Based on Fourier and Fick law, a modified Budaiwi model considering phase change within wall and the effect of solar radiation was established by using air humidity ratio in wall and temperature as the driving potentials.The term omitted in the mass transfer equation of the original model was added in the modified model.To evaluate the accuracy of the model, a test system was set up to measure relative humidity and temperature within a porous wall under actual weather conditions and compared with the prediction of the model.It was found that the predictions were in good agreement with the experimental results.At the interface close to outdoor, the average deviation of the relative humidity was 4.44% and average temperature deviation was 1.31K.At the interface close to indoor, the average deviation of the relative humidity was 6.3% and average temperature deviation was 1.26K.