Words: Paul Potts
Cavity Drainage Walls
The initial impression of brick and block cavity walls is they are simple masonry constructions; however, these simple assemblies have highly complex physical properties and performance characteristics that make a comprehensive description of them challenging, leaving many readers stumbling.
For clarity, discussions in this article are limited to masonry designs for climate zones 5 – 7 (northern United States and Canada.)
Moisture and water vapor enter the cavity from both the exterior and interior depending on indoor and outdoor temperatures and wind conditions. Wind driven rainwater penetrates the brick veneer by diffusion through the solid brick and by penetrating the mortar joints. Without moisture barriers moisture will also enter the cavity from the interior space in winter where it is warm inside and cold outside. The primary function of modern cavity wall designs is to (a) deflect wind driven rainwater, channel penetrating moisture to the cavity drainage system and (b) purge the cavity of water vapor and moisture with air circulation.
Simple Cavity drainage walls of the early Twentieth Century have evolved into two major classifications (1) air vented cavity drainage walls and (2) Pressure Equalized Rain Screen Walls. To understand the principles of modern cavity walls, enhanced with air barriers, moisture retarders and insulation, it will be useful to review developments from the earliest cavity drainage wall design to the most complex pressure equalized rain screen cavity wall designs.
Simple masonry cavity drainage wall
The simple cavity drainage wall has four components (1) exterior brick veneer that takes the brunt of wind and rain pressure (2) a drainage plane on the back of the veneer with drainage openings at the bottom of the cavity, (3) an inner structural wythe of 8-inch concrete masonry units (CMU) or other material supporting the veneer and (4) a cavity between the brick veneer and the structural wythe. The cavity provides space for drainage, air circulation, insulation, air barrier and moisture retarder.
Setting aside earthquakes, the principal weakness of brick veneer masonry construction is leaky mortar joints and the sponge-like affinity brick has for rainwater. Rainwater driven by wind pressure passes through solid brick by diffusion leaving some moisture in the brick and, also, penetrates the mortar joints by capillary suction and convection; however, leakage through the mortar joints by convection passes 50 to 200 times more water than diffusion through the brick and capillary suction combined. 1
Water that penetrates brick by diffusion or leaks through the mortar joints by capillary suction does so as liquid water molecules that appear as drops of water on the cavity side of the veneer and ideally runs down the drainage plane and out the drainage holes. Seeing drops of water on the cavity side of brick veneer after a rainstorm is not abnormal. The cavity side of the brick veneer should be free of mortar bulges that interferer with drainage. A signature weakness of this kind of drainage is that much moisture remains trapped in the cavity because mortar droppings plug the drainage holes or accumulates on wall ties and other bridging where moisture collects. This is a workmanship issue that we shall discuss later.
Water vapor, the gaseous phase of water, is propelled into the cavity by wind pressure (convection) through leaks in the mortar joints. As wind pressure increases, pressure in the cavity increases, forcing
wind and water vapor to retreat into the occupied space through openings in the backup wythe. In cold weather, the wind carries less water vapor, but carries cold air into the occupied space instead robbing the HVAC system of its energy efficiency. According to a study by the National Institute of Science and Technology (NIST), the energy savings from a well-constructed air barrier, that prevented this cold air infiltration, can be as much as 33-percent of the annual energy cost. 2
Air Vented Cavity Drainage Wall
Circulating air through the cavity removes moisture by reverse-convection that would otherwise remain in the cavity to corrode reinforcement and other metal parts and wet the insulation. The air vented cavity design promotes air circulation by adding open head joint air vents at the top and bottom of the veneer wall 61-cm (24-inches) O.C. apart. This arrangement of openings induces air cycling by differential air pressure entering at the lower openings where the pressure is higher and exiting at the top openings where the pressure is lower.
Early in the development of cavity wall design, moisture retarders were added to the cavity face of the CMU to prevent cavity moisture from entering the building. The term moisture retarder gave designers a false sense of confidence that moisture was no longer a problem. Their mistaken perception lay in the fact that moisture retarders slow moisture but do not stop air movement which carries up to 200 times as much water as vapor than does diffusion and capillary suction combined. Thus, air movement can carry copious amounts of water vapor and push energy robbing cold air into the occupied space in winter even with a moisture retarder (but no air barrier) in place. Unfortunately, many designers have become comfortable with the moisture retarder solution and haven’t kept up with further developments in preventing water vapor and cold air out of the building.
Pressure Equalized Rain Screen Walls
As wind pressure increases during a rainstorm, wind driven rainwater and water vapor carried on air movement begins to enter the cavity through hairline shrinkage cracks in the mortar joints and fine gaps between the mortar and the brick that result from poor craftmanship.
Cavity air pressure begins to increase on the windward side of the building and slips around building corners to leeward sides of the cavity where the pressure is lower. Eventually the entire perimeter cavity becomes a pressurized reservoir of water vapor and cold air waiting its turn to enter the occupied space. The first line of defense against this intrusion is a continuous, durable air barrier that prevents air pressure from finding its way into the occupied space and a second defense is to confine the developing air pressure to the windward side of the building with cavity battens at the corners of the building.
The differences between an air barrier and a moisture retarder are significant. While air barriers are defined by their continuity and resistance to air movement, moisture retarders were never intended to be a continuous barrier to wind borne water vapor and cold air movement. Designers had developed a false sense of security that moisture retarders would keep their buildings energy efficient and dry; but lacking an air barrier this is not true.
In 1963, the Canadian National Research Council’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 rain screen cavity walls.
The premise of pressure equalized cavity wall design is that wind driven air laden with water vapor or cold air will continue to enter the cavity until air pressure in the cavity equals the wind pressure and a temporary equilibrium is established. Once there is equilibrium no more air enters the cavity until the wind abates allowing cavity air pressure to exhaust carrying moisture out with it. The pressure equalized system turns wind pressure into an ally for drying the cavity, but it requires extraordinary design and workmanship collaboration to make it happen.
The pressure equalized cavity wall design requires dividing the cavity into small compartments using battens and a continuous air barrier so that wind can pressurize each compartment until the compartment cavity air pressure is equal to the wind pressure. The engineered cavity enclosed by battens and air barriers is pressurized by openings at the bottom of each compartment allowing the compartment to quickly pressurize equalizing the wind pressure and no more water laden air enters the cavity. Wind pressure is dynamic, rising and falling in gusts when the wind pressure drops below the pressure in the cavity compartment the air exhausts taking moisture out with it and the cycle starts over again. 3 There are many versions of the compartmented cavity, some have small compartments 4-foot square and others have battens at the corners only. Corner battens only are probably the most practical.
Water vapor laden air pushed by wind pressure will move through leaks in masonry joints. Any hole or penetration in the building envelope will allow this undesirable air to move through the backup wythe and into the interior of the building. While moisture retarders slow the movement of moisture diffusion across the retarder; only continuous air barriers stop air movement which carries 50 to 200 times as much moisture as that moisture stopped by the moisture retarder. Air barriers also prevent cold air from being pushed into the building by wind pressure or sucked into the occupied space by differential building pressures created by the HVAC system or stack effect. By stopping air borne water vapor, continuous air barriers have turned out to be great water control layers in masonry cavity construction, but perhaps their larger contribution is preventing cold moist air from entering the building.
It’s instructive to note that air barriers won’t stop warm humid air from escaping the building and condensing in the cavity insulation under winter conditions unless manufactured as combination air barrier/vapor impermeable products. 4 To be effective, air barriers must be continuous and durable.
Finding an effective air barrier material is not that difficult. The challenge is the necessary architectural detailing, inspection and craftsmanship required to provide a continuous barrier, considering break outs for mechanical and electrical hardware and structural elements. Proper sealing is done with meticulous attention to flashing and caulking details, and correct door and window sealing. Unless the designer wants to start with a simple square or rectangular building, creating an air barrier for complex architecture is difficult. Completing an airtight perimeter on a large commercial project, a hospital or a high school e.g. with numerous breakouts in the cavity is a design and workmanship challenge. It would be advisable to have a continency budget to enlist the contractor’s assistance locating and sealing openings that are not anticipated in the construction documents.
Air barriers stop air movement and thus prevent water vapor laden air from moving through the barrier. Moisture retarders are placed on the cavity side of the CMU to slow the movement of moisture driven in either direction by temperature and moisture gradients between the interior and exterior. There will always be a certain amount of moisture entering the cavity by differential thermal pressure when the interior is warm and the exterior cold. Moisture vapor measured as humidity will naturally move from the warm side of a wall to the colder side. If the temperature is high inside the building and lower outside the building, the vapor drive will be directed outward. Moisture that moves in this way will reach the insulation unless there is a moisture retarder or combination air barrier/moisture retarder to stop it. At some point along this temperature gradient where the temperature of the insulation gets cold enough moisture vapor will begin to condense and turn to water. While this involves tiny amounts, eventually it will rot the insulation and turn to mold.
This is the end of Part I Cavity Wall Design, Materials and Workmanship – by Paul Potts
Part II will start with Design and Workmanship
This article represents the research and opinions of the author and is intended for general information purposes only and does not constitute professional advice. If you are intending to design or construct with masonry consult an architect.
- Richard L. Quirouette in his paper “Difference Between a Vapor Barrier and an Air Barrier,” Building Practice Note No. 54 (National Research Council of Canada, Ottawa, Ontario, 1985) provides a steady state example to demonstrate that air leakage by convection transfers more than 200 times the amount of water transferred by vapor diffusion and capillary action combined.
- NIST Savings from More Airtight Buildings: 1995 study performed by NIST (National Institute of Standards and Technology) found that 15% of the heating load in commercial buildings nationwide is caused by air leakage.
- E Building Envelope Forum Solving the Air Barrier Riddle: Permeable or Impermeable? By Sonya Santos http://graycoconsulting.com/grayco/pdf/BE_article_air_barrier.pdf
Paul Potts is a freelance writer published in design and construction magazines. He has worked as owner’s representative, specification writer and construction administrator for architects, engineers and owners. Potts can be contacted via e-mail at firstname.lastname@example.org or visit his website at www.pauljpotts.com or at LinkedIn https://www.linkedin.com/in/pauljpotts/