The short answer
Cavity wall insulation beads are small expanded polystyrene beads blown into the cavity. Most are coated with a binder as they go in so they lock together into a solid, water-resistant fill. Bonded bead rarely fails on its own; loose bead without binder can settle and pour out of any hole drilled in the wall, and either can sit wet if the wall is badly exposed or bridged.
On this page
What the beads are
Expanded polystyrene (EPS) beads are a few millimetres across, white or silver-grey, and very light. They are blown through holes drilled in the mortar joints until the cavity is full. Silver-grey beads contain graphite, which improves their insulation slightly.
Bonded bead
As the beads leave the hose they are sprayed with an adhesive binder. Once the cavity is full the binder sets, and the beads hold together in a loose-textured solid. They cannot settle, and they stay put if a hole is later drilled in the wall.
Loose bead
Older installations, and some poor ones, used beads without binder. They behave like a bag of dry rice: over time they settle, leaving a gap at the top of the wall, and they pour out of any hole drilled for a new cable, a satellite dish or a vent.
Bead compared with other fills
| Bonded bead | Blown mineral wool | Polyurethane foam | |
|---|---|---|---|
| Installed cost, per m² | About £18 to £30 | About £15 to £22 | Higher; used for narrow cavities |
| Behaviour if wet | Does not wick; water runs between beads | Can mat and hold water | Closed cell; resists water |
| Settles over time | No, if bonded | Can, if under-filled | No |
| Removal | Vacuums out; bonded takes longer | Vacuums out; slow when matted | Hard to remove |
Prices are installed guide prices from a UK builders’ cost guide, April 2026, before VAT. Government-commissioned research on non-standard cavity walls recorded material costs for EPS bead of about £2 to £8 per m², with closed-cell foam several times that.
When beads fail
- Exposure. Even bonded bead sits in water if a wall takes heavy wind-driven rain and the outer leaf is saturated. Water runs between the beads to the base of the cavity, where it can collect.
- Bridging. Mortar on ties and rubble at the base still bridge the cavity whatever the fill.
- No binder. Loose bead settles, leaves cold spots and spills out of holes.
- Narrow cavities. Historic England advises that insulation in cavities under 50 mm is generally not cost-effective.
“Beads coming out of my wall”
Beads trickling out of an air brick, a cable hole or a gap at a window frame usually mean loose, unbonded bead. It is a sign the fill has lost volume, and the top of the wall may now be empty. Have the cavity checked with a borescope survey before sealing the hole.
Removing beads
Loose bead is the easiest fill to extract: it flows to openings at the base of the wall and is vacuumed into sacks. Bonded bead has to be broken up with compressed air first and takes longer. See extraction for how it is done and the removal cost calculator for a budget.
Refilling with beads
Bonded bead is often chosen when a failed wool fill is replaced, because it does not wick. HMRC zero-rates installing insulation in homes until 31 March 2027. Whether a refill is wise at all depends on why the first fill failed: see removal and refill.
Seeing beads where they should not be? Tell us about the house and we will find a specialist to look inside the wall.