What is a cavity brush?

A cavity brush is a long, cylindrical brush inserted vertically into the cavity of a cavity wall to keep loose blown-in insulation in place. When an existing cavity wall is insulated, beads (such as EPS beads) or mineral wool fibres are blown into the cavity. In terraced and semi-detached houses the cavity often continues into the neighbouring house. Without a barrier, the material flows there under pressure. The brush forms that boundary. The term translates the Dutch spouwborstel.

Properties

• Construction. A core of twisted metal wire with many stiff plastic bristles all round. The bristles fill the cavity and deform around mortar snots and wall ties.
• Sizes. Short brushes of about 1 metre, often around 90 mm in diameter, and long brushes of just over 5 metres for a full facade height. Other diameters are available to order.
• Other uses. Sealing around roller shutter boxes and window frames, and as a bird guard under the bottom row of roof tiles or under solar panels.

Application

1. Set the boundary. Fit the brush at the party wall, on your own side, and next to shutter boxes or other spaces that must not be filled.
2. Choose the right diameter. The brush must be clearly larger than the cavity width so the bristles grip both leaves.
3. Drill in a perpend joint. At the top of the facade drill at a downward angle, at the bottom at an upward angle, through the outer leaf.
4. Pull the brush in with a chain. Lower a chain down the cavity from the top, pull it out at the bottom, attach the brush at the top and pull it into the cavity in one go. Then remove the chain.
5. Make good the holes with mortar matching the joints.
6. Think of birds. Brushes under roof tiles remove nesting places. Provide an alternative and respect protected species.

Common mistake: filling the cavity without a brush at the party wall. The insulation then escapes into the neighbour’s cavity: your own wall is filled unevenly, you use more material and you end up in a dispute.

Related terms

• Cavity wall
• Cavity wall insulation
• EPS (airpop)
• Perpend joint
• Roller shutter
• Cavity wall tie
• Outer leaf

Want to know more about building terms? Visit our knowledge base at fredsdiyplans.com.