Cavity wall

Cavity walls consist of two 'skins' separated by a hollow space (cavity). The skins are commonly masonry such as brick or concrete block. Masonry is an absorbent material, and therefore will slowly draw rainwater or even humidity into the wall, as well as from the inside of the house as from outside. The cavity serves as a way to drain water back out through weep holes at the base of the wall system or above windows. The weep holes allow wind to create an air stream through the cavity and the stream removes evaporated water from the cavity to the outside. Usually weep holes are created by intentionally leaving several vertical joints, also open head joints, open about two meters apart at the base of in every story. Weep holes are also placed above windows to prevent dry rot of a wooden window frame. A cavity wall with masonry as both inner and outer skins is more commonly referred to as a double wythe masonry wall.