Air-vented cavity drainage walls
Richard L. Quirouette in his paper “Differences Between a Vapour Barrier and an Air Barrier” explains how air leakage by convection is much more than the amount of water transferred by vapour diffusion. The exterior wythe of brick is referred to as a rainscreen because it acts as a screen, not a barrier. Rainwater driven by wind pressure penetrates the brick veneer joints by diffusion, capillary action, and convection—not through the brick itself, which admits almost no water into the cavity but by convection (water being carried by air movement) through hairline cracks in leaky mortar joints transfers 200 times as much moisture into the cavity and the building as the other two sources combined. It is fundamental to masonry to reduce convection by fully buttering the head joint before setting the brick into a bed of mortar. Once in the cavity, moisture streams into the building through openings in the CMU, which could be prevented with an effective air barrier.
Open head joint air vents measuring 609 mm (24 in.) on centre (o.c.) horizontally—one at the bottom of the wall (sometimes combined with a weep joint) and another directly above, at the top of the wall—induce air circulation in the cavity by differential air pressure. Air venting cycles through the cavity, carrying moisture away by convection.
Rainscreen pressure-equalized cavity walls
In his 1962 paper, “Curtain Walls,” Øivind Birkeland of the Norwegian Building Research Institute wrote:
“The only practical solution to the problem of rain penetration is to design the exterior finishing (veneer) so open that no super-pressure can be created over the joints or seams of the finishing. This effect is achieved by providing an air space behind the exterior finishing, but with an open connection to the outside air. The surges of air pressure created by the gusts of wind will then be equalized on both sides of the screen and no more moisture-laden air can enter the cavity.”2
In 1963, following up on Birkeland’s assertions, the Canadian National Research Council’s (NRC’s) Division of Building Research issued Canadian Building Digest (CBD) 40, “Rain Penetration and Its Control.” Thus began the scientific engineering and design of pressure-equalized rainscreen walls.
The non-scientific explanation of pressure-equalized cavity design is most of the moisture getting into the cavity is carried there by air movement (convection), propelled by wind pressure entering the cavity through leaky mortar joints on the windward side. The moisture laden air pressure on the windward cavity travels around the corners of the cavity pressurizing the cavity on the leeward sides of the structure, escaping into the building through breaches in the CMU backup. This depressurization allows a continuous movement of moisture-laden air to circulate throughout the cavity and into the building through large mechanical and electrical openings in the CMU.
The pressure-equalized system works by allowing wind gusts to enter the cavity through openings at the bottom of the brick veneer. However, it is stopped from flowing into the building and leeward walls by an effective CMU air barrier, as well as battens compartmentalizing the cavity. This system causes the cavity to pressurize, equalizing with the wind pressure outside. Therefore, no more moisture-laden air can enter the cavity.8 Wind pressure is dynamic, rising and falling in gusts. This helps to keep the cavity dry, as each time the wind pressure drops, the pressurized cavity breathes out and carries moisture with it.
The pressure-equalizing technique requires:
- Openings into the bottom of the cavity compartments to allow wind pressure to enter the cavity.
- An effective and continuous air barrier between the cavity and the building interior.
- Battens to compartmentalize the cavity.
The combined area of the open joints in the bottom of the cavity wall must be calculated based on the size of the cavity. Therefore, it is important to keep the cavity as narrow as practical.
While designing an airtight backup wall to prevent air pressure from escaping into the building, calculating the area of air openings at the bottom of the wall, and sizing the compartments in the cavity is challenging enough for architects, it is also difficult to prepare construction documents anticipating every breach between the cavity and the building. It is more likely many unanticipated breaches in the inner wall would have to be covered by a contingency budget and change orders.
