Designing masonry cavity walls

By Paul Potts

Photo © Paul Potts

Cavity drainage masonry construction consists of a rainscreen wythe of brick or thin stone veneer on the exterior, a supporting wythe of 203-mm (8-in.) concrete masonry units (CMUs) on the interior—sometimes used as an infill in conjunction with a structural steel frame—and a separating cavity between the veneer and supporting masonry partly filled with insulation. While the cavity space may be between 50 and 114 mm (2 and 4.5 in.)  according to building code, the minimum recommend clear space is 50 mm (2 in.), plus space for the insulation. The space is needed to promote air circulation and drainage, to keep the cavity dry and avoid mortar bridging.

There are three types of masonry cavity walls considered in this article:

  1. Simple cavity drainage walls, which have an inner and outer wythe of masonry, an inner cavity, inner cavity insulation, a vapour barrier, and weep holes at the bottom of the exterior brick veneer to drain moisture out of the cavity.
  2. Air-vented cavity drainage walls, which have the same components as simple cavity walls with the addition of vented openings at the top and bottom of the brick veneer, to allow air to ventilate the cavity and encourage any moisture to escape by convection.
  3. Rainscreen pressure-equalized cavity walls are the most sophisticated of the three.

Masonry cavity construction became commonplace when architects, engineers, scientists, and masons discovered that a void space separating two wythes of masonry, with a layer of insulation and a moisture retarder covering the insulation provides as much comfort and conserves as much British thermal unit (BTU) energy as several feet of solid barrier masonry.

Geographically, this discussion is limited to commercial construction in Canada and the northern U.S., where the winters are cold, and the summers are warm to hot.

Simple cavity drainage walls

A simple cavity drainage wall consists of:

  • A rainscreen exterior brick wythe which takes the brunt of the rain, sleet, snow and other elements that are windblown at the exterior of the building attached to an interior supporting structure of CMU or other supporting material. The weaker brick veneer is supported by ties between the brick and the supporting CMU. The brick veneer is called a rainscreen because it does not prevent all pressurized moisture from entering the wall.
  • A cavity with weep holes at the bottom of the cavity provides a pathway for rainwater that penetrates the brick joints to drain to the exterior.
  • Cavity insulation covered with an effective moisture retarder which prevents cavity moisture from entering the building, as well as an air barrier which prevents free moisture from entering the building by convection.
A simple cavity drainage wall. Illustration © International Masonry Institute

Wind pressure is shared by the brick veneer and backup CMU connected by stainless steel wall ties. The use of wall ties grew after it was conclusively shown metal-tied walls were more resistant to water penetration than masonry-bonded walls.1

Water collecting in the bottom of the cavity is drained to the exterior through a  weep system. Weep holes are either small, round holes filled with rope wicking or full head joint weeps with screens to keep out insects. The traditional round weep hole filled with rope wicking is rather inefficient. The rope is intended to drain water and discourage the infestation of insects, but it fails over time to do either function. The weep hole has been largely replaced with the open head weep joint, with a screen made specifically to fit the space of a full brick head joint.

A large amount of water remains in the cavity because mortar droppings accumulate on wall ties, reinforcement, and other bridging and absorb it. Mortar droppings also plug the weep openings. Placing a drainage media such as Mortar Net or a similar product at the bottom of the cavity helps break up the mortar droppings before it reaches the weep openings.

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