An experimental and theoretical analysis of the water vapor adsorption in several types of porous building materials is presented. For the measurement of adsorption isotherms, a DVS-Advantage water sorption device is used. The experimental data is analyzed using theoretical formulas based on the BET, BSB, BDDT, and FHH isotherms, assuming a mono as well as multi-layer water vapor adsorption. The BSB equation is found to provide a good approximation for the relative humidities below 0.6–0.7, whereas the FHH equation shows a sufficient accuracy for the relative humidities above 0.4–0.5. Based on a combination of BSB and FHH isotherms, a semi-empirical formula is proposed that allows one to obtain a very accurate approximation of experimental data for all analyzed materials and all values of the relative humidity.