Measured hourly data on indoor humidity are compared with the data obtained from calculated values for NBS Houston test houses and for a high mass test building in an environmental chamber. The measured values are usually very much different from the calculated values if no considerations are given to the moisture absorption and desorption phenomena that take place at the interior surfaces. The paper also introduces the Tsuchiya model that permits the evaluation of room surface moisture absorption capability. The model is based upon the detailed simulation calculation for room moisture balance that includes the surface condensation, evaporation, and absorption/desorption coefficients. These coefficients were determined in such a manner that the measured room humidity levels coincide with the calculated values.