store thermal energy. By embedding the fluid network into a massive layer, TABS can effectively reduce indoor temperature variation and cooling demand. However, their application is limited to new constructions, and there is a need to adapt this concept for refurbishment. One solution to improve cooling efficiency is to use Phase Change Materials (PCM). These materials rely on latent heat for phase change instead of sensible heat only. In this paper, a suspended ceiling that supports PCM modules is considered, with air circulating between the PCM and an existing concrete slab. Heat transfer at different stages was modelled, progressively increasing the complexity of the system. The results demonstrate that the melting temperature should be between 15 to 17°C for effective cooling. The need for an additional thermal resistance between the PCM and the room was highlighted, as it has been discovered that it plays a crucial role in the thermal behaviour of the entire system and can promote energy storage when it reaches a certain level. Conversely, lower values should be chosen if a radiative heat flux through the ceiling is desired.