Mathematical modelling of water sorption in porous building materials is considered. The explanations of inadequacies of both Brunauer-Deming-Deming-Teller model for the high relative humidities range and Frenkel-Halsey-Hill model for the low relative humidities range reported by Pavlik et al. (Transp. Porous Med. 91:939-954, 2012) are proposed. The generalized D'Arcy and Watt (GDW) model is proposed as a simpler alternative for a procedure of experimental isotherms fitting proposed by Pavlik et al. The suitability of the GDW equation to describe the water sorption isotherms in the building materials for the whole range of relative humidities is confirmed.