<P> 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 . </P> <P> 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 . </P> <Ul> <Li> Resist wind driven rain </Li> <Li> Insulation provided by slow moving airfilms & airgap </Li> <Li> Enables use of low cost nonrigid insulation batts </Li> </Ul>

When did cavity wall insulation become part of building regulations