An in situ experiment designed to monitor the thermal behaviour of a real heated slab-on-ground floor is described. Some aspects of this experimental investigation are discussed. This paper also reports on the results of a numerical simulation of heat transfer beneath the slab and the surrounding ground, A three-dimensional finite volume model, integrated in the building energy simulation program APACHE, was used to simulate the transient temperatures in the soil. The results showed that this model is well adapted to this type of problem. A differential sensitivity study suggested that uncertainties in input parameters may account for the small discrepancies noticed between measured and calculated temperatures.