Houses in the UK mostly have either cavity walls or solid walls:
- Cavity walls are made of two layers with a gap or ‘cavity’ between them. If your home was built after the 1920s, it's likely to have cavity wall.
- Solid walls have no gap, so they can’t be filled with cavity wall insulation. If your home was built before the 1920s, it's likely the external walls are solid walls rather than cavity walls
Cavity walls can be insulated by injecting insulation into the cavity from the outside through drill holes in the outside walls.
Insulation is usually mineral wool or polystyrene beads or polyurethane foam.