The paper describes a simplified method of calculating moisture flows between indoor air and porous materials. The simplified method is based on dynamic heat transfer through the building partition – Thermal Response Factors (TRF). The paper presents examples of humidity step and impulse responses for different porous materials. Formula based on impulse responses was developed and simplified for practical calculation. Presented solution based on TRF methods is close to equation developed by A. TenWolde. Proposed simplified method was implemented for calculating the whole year moisture flows between different building materials and indoor air. The paper presents comparison between proposed method and more accurate calculations. In the comparison the simulating software like MOIST 3.0 and WUFI ver. 4.0 were used. It was showed that proposed simplified method is accurate enough for indoor air moisture balance (the difference for whole year sum of absorbed and desorbed water vapour is about 10%). Presented method may be useful for indoor moisture balance calculation. The method showed that moisture accumulation may have significant impact on indoor humidity balance and indoor relative humidity.