A wall type with a hollow inside is known as a cavity wall. They can be thought of as having two “skins” that are separated by a hollow area (cavity). Typically, the skins are made of masonry materials like brick or cinder block. Masonry is an absorbent substance that may gradually pull moisture from the air or even rainwater into the wall. Water can be drained from the cavity through weep holes above windows or at the base of wall systems. By allowing wind to create an air stream across the cavity, the weep holes enable the cavity to export evaporated water to the outside. Weep holes are often made by separating multiple vertical joints at the base of each level, spaced about two meters apart. There are also weep holes.

In the United Kingdom, cavity wall building was first used in the 19th century, and it became widely used in the 1920s. Early examples involved tying the two skins together with stones, while in the 20th century metal ties were introduced. The purpose of the initial implementation of narrow cavity widths was to limit the entry of moisture into the building’s interior. In the 1970s and 1990s, adding insulation to the cavity was considered normal and then required.

Figure 1.A typical cavity wall with mineral wool insulation during construction.

Source: https://en.wikipedia.org/wiki/Cavitywall

Advantages

• Resist wind driven rain
• Insulation provided by slow moving air films and airgap
• Enables use of low cost nonrigid insulation batts

Tie types

The interior and external walls (or leaves), which are made of bricks or cement blocks, are tied together in a hollow wall. They can be produced using:

• Stone
• Brick
• Iron – prone to rusting & expanding
• Stainless steel
• Plastic
• Elastic

Figure 2. Components on a concrete masonry unit and brick cavity wall

Source: https://en.wikipedia.org/wiki/Cavitywall

Two masonry walls that are separated by an air space form a cavity wall. Brick makes up the exterior wall, which faces the outside of the building structure. Masonry components such as concrete blocks, structural clay, bricks, or reinforced concrete may be used to build the inner wall. Metal ties or bonding blocks are used to join these two walls. The cavity wall is strengthened by the ties.

The inside wall’s cavity side is protected from moisture by the water barrier, which is a thin membrane.

Figure 3. Weep holes in masonry wall

Source: https://en.wikipedia.org/wiki/Cavitywall

The flashing element is crucial. The basic objective of it is to deflect water away from the hollow. Typically, metal flashing extends from the inner wall through the exterior wall, and a downward-curving weep hole allows the water to drain. In cavity walls, flashing systems are often placed near the base of the wall so that they may catch any water that slides down the wall.

Weep holes are drainage holes that have been drilled into the cavity wall’s outside to allow water to escape. In cavity walls, expansion and control joints do not have to line up.

Insulation for the cavity is frequently included in contemporary cavity wall construction. This construction makes it possible to add a continuous insulation layer between the two wythes and, vertically, through the slabs, which minimizes thermal bridges.

Insulation

Insulation is used to reduce heat loss through a cavity wall by filling the air space with material that inhibits heat transfer. This immobilises the air within the cavity (air is still the actual insulator), preventing convection, and can substantially reduce space heating costs.

Figure 4. A wall that has had cavity wall insulation installed (after construction), with refilled holes highlighted with arrows

Source: https://en.wikipedia.org/wiki/Cavitywall

Issues

• Problems with moisture retention in older buildings due to installation of Cavity wall insulation.
• The orientation or design of a building may affect the performance of building façades.
• Moisture problems in materials weathering.