Why retrospective cavity wall insulation can be a bad idea

Lots of people have had cavity wall insulation done. Some will never be affected but others are and will be.

Retrospective cavity insulation is when insulation (usually wool-fibres or polystyrene beads) is injected under pressure through drill-holes with the intention of completely filling the cavity of external walls. The idea is to improve the thermal performance of the walls i.e. prevent heat-loss.

Cavity walls became common in England around late 1920’s. They are composed of an external leaf of brick and an internal leaf of brick or block. The first cavities were around 20mm wide and the intention was to provide a break in the structure so that water would not track across from the external face to the internal face.

Cavity walls need steel wall-ties to tie the two leaves of masonry together. Prior to mid 1980’s wall-ties were not stainless steel and so often corrode and expand when exposed to moisture. Wall-ties are at greater risk of moisture exposure if the cavity is tightly packed with insulation which can act as a conduit between external and internal leaves. If the external wall is in poor condition i.e. defective mortar-pointing, heavily weathered bricks, defective render or incorrectly gauged mortar, then the risk of moisture penetration is higher.

(On Merseyside) north-west facing elevations are more exposed to driving wind and rain. If lateral moisture penetration occurs the insulation becomes damp and you have wall-ties permanently exposed to moisture, which encourages further wall-tie corrosion and expansion which damages the structure.

You can get water-proof insulation beads, but it is very rare that the numerous contractors who jumped on the various government grant schemes would offer this to you above the cheap wool-fibre alternative.

Another consequence of filling cavities is that if the elevation is particularly defective or exposed e.g. on the coast, certain types of insulation such as wool-fibres can become saturated and sink to the bottom of the cavity where it can cool the internal wall surface. If this happens the opposite of what was actually intended occurs because colder walls will make the house colder. In extreme cases it can cause condensation and black mould. This is a real problem and claims are being made all over the UK.

If you are buying a house and you see drill holes in the ‘T’ junction of mortar-beds/joints, like those in the photo, it is likely that the wall has had either retrospective insulation injected or wall-ties replaced or both.

Always ask the vendor and ensure that retrospective insulation works are covered by a transferable warranty from CIGA (The Cavity Wall Guarantee Agency).

Fully filled cavities are bad practice in certain properties. Cavities are supposed to be kept clear.