Why was the cavity wall introduced?

Cavity wall construction became standard in UK housebuilding from around the 1920s onwards, largely replacing solid 9-inch brick walls. The primary purpose of the cavity was to prevent rain penetration - the gap breaks the path water must travel to reach the interior. The cavity also provides a space for insulation, significantly improving thermal performance compared to solid masonry.

What width should a cavity wall cavity be?

The minimum clear cavity width for a new cavity wall is 50mm under Approved Document C, though 75-100mm is standard in modern construction to allow full-fill insulation while maintaining a clear residual cavity. Wider cavities up to 150mm are used in some high-performance builds. The cavity must remain clear of mortar droppings, which can bridge it and conduct water to the inner leaf.

How do wall ties work in a cavity wall?

Wall ties are metal connectors embedded in the bed joints of both leaves as the wall is built, spanning the cavity to tie the two leaves together structurally. They must slope downward toward the outer leaf (or have a drip) to prevent moisture tracking back to the inner leaf. Corroded or failed wall ties are a serious structural issue requiring specialist repair.

Cavity Wall - Construction Glossary

Q: What is a cavity wall?

A cavity wall is an external wall built from two separate leaves of masonry - typically a 102mm outer leaf of facing brick and a 100mm inner leaf of dense concrete blockwork - with a gap between them called the cavity. The two leaves are connected by wall ties. The cavity prevents rainwater penetrating the outer leaf from reaching the inner leaf and the interior of the building.

Definition

A cavity wall is an external wall constructed from two separate leaves of masonry - typically a 102mm outer leaf of facing brick and a 100mm inner leaf of dense concrete blockwork - with a gap between them called the cavity. The two leaves are connected by metal wall ties. The cavity prevents rain that penetrates the outer leaf from crossing to the inner leaf and the building interior, and provides a space for insulation.

In practice

Cavity wall construction became standard in UK housebuilding from the 1920s. The standard modern cavity is 75-100mm wide, allowing full-fill insulation (mineral wool batts or expanded polystyrene beads) while maintaining a residual cavity or using a partial-fill board insulation system. The minimum clear cavity is 50mm under current regulations.

The main vulnerabilities in a cavity wall are mortar droppings on wall ties (which bridge the cavity and carry water to the inner leaf), cavity trays and weep holes at lintels and openings (which must be installed to drain water back out), and failed insulation retrofit that partially blocks the cavity. At window and door reveals, vertical DPCs prevent water crossing the cavity at the narrow point where both leaves come together.

Building Regulations

Relevant PartsPart A (structure), Part C (moisture), Part L (energy)

Minimum cavity width50 mm clear (75-100 mm typical)

Wall tie standardBS EN 845-1

U-value target (new build)0.18 W/m²K (AD L 2021)

Approved Document C requires cavity walls to prevent moisture reaching the interior. Approved Document L sets U-value targets for new external walls - 0.18 W/m²K for new dwellings. Cavity wall insulation retrofit using blown fibre or bead is a BBA-certified process requiring a registered installer and a survey to confirm cavity suitability.

Full Building Regulations guidance

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Cavity Wall

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