The external walls of a masonry build hose will typically have a cavity, with an external brick or rendered blockwork face and an internal wall of clockwork which, combined with certain partition walls, provides the structural support for the floors and roof.
The two skins are stabilized by being tied together with corrosion-resistant wall-ties. The Bottom of the cavity is filled with lean mixed concrete with a slant towards the external skin, or have a cavity tray installed which directs any moisture away form the inner skin.
The cavity space between the two walls should be a minimum of 100mm wide, and usually accommodates insulation to improve thermal performance, as well as creating a rain barrier and preventing heat loss. The typical build process in the Uk involves building both walls in parallel, then adding insulation either as the wall is built, or injecting ti in once the cavity is completely.
Historically, the size of the cavity has grown in accordance with the energy performance and subsequent insulation levels required by Building Regs, which has led to and increase in width from 35mm to 50mm, and again to 100mm. In the self-build market, cavities of 125-150mm and beyond are now common, largely due to the priority placed on energy efficiency.
Cavity masonry construction provides excellent protection against moisture ingress in almost all conditions commonly found throughout the Uk, and the standard full-fill cavity can be used in all but the most exposed areas. In particularly wet and windy locations partial-fill cavities are used in order to carry away any water that penetrates the outer leaf.
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