A cavity wall is a type of wall that has a hollow center. They can be described as consisting of two "skins" separated by a hollow space (cavity). The skins typically are masonry, such as brick or cinder block. Masonry is an absorbent material that can slowly draw rainwater or even humidity into the wall. One function of the cavity is to drain water 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 that exports evaporated water from the cavity to the outside. Usually, weep holes are created by separating several vertical joints approximately two meters apart at the base of each story. Weep holes are also placed above windows to prevent dry rot of wooden window frames. A cavity wall with masonry as both inner and outer skins is more commonly referred to as a double wythe masonry wall. History Cavity wall construction was introduced in the United Kingdom during the 19th century and gained widespread use in the 1920s. In some early examples, stones were used to tie the two skins together, while in the 20th century metal ties came into use. Initially cavity widths were narrow and were primarily implemented to reduce the passage of moisture into the interior of the building. The introduction of insulation into the cavity became standard in the 1970s and compulsory in the 1990s. Advantages Resist wind driven rain Insulation provided by slow moving airfilms and airgap Enables use of low cost nonrigid insulation batts Tie types A tie in a cavity wall is a used to secure the internal and external walls (or leaves)—constructed of bricks or cement blocks. They can be made from: Stone Brick Iron - prone to rusting & expanding Stainless steel Plastic Elastic Components A cavity wall is composed of two masonry walls separated by an air space. The outer wall is made of brick and faces the outside of the building structure. The inner wall may be constructed of masonry units such as concrete block, structural clay, brick or reinforced concrete. These two walls are fastened together with metal ties or bonding blocks. The ties strengthen the cavity wall. The water barrier is a thin membrane that keeps moisture away from the cavity side of the interior wall. The flashing component is important. Its main purpose is to direct