A cavity wall is a type of wall that has an airspace between the outer face and the inner, usually structural, construction. The skins typically are masonry, such as brick or cinder block. Masonry is an absorbent material that can retain rainwater or condensation. 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 provide a drainage path through the cavity that allows accumulated water an outlet to the exterior of the structure. Usually, weep holes are created by leaving out mortar at the vertical joints between bricks at regular intervals, by inserting tubes, or by inserting an absorbent wicking material into the joint. Weep holes are placed wherever a cavity is interrupted by a horizontal element, such as door or window lintels, masonry bearing angles, or slabs. A cavity wall with masonry as both inner and outer vertical elements is more commonly referred to as a double wythe masonry wall.

Cavity walls were first used in Greco-Roman buildings, but fell out of use until the 19th century, when they were reintroduced in the United Kingdom, gaining widespread use in the 1920s. In the 20th century metal ties came into use to bind the layers together. 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 then required by most building codes in the 1990s.

A tie in a cavity wall is used to secure the internal and external walls, typically using metal tie straps or truss-like assemblies of welded wire that link the masonry thicknesses together.

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 water-resistant membrane, either applied as a film or as a troweled or sprayed liquid to the inner side of the cavity.

The flashing component is important. Its main purpose is to direct water out of the cavity. Metal flashing usually extends from the interior wall through the outer wall and a weep hole with a downward curve allows the water to drain. Flashing systems in cavity walls are typically located close to the base of the wall, so that it will collect the water that goes down the wall.

Weep holes are drainage holes left in the exterior wall of the cavity wall, to provide an exit way for water in the cavity.

Expansion and control joints do not have to be aligned in cavity walls.^{[page needed]}