This article is aimed at determining the absorption of moisture by diffusion of mineral wool and polyisocyanurate over a long-term period. The task of increasing the thermal insulation properties of enclosing structures is most relevant for the erection of new buildings or structures, as well as for the repair of existing. The method models operating conditions in which thermal insulation product absorb moisture from both sides at high relative humidity of air (100%) and pressure difference of water vapour over a longterm period. The moisture absorption by diffusion of mineral wool and polyisocyanurate were obtained after 28 days of exposure to temperature and pressure drop of water vapor. Significant changes in moisture content of mineral wool were observed. From the results obtained, it can be concluded that polyisocyanurate has a lesser absorption property of water vapor, which is an important attribute in its operation.