Cavity wall insulation removal and replacement

Refill with bonded bead

Expanded polystyrene beads coated with a binder as they are blown in, so they lock together and cannot slump. Beads do not wick water the way loose wool can, which is why they are often chosen for a second attempt. They suit cavities down to about 50 mm.

Refill with blown mineral wool

Cheaper per square metre, and fine in a sheltered wall that suits it. It is the fill most likely to have matted in the first place on an exposed wall, so think hard before putting it back where it failed.

Leave the cavity empty

The cavity was invented to keep rain out: water that gets through the brick runs down the inside face of the outer leaf and out at the base. On a very exposed elevation, or with a narrow or irregular cavity, putting that gap back can be the most reliable choice. Historic England advises that early cavity walls in areas of severe driving rain are prone to damp crossing the fill, even where the cavity is wider than 50 mm.

Insulate one wall, not all of them

Failure is often on one or two elevations, usually the ones facing the prevailing wind. It is common to extract everywhere but refill only the sheltered walls, and treat the exposed one differently.

Insulate inside or outside instead

Internal wall insulation (boards on the inside face) or external wall insulation (boards and render on the outside) keep the cavity clear and still cut heat loss. Both cost far more than a refill and need careful detailing to avoid trapping moisture, so they are usually for the problem walls only.

Before anything goes back in

• The cavity must be dry. Ask for moisture readings.
• Debris and mortar snots must be cleared, and ties checked.
• Cavity trays over windows and doors must work.
• Pointing on the outer leaf should be sound; open joints let in far more water.
• Get the new installation guaranteed, and keep the paperwork with the house deeds.