A coupled heat and moisture transfer model which takes the coupled effect between heat transfer and moisture transfer into consideration is developed to model the temperature and humidity distribution within building envelopes. The continuous variables, relative humidity and temperature are chosen as the driving potentials. And a simple solution method based on a multi-physics simulation soft, COMSOL, is proposed to solve the coupled heat and moisture transfer model simply. The numerical results of this model agree well with HAMSTAD benchmarks which are international accepted.