As simultaneous heat and mass transfer between building envelopes and indoor air is complicated and expensive to measure in laboratory and field experiments, a numerical model is important in understanding and extrapolating experimental results. In this paper a numerical model that solves simultaneous heat and mass transfer between building envelopes and indoor air is verified using the field measurements presented in Part I of this paper. The verification results show that the model is able to predict the transfer of water vapor, CO2, and SF6 between the building envelope and air. The model is then applied to investigate the humidity, comfort, and air quality in a bedroom of a wooden building located in four European countries (Finland, Belgium, Germany, and Italy). The numerical results show that moisture transfer between indoor air and the hygroscopic structure significantly reduces the peak indoor humidity (up to 35% RH), percent dissatisfied with warm respiratory comfort (up to 10%) and the percent dissatisfied with indoor air quality (up to 25%).