What it is called, by market
One concept, four markets. Where a name means something else locally, the card says so rather than leaving you to find out on site.
United Kingdom
Close, not identicalcavity wall
also twin-leaf wall, cavity
Names a whole wall TYPE — two masonry leaves tied together — which has been the default British external wall for a century, not a detail within another assembly.
Australia
Close, not identicalcavity wall
also brick veneer cavity, cavity
Used for both the twin-masonry wall and the gap behind a brick veneer on a framed wall, so the phrase covers two quite different constructions.
United Statesyours
Close, not identicaldrainage cavity
also air space, brick veneer cavity, rainscreen gap
The gap behind a masonry veneer over a framed and sheathed wall. The structure is the frame, not the outer leaf, which is the opposite arrangement from a British cavity wall.
Canada
Close, not identicaldrainage cavity
also air space, rainscreen gap
As the United States, with the cavity width and the drainage provisions set by the Canadian code and by climate zone rather than by the British convention.
The governing standard, by market
United Kingdom
BS EN 1996 / Approved Document C
Masonry design, and resistance to moisture from the ground and weather.
United States
TMS 402 / ASTM C1063
Masonry structures and veneer anchorage requirements.
Calculators for this
Each works in either measurement system, and the terminology on the page follows whichever market you have selected.
Frequently asked questions
- Is a cavity wall the same as a brick veneer wall?
- They share the cavity and differ in what holds the building up. A British cavity wall is two masonry leaves — traditionally brick outside and block inside — tied together, with the INNER leaf carrying the floor and roof loads. A North American brick veneer wall is a timber or steel frame, sheathed and weather-barriered, with a single leaf of brick hung outside it on ties; the frame carries everything and the brick carries only itself. So the outer leaf looks the same from the street and does a completely different structural job, which changes the ties, the movement provisions, the support at openings and what happens if the veneer is removed.
- What are the ties actually for?
- Transferring wind load between the leaves while letting them move independently, and they are the component most often got wrong. A tie has to be stiff enough in compression and tension to carry wind pressure and suction from the outer leaf into the structure, and free enough to allow the differential movement between two leaves of different materials at different temperatures and moisture contents. It also has to shed water: a tie sloping the wrong way, or one with mortar droppings bridging it, carries water across the cavity to the inner leaf, which is the commonest single cause of damp in a cavity wall. Their type, spacing and embedment are specified, and around openings and at edges the spacing tightens.
- Does filling the cavity with insulation defeat it?
- It changes the wall from a drained cavity to a filled one, which is a different construction with different rules rather than a simple upgrade. A clear cavity works by letting water run down an open gap and out through weeps; a full-fill insulation has to be a material and an installation approved for that use, which resists water transfer across itself rather than relying on an air gap. Partial fill keeps a residual clear cavity of a specified width, which is why board insulation is held against the inner leaf rather than filling the gap. Retrofitting full fill into a wall built as a drained cavity — particularly in a severely exposed location, or where the outer leaf is cracked or the ties are already bridged — is how a dry wall becomes a damp one.