Cavity masonry walls were introduced in the nineteenth century to stop wind-driven rain from penetrating to the inside surfaces of a building. The main reason for building cavity walls has always been to keep the rain out, but it was found that the air layer trapped in the cavity also provided a degree of thermal insulation.
To increase this thermal insulation Building Regulations since the 1980’s have required new houses to be built with insulation material in the cavity. As long as they are built properly, this insulation should not compromise the walls’ resistance to rain penetration.
In the last few years, there has been a drive to “retro fill” cavity walls of houses built before 1980 with materials blown into the cavity.
Since this started there have been an increasing number of complaints about cavities filled with blown mineral-wool fibre. This consists of loose fibres blown through holes drilled in the brick outer leaf. The manufacturers and installers claim that the material is water-repellent, and that it cannot allow rainwater to penetrate across the cavity. This appears to be incorrect and the fibres can soak up water like blotting paper.
There are several consequences of this effect: