Mineral wool and Styrofoam are two basic materials used for thermal insulation of external walls. Both materials are characterised by a very low heat transfer coefficient λ of around 0.04 W/m•K. Mineral wool, as compared to Styrofoam, offers great resistance to fire (EuroClass A1) and very good permeability to steam. Therefore, in order to maintain consistency of the entire thermal insulation system and good air circulation ensuring proper microclimate to be maintained inside the rooms, plaster of good permeability to steam should be used. Such properties are present in mineral plaster with cement bonder or in silicate, silicone and silicate-silicone plaster. Acrylic plaster with acrylic dispersion-bonder exhibits very low permeability to steam. For that reason, such material is recommended especially for thermal insulation with Styrofoam, which is practically non-permeable to steam.