Three materials fill the great majority of British cavities, and they are not interchangeable. Blown mineral wool, expanded polystyrene bead and injected foam each behave differently in a wet, exposed or irregular cavity, and the choice between them is a survey decision rather than a preference. What the survey is looking at is the cavity width, whether it is clean, and how exposed the wall is to driving rain.
Blown mineral wool
Fibre blown into the cavity through drilled holes. It is the most widely used fill in Britain, it copes with irregular cavities, and it is water repellent rather than waterproof, which matters in an exposed wall. It relies on being installed at the right density throughout, and slumping in a poorly filled cavity is the classic failure.
Polystyrene bead
Small beads blown in with a binder so they hold together. Beads flow well into awkward cavities and do not slump, and the binder is what stops them pouring out when a wall is later drilled or opened. They are usually the choice where a cavity is narrow or obstructed.
Injected foam and where caution belongs
Foam expands to fill and sets rigid, which makes it good at sealing an irregular cavity and difficult to remove later. In an exposed wall, or one where the outer leaf is in poor condition, any fill can carry moisture across the cavity, and that is the risk a competent surveyor weighs before recommending anything at all.
Questions people ask about cavity wall insulation types
How do I know which type my wall needs?
From a borescope survey that measures the cavity, looks for debris and existing fill, and considers exposure. A recommendation made without one is a product being sold rather than a wall being assessed.
Can cavity wall insulation be removed?
Yes, by extraction through drilled holes, and it is a real job with a real cost. It is the remedy where a fill has slumped or is carrying water, and it is usually followed by a decision about whether to refill.
Does the guarantee cover damp?
Installations under the recognised schemes carry an independent guarantee, and what it covers varies. Read what the guarantee actually promises and who stands behind it before the holes are drilled.