Pitfalls with injected cavity wall insulation


Cavity wall construction is the modern method for house building, consisting of a block inner leaf and brick on the outside, which has a cavity, usually between 50-100mm in width. The cavity wall was first used in coastal locations to keep out rain but soon spread to other drier locations, as it was identified that the cavity also provided a degree of thermal retention.

Reflecting increasing attitudes towards energy conservation, the modern cavity wall was further enhanced in the 1980’s as Building regulations stipulated that all cavity walls should incorporate thermal insulation. This usually took the form of rigid insulation batts, fixed to the inside of wall, which if undertaken correctly, should not compromise the buildings resistance to water penetration.

A cavity is essential to preventing water penetration, as the single 1 brick thick external wall will always allow water through when exposed to wind driven rain, due to the drying process as the mortar shrinks at the perp vertical joints.
 
The trick with cavity walls, is that by ensuring there is a gap, this allows water to penetrate and run down the inside face of the brick to the foundations. However, if cavity’s are filled, this allows a direct path for water to penetrate, and can lead to dampness showing on internal finishes.

There are many different cavity fill insulation types, however the most widespread is blown mineral wool fibre, which is not water resistant, being able to hold 2 ½ times volume of water than its own weight, and can lead to damp patches and mould growth. In addition, when mineral wool fibre becomes wet, it can act to draw heat from inside a building, increasing your fuel bills. The insulation may also have voids/ gaps which can directly cause or lead to condensation problems, and as it is difficult to inspect the cavity, this can often go unnoticed and be hard to detect.

Insulation can also sink under its own weight, and or insufficient insulation material may be injected, which in turn create the ideal conditions for mould and condensation growth, as this leads to cold spots, and thermal bridging to occur at high level.

Buildings built with galvanised fish tail or wrought iron wall ties may have problems, as the damp conditions within the injected cavity can lead to corrosion and subsequent wall tie failure, which can be costly to remediate.

Other insulation types include foam and bonded polystyrene beads which may be used, although not as popular as mineral wool fibre as they are generally more expensive, requiring application over many days, they still, however suffer from similar problems.

Bonded Polystyrene beads have been known to transmit water across the cavity, as although the beads themselves are waterproof, they have been known to leave voids, which fill up with water. Another problem encountered is that the beads themselves tend to escape easily, coming out of vents and such like.

Foam insulation is also subject to problems and prone to human error, as it needs to be ready mixed on site, and is unproven in its longevity. If the mix is too strong, it can cause pressure on the walls, leading to cracks and further wall damage.


In conclusion, there is no doubt that injecting insulation into wall cavities retrospectively can lead to problems, which may far outweigh any thermal retention benefits. It is encouraged to seek other options, which are often cheaper and easier, such as insulation your loft, hot water cylinder jacket, installing double-glazing or even application of a thick external render.