Analysis of the Durability of Thermal Insulation Properties in Inverted Foundation Slab Systems of Single-Family Buildings in Poland

This manuscript is aimed at analyzing how operating factors may affect the durability of thermal insulation in building partitions located underground. It examines the durability of inverted insulation systems where thermal insulation is installed above the waterproofing layer and used in residential foundation slabs. The article demonstrates that, despite their popularity due to cost efficiency, the long-term success of these systems depends on thorough investigations of thermal isolation, especially considering different climate conditions. The analysis was based on an extensive literature review (2016–2024), supplemented with laboratory test results on extruded (XPS) and expanded (EPS) polystyrene boards. Additional tests examined the water penetration mechanism into insulation layers that are in direct contact with groundwater, revealing that cyclic freezing and thawing significantly increase moisture levels. The findings highlight the need for updated region-specific guidelines for the underground insulation in Central and Eastern Europe.