This article presents the results of water sorption properties investigation and thermal conductivity measurements of expanded polystyrene thermal insulation materials with different mass densities. The sorption behaviour of the expanded polystyrene materials was achieved in a Climacell 111 type climatic chamber, after drying in a Venticell 111 type desiccator apparatus. The relative humidity varied from 25% to 90% at 293 K for 240 min. The thermal conductivity of each sample was determined using a Holometrix 2000 (HLS) heat flow meter. In this article, the sorption isotherms, sorption kinetics, thermal conductivities and the prediction of changes in function of water content of four pure expanded polystyrene (30, 100, 150, 200 and grey) slabs with different densities (14, 17.5, 23.7 and 27.5 kg/m ³ ) and one expanded polystyrene mixed with graphite are given (grey expanded polystyrene). Practical application: The thermal conductivity as well as the moisture content are key thermal transport properties of building materials. The role of insulating materials in the building energy and moisture balance is more significant when compared with the other materials of the building structures. The laboratory measurements of these values of the insulating materials are very important either for the manufacturers or the contractors. The available bibliographic data for these materials are strongly incomplete and somewhere out of date.