This study presents the example of using extruded polystyrene for thermal insulation under the foundation slab of a building. Under the foundation slab of the thickness of 500 mm of a five- storey reinforced concrete building there was applied a layer of XPS of the thickness of 150 mm. The authors monitored the deformation of insulation under the load of the building from the moment of concreting the foundation slab until the completion of its development. Additionally, there were conducted laboratory tests of the σ−ε dependence and of material creeping. On the basis of the results of material properties, for the purpose of conducting the analysis of XPS deformations and effort, there was constructed a numerical model in the FEM system. This work includes the results of the conducted measurements and analyses with proper comments. They became the basis for drawing the conclusions which will be of use for future work.