The use of circular economy criteria for designing new materials is becoming a key factor for the sustainable development of the building sector and the improved performance of construction companies. This research introduces a new plaster composite material in which the original raw material is partially replaced by expanded polystyrene waste in solution and end-of-life tyre textile fibre. This new material has been tested to verify its behaviour in humidity-drying cycles, recording good mechanical properties with strengths well above the values required by current regulations, even after being subjected to the severe effects of these accelerated ageing cycles. A lightened plaster composite has therefore been developed in which the recycled material is fully integrated into the matrix, thereby obtaining a technically viable product for the manufacture of prefabricated products for wet rooms. In addition, a simulation has been carried out using FlexSim 3D modelling and analysis software, which has corroborated the improvement in the further distribution process of a prototype of a prefabricated false ceiling panel made with this new product, as well as the reduction in costs due to more efficient transportation and a lower environmental impact.