Masonry Cavity Wall System

A twin-leaf cavity wall is built as separate masonry skins with a continuous void between them, held together by ties that cross the void. The outer skin takes the weather and is accepted as permeable. The void behind it breaks the path that water would otherwise follow across the wall, so that moisture arriving at the back of the outer skin runs down and is returned outward rather than reaching the inside.

The inner leaf is the structural leaf, carrying floor and roof loads to the foundation and giving the wall its stiffness, while the outer leaf supports only itself between its bearings at openings and at its base. Both leaves are masonry, but they move for different reasons and not in the same sense: fired clay units tend to expand as they take up moisture over time, while concrete-based blockwork shrinks as it dries, and the outer leaf follows the whole external temperature swing while the inner sits near internal conditions. The ties and the outer leaf's own movement joints have to tolerate that difference.

The test that separates this wall from its nearest sibling, masonry-veneer-over-frame-system, is what the inner skin is made of. At a survey opening or during construction, look inward across the void: blockwork or brickwork carrying the floors means a twin-leaf cavity wall, and a studded frame with a sheathing board means the outer masonry is a veneer with a different job for its anchors. The unqualified phrase cavity wall is deliberately not treated as an equivalent name here, because in North American practice it usually describes the veneer wall.

The void may be left clear, partly occupied by insulation held against the inner leaf, or filled across its width. That choice changes what the void is being asked to do at the same time as draining, and it is settled at design stage against exposure and the products involved rather than read off a finished wall. Insulating a cavity that is already closed is a different problem, covered by retrofit-cavity-wall-insulation-system.

A tie has to do two contradictory things at once: transfer lateral force across the void and offer water no route across it. That is why the way a tie sits in the wall matters as much as what it is made of. One laid falling inward, or carrying a bead of mortar on its back, converts a structural connection into a bridge and delivers water to the inner leaf at exactly the points where the leaves are most firmly connected.

Insulation changes what the void can still do. Where fill occupies the whole width, resistance to water crossing the wall passes from an open space to the fill's own behaviour; where the fill is partial, the retained gap is worth only as much as the care taken to keep boards tight to the inner leaf and their faces clear. Neither arrangement is better in the abstract - each moves the burden onto a different component.

A tray without an exit does not drain, and outlets without a tray above them drain nothing. The pair works only where the tray runs across the full width of the interruption, is turned up behind whatever sits above it, is stopped at its ends, and has clear openings below. Every place the void is interrupted - a lintel, a floor bearing, a change of construction, an abutment - re-creates that same demand.

Closers sit where three demands meet: stopping air moving through the void, shortening the thermal path around the insulation, and not reconnecting the leaves for moisture. A closer that answers the first two by simply packing the gap with something conductive or absorbent has met them at the expense of the third, which is why reveals and the wall base are so often where internal staining appears first.