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.

The typical cavity wall method of construction was introduced in Northwest Europe during the 19th century and gained widespread use from the 1920s. In some early examples stones were used to tie the two leaves of the cavity wall together, while in the 20th century metal ties came into use. Initially cavity widths were extremely narrow and were primarily implemented to prevent the passage of moisture into the interior of the building. The widespread introduction of insulation into the cavity began in the 1970s with it becoming compulsory in building regulations during the 1990s.

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