Masonry - Resistance to moisture
The ability of concrete masonry walls to resist rain penetration will depend on the wall construction, block type and exposure. Unless reasonably thick, single leaf walls will normally need to be rendered or clad to provide the necessary rain resistance. While some water is expected to penetrate the outer leaf of a cavity wall, it will not pass through to the inner leaf in most exposure conditions, provided a residual 50 mm clear cavity is maintained. In severe or very severe exposure conditions it may be necessary to increase the cavity widths. Cavity walls fully filled with insulation can increase the risk of rain penetration. Some blocks are unsuitable for use as outer leafs of cavity walls unless protected with a render or cladding.
For further information see the Concrete Block Association or the Autoclaved Aerated Concrete Products Association