Building science
Cavity wall insulation and damp
The three common types of retrofit cavity wall insulation, and why wet insulation can be worse for a building than no insulation at all.
Published 29 February 2024
The three types of retrofit insulation
Injected foam
This gives the best U-value and best resistance to moisture penetration. It’s injected under pressure through drill holes in the external brickwork.
Polystyrene beads
These are either white or graphite beads. Graphite beads are often coated with glue as they’re injected, so they stick together, giving a better U-value and reducing the chance of settlement.
Blown fibre
Typically glass fibre or mineral wool. It offers the lowest U-value and is easily caught on wall ties and mortar protrusions, which can lead to cold spots.
Should you have cavity wall insulation?
Most homes built after 1920 are likely to have a 25mm cavity, and homes from 1930 onwards a 50mm cavity.
Insulation problems
As of 2020, an estimated 6 million homes in the UK had cavity wall insulation, with around 1.5 million (25%) reported to have problems with damp and mould.