Cavity wall ventilation

There was a period between the wars when cavities were deliberately ventilated

The cavity in modern cavity wall construction is not ventilated. Terry Bryant refers to “ventilated” cavities (Letters, PE December).

There are tiny weep holes in the outer brick leaf at the bottom of the cavity but these cause negligible air movement in the cavity. The only air movement is that by natural convection and is minimal.

There was a period between the wars when cavities were deliberately ventilated. This was achieved by having only an air brick in the outer leaf, with no bridging sleeve across the cavity, and, if necessary, alternate bricks removed in the inner leaf to allow air flow under a wooden floor. This eliminates the insulating effect that would be achieved by having the cavity non-ventilated.

UK building regulations require insulation panels to be attached to the inner leaf in the cavity. Although the cavity gap is reduced, it is not usually eliminated altogether. A cavity is essential to deal with possible ingress of moisture through the outer leaf and should not be filled in.

Raymond Smyth, Sheffield

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