The issue is will the cavity system allow water to penetrate to the inner leaf which is bad and will cause internal dampness.

Cavities can be carefully partially filled with some type of insulation which allows the water soaking through the brick to evaporate and drain back out.

The standard technique is to use blown in EPS beads which allow for drainage downwards, reducing the chance that any water loading will make it through the insulation layer and to the inner leaf of the wall.

Problems with cavity insulation are often caused by:

-Treating walls which have a large amount of debris in the cavity, leading to water bridging

-Over-filling cavities which also leads to water bridging

-Treating walls with very large amounts of rain exposure, especially to driving rain for extended periods of time since this will lead to saturation of the cavity insulation system which may be unable to dry for months at a time. These walls should be externally insulated and clad instead.

In the UK context for example, houses in Wales and parts of the English South coast, the whole Southwest, the East of Scotland, and most of NI are in zones 3 and 4 in terms of wind driven rain exposure.

In zone 3 and 4, installers are required to carry out an assessment of site conditions and of cavity thickness as well as exterior wall finish. Older houses might have 50mm cavities, new properties often have 100mm or 150mm cavities. Obviously that reduces the changes of water soaking through.

Approved Document C allows a full fill of insulation for a 75mm cavity on a fully rendered wall and on a 50mm cavity if clad with rain impervious material but requires 150mm for bare masonry.

Thanks for the detail! I have a 1930s house with, I think, unfilled cavities. I’ve always been put off looking into insulating because it seemed potentially harmful.