This paper presents the results of a series of specially designed experiments to study moisture transport across bonded or natural contact interfaces between building materials. The results showed that the bonded or natural contact interfaces between aerated concrete and mortar are imperfect hydraulic contact. In addition, all the tested cases were simulated using a numerical model. The results showed that there was good agreement between experimental results and predictions made by the model. Based on predictions of the model and experimental results, the present study found that imperfection of an interface between different types of building materials significantly depends on the direction of the moisture transport. Moreover, the bonded interface between AC and PCLMS showed similar characteristics as the natural contact interface between AC and PCLMS: indicating that the bonding has no critical effect on moisture transport, and the bonded interface can be approximately treated as the natural contact interface.