This article examines the validity of using polyurethane boards (PUR and PIR) in groundbased floor construction. Currently, polystyrene (EPS or XPS) boards are most commonly used. The use of PUR and PIR boards raises some concerns among designers and contractors regarding their performance under load, such as a cement screed layer. The aim of the study was to assess the effect of simulated temporary loading of polyurethane boards on their ther-mal insulation performance. The performance of polyurethane samples was also compared with that of XPS polystyrene. A lambda meter was used to measure thermal conductivity. The -value of samples cut from the plates was measured and then the test was repeated after 30 days of seasoning these samples under load. Changes in sample thickness were also moni-tored. It was found that all seasoned samples showed a slight decrease in insulating perfor-mance, however this was not due to a reduction in sample thickness under load. Thickness changes were minimal in all cases. No significant differences were observed in the perfor-mance of polyurethane and polystyrene samples under load. The advantage of polyurethane is that its -value is over 30 % better than that of XPS, but when selecting the material, economic and environmental aspects should also be taken into account.