Moisture transport through a building envelope normally involves interface phenomena, i.e., moisture transport across interfaces between building materials. Therefore, the knowledge of the interface phenomena is essential for the prediction of moisture behaviour in a building envelope. Most hygrothermal models treat materials as individual layers in perfect hydraulic contact, i.e., the interface has no effect on the moisture transport. However, in practice, this might not always be true. Therefore, to appropriately evaluate the performance of a building envelope on moisture transport that lead to building envelope design guidelines, it is imperative to obtain a good understanding of the interface phenomena. The most common types of interfaces are: “Hydraulic contact” when there is an interpenetration of both layer’s porous structure; “Perfect contact” when there is a contact without interpenetration and “Air space” between layers when there is an air box of a few millimetres wide between the layer’s porous structure.