A cavity wall consists of masonry built in two leaves or skins, with a void or cavity between them. Cavity walling began to replace solid walls for the external walls of domestic buildings during the latter parts of the nineteenth century but did not become common until the 1920s. The primary function of the cavity was to act as a barrier to prevent the passage of moisture from the outside to the interior of the building, the fundamental principle being that the inside and outside of external walls are kept separate, resulting in a dry interior. Initially the cavity tended to be very narrow but by the 1970s, the width had increased (to 50-75mm), as designers began to recognize the additional benefits of insulating the cavity to achieve a better balance of temperature for the inside environment of the building, keeping the interior cool in the summer and retaining heat in the winter.
Cavity insulation became compulsory under the Building Regulations in the 1990s and today’s modern insulation requirements necessitate a cavity width of up to 100mm. Insulation materials are introduced into cavities during construction. The cavity is either partially or fully filled in order to improve the thermal performance of the wall and to meet regulatory requirements.
A typical cavity-wall cross-section shows the minimum dimensions in millimetres for cavity-wall construction. The
thermal insulation requirements of the Building Regulations and practicality dictate that the external skin is typically constructed in facing brickwork and the inner skin in concrete blockwork that is at least 100mm thick.
Fig. 167 Typical cavity-wall cross-section, showing the minimum requirement for its construction, in millimetres.
The masonry below ground level can be constructed either of concrete blocks or of common bricks that have been designated as suitable for use below ground. The brickwork between finished ground level and the level of the horizontal DPC will be in contact with moisture from the ground, and this will make it vulnerable to frost damage in winter. In addition, any soluble salts present in the ground will be absorbed during periods of wet weather, resulting in white salt deposits forming on the face of the bricks (efflorescence) during dry weather. Externally, between finished ground level and horizontal DPC, therefore, it is good practice to use a brick that is of engineering quality, with low levels of water absorbency. Facing brickwork starts immediately above DPC level.
Cavity walling below ground level must be capable of withstanding the lateral pressures exerted by the sub-soil. To this end, when the construction has reached ground level, the cavity is filled with a lean mix of concrete, which is then given a chamfered finish at the top in order to direct any moisture in the cavity towards the external leaf. Weepholes (see Figs 190 and 191) should be introduced into the external leaf at ground level at 890mm intervals (in other words, four bricks) to allow any moisture build-up to be expelled.