The literature highlights that design and composition of hydronic Radiant Floor Systems (RFS) are critical for an adequate thermal behaviour, ultimately providing high indoor thermal comfort. The thermal performance of RFS is strongly influenced by the thermophysical properties of the heat transfer medium embedding the water pipes. In these systems, mortars with high conductive properties enable faster thermal responses. However, this can lead to peak heating loads and significant thermal fluctuations, potentially compromising thermal comfort and increasing the energy demand. The objective of this study is to investigate the impact of mortar thermal conductivity on the thermal performance of RFS. A 3D numerical simulation model was developed in Ansys Fluent®, replicating an experimental setup of conventional heavyweight hydronic RFS. The performance indicators used in the parametric analysis were surface temperature and total heat flux. The results obtained provide theoretical support for the improvement of the thermophysical properties of the heat transfer medium in RFS during the design phase.