Abstract The paper summarizes the results of laboratory tests that were performed on samples of extruded polystyrene foam (XPS). The aim of the investigation was to determine the mechanical characteristics which are essential for the seismic analysis of modern low-energy building structures, where XPS boards are used as a thermal insulation layer beneath the building's foundations. As well as this, the shear behaviour of differently composed thermal insulation (TI) foundation sets was investigated, and their friction capacity estimated. The results showed that, in general, XPS material with higher compressive strength also possesses higher shear strength. Sliding between individual components of TI foundation set was revealed to be a likely failure mechanism in the case of building structures that are founded on a TI layer when subjected to strong earthquake loadings.