What Is a Cavity Wall?

A cavity wall is a wall made up of two separate leaves (layers) with a gap — the cavity — between them. In the UK, the outer leaf is typically facing bricks and the inner leaf is concrete blocks. The two leaves are tied together with metal wall ties at regular intervals.

Why Have a Cavity?

The cavity serves two main purposes:

• Moisture protection — Rain can penetrate the outer leaf of brickwork. The cavity prevents this moisture from reaching the inner leaf and the interior of the building. Any water that crosses the outer leaf runs down the inside face and drains out through weep holes at the base.
• Thermal insulation — The cavity can be partially or fully filled with insulation material, significantly improving the wall's thermal performance and reducing heating costs.

Standard UK Cavity Wall Construction

• Outer leaf: 102.5 mm facing brickwork
• Cavity: 50–150 mm (modern walls use wider cavities for more insulation)
• Insulation: Partial or full fill (PIR boards, mineral wool batts, or blown insulation)
• Inner leaf: 100 mm concrete blocks (lightweight or aircrete for better insulation)
• Internal finish: Plaster skim, dry lining, or dot and dab

Cavity Wall Insulation

Older cavity walls (pre-1990s) were typically built with an empty cavity. These can be retro-filled with blown insulation (mineral wool, EPS beads, or polyurethane foam) by drilling small holes in the outer leaf. Modern cavity walls are built with insulation installed during construction, either as partial fill (boards held against the inner leaf with clips, leaving a clear cavity) or full fill (insulation filling the entire cavity width).

History in UK Construction

Cavity walls became standard in UK construction from the 1920s onwards, replacing solid walls (typically 9-inch or 225 mm solid brick). If your property was built before the 1920s, it almost certainly has solid walls. Properties from the 1920s to 1970s usually have uninsulated cavities. From the 1990s onwards, cavity insulation has been a building regulations requirement.