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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. The cavity serves as a way to drain this water back out through weep holes at the base of the wall system or above windows. A cavity wall with masonry as both inner and outer skins is more commonly referred to as a double wythe masonry wall.
The 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. 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.