Why XPS Works Well for Underdeck Applications XPS has a closed-cell structure that provides useful thermal and moisture-resistant characteristics. Thermal Scientific Works supplies XPS Boards in a standard size of 1250 × 600 mm, with commonly supplied thicknesses of 25 mm and 50 mm and density in the 35–37 kg/m³ range. The appropriate board thickness should be determined according to the required thermal performance and project design rather than selected on thickness alone.