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The interest in passive construction that has low demand for thermal energy continues to grow every year. It is related to both the reduction in maintenance costs of buildings and the growing environmental awareness in societies. Passive houses are characterised by very good thermal insulation properties of their external partitions. This paper presents the results of tests on the thermal conductivity for three different types of building materials, assuming their use as thermal insulation. The materials were subjected to water absorption tests during long-term immersion. The purpose of this study was to simulate water absorption caused by long-term water exposure. The tests were carried out on a perlite concrete block, a sheet of polyurethane foam applied by spraying and expanded clay in a loose form in three different fractions. The tested insulation materials were selected due to their different form, structure, and porosity characteristics, which largely determined the value of the thermal conductivity. A perlite concrete block is a new, innovative product, of which manufacturer's data are not yet available as the product is currently at the stage of detailed research. The results were analysed statistically and used in graphs to show the dependence of the value of the thermal conductivity on moisture content of the samples. The purpose of this study was to indicate the importance of the proper incorporation of insulation materials into buildings, their storage before construction and use in the post-construction phase. Building standards for passive houses place high demands on materials used. In order to fully enjoy the advantages of passive buildings, i.e., lower energy consumption and benefits for the natural environment due to lower consumption of energy generated from non-renewable energy sources, it is necessary to use certified building materials and ensure proper use of objects. The studies quoted in this paper indicate to what extent the influence of moisture content negatively affects the properties of insulation materials in external partitions. [ABSTRACT FROM AUTHOR]