Implementation of phase change materials (PCMs) into building components allows their thermal performance to be enhanced, reducing indoor temperature fluctuations and improving the occupant comfort. A practical problem to solve is the possibility of leakage when the PCM is under the liquid state. In this study, a new component using PCMs is described. This component derives from an existing slab having cylindrical cavities which is used in floors/ceilings. The cavities are filled with a polymer-paraffin composite and leaks are avoided, due to, both the use of the composite and the insertion inside cavities. This study is based on numerical simulations whose results are compared to experimental ones with the same boundary conditions in order to validate the model. The PCM was introduced inside the cavities with an annular repartition and different percentages. A new indicator was introduced (PCM activity) that allows an evaluation of the amount of PCM to be used in such a component. With this indicator the optimal percentage of PCM has been determined. (C) 2014 Elsevier B.V. All rights reserved.