Designing masonry cavity walls

by arslan_ahmed | August 29, 2022 10:39 am

By Paul Potts

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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[2], the minimum recommend clear space[3] is 50 mm (2 in.), plus space for the insulation. The space is needed to promote air circulation and drainage[4], 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.

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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.

Air-vented cavity drainage walls

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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[7] 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[8].

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[9].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.

Air barriers versus moisture retarders

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Better workmanship can improve the watertightness of mortar joints, but an air barrier and a moisture retarder are still needed to prevent moisture from entering the building.

Many designers recognize the need for a moisture retarder on the exterior face of the CMU, but they overlook the importance of an air barrier. According to a study[11] by the National Institute of Science and Technology (NIST), the energy savings from a well-constructed air barrier can be as much as 33 per cent of the annual energy cost.

Insulation in cavity walls

Three types of insulation are used for masonry cavity walls:

• Extruded polystyrene (XPS) moisture-resistant rigid board.
• Foil-faced polyisocyanurate (polyiso) rigid foam.
• Closed cell sprayed polyurethane foam (SPF).

XPS insulation offers moisture resistance and long-term durability. It is lightweight, easy to handle, and can be cut into smaller pieces. XPS comes in 406 mm (16 in.) widths and
1.2 m (4 ft) lengths which can be pressure-fit between wall ties. A misgiving about XPS is its rigid configuration, making it difficult to enclose irregular shapes.

Dual foil-faced polyiso rigid foam insulation in 406 mm widths and up to 3 m (10 ft) lengths is recommended for cavity wall insulation. It has a higher R-factor than XPS at the standard test temperature of 24 C (75 F). However, studies[12] have shown the R-value of polyiso steadily declines as the temperature goes below 15 C (59 F).

Closed-cell SPF insulation has sound/air barrier and moisture retarder properties. The adaptability of SPF to foam into irregular openings and cover irregular shapes makes it a potential complement to the rigid products in the development of air barriers. This adaptability helps simplify the design of cavity drainage walls, air-vented cavity walls, and especially pressure-equalized cavity walls. Cautions about SPF have been published in ASTM STP1549, Dimensional Stability Considerations in Spray Polyurethane Foam Air Barriers.

In a paper[13] published in 2014, the ASTM made note of “the short- and long-term shrinkage variations in foam systems that cause problems when the SPF product is used as an air barrier particularly when applied to the exterior of the building.” Regardless of these concerns, SPF has an important place in the design of pressure-equalized rainscreen walls and air-vented cavity walls to make airtight building envelopes.

Understanding the extent of the bond between brick and mortar

There are two characteristics of the bond between brick and mortar: the extent of bond and the strength of bond. The more important of the two is the extent of bond, which refers to how completely the mortar is spread in intimate contact with the brick across the entire surface of the head joint. The extent of the bond between the brick and mortar is a direct result of the initial rate of absorption of the brick, the water retention of the mortar, the skill of the mason, and the workability of the mortar. Mortar bond can be a problem with flat stone or manufactured stone veneers due to lower stone porosity. An experienced mason can feel if the mortar is sticking to the brick or stone properly and adjust, if necessary, by rewetting the brick or retempering mortar which has gone dry on the board.

The initial rate of absorption of brick

When brick and mortar are brought together, a tug of war begins between the suction of the brick (trying to draw water out of the mortar) and the water retention qualities of the mortar (trying to keep its water to itself). Water retention is a manufactured quality and can vary between manufacturers and types of mortar. The objective is to keep the water and cement together, so they can be absorbed as a paste into the brick, and let the brick absorb as much of the paste as possible. This makes a good extent of bond—whether it is Portland cement/lime mortar, masonry cement, or mortar cement.

The suction properties of brick, measured as the initial rate of absorption (IRA), produce the bond between brick and mortar when a certain amount of cement/lime paste is absorbed into the pores of the brick. However, if the brick is too dry the brick suction prematurely draws too much water from the mortar, leaving a dry paste behind. If it is too wet, the brick has too little suction—reducing the extent of bond and weakening compressive strength. An experienced mason can feel if the mortar is not sticking properly to the brick because of mismatched moisture properties and adjust by rewetting brick or retempering mortar which has gotten too dry on the board.

Workmanship

Mortar droppings in the cavity can cling to the cross member of the reinforcement and absorb water. The mason can reduce mortar dropping into the cavity by sloping the bed joint mortar somewhat away from the cavity. Moisture that penetrates the veneer brick does so through the head joint; Fully buttering the head joint will reduce this defect. Additionally, one must not slush mortar on the joints. CMU mortar joints on the cavity side of the CMU must be cut flush to allow rigid insulation to fit tight to the wall and prevent creating a hollow space where moisture can collect.

Conclusion

When masonry walls encounter problems, water-related issues are often one of the primary factors. Brick masonry exposed to a disproportionate amount of water may have dimensional changes; efflorescence on exterior surfaces; and cracking, crazing, spalling, or disintegration due to repeated freeze-thaw cycling. Water may also cause metals to corrode, insulation to lose its effectiveness, and interior finishes to deteriorate.

Author’s note: This article is a presentation of the author’s experience and research and does not constitute legal or design advice. If the reader is contemplating construction, it is their responsibility to hire architects and engineers to design the work, prepare construction documents, and give the owner professional design advice during construction.

Notes

¹_{Read The Cavity Wall: A Brief History, published by the London Amp Company. londondampcompany.co.uk/the-cavity-wall-a-brief-history[14].}

²_{In the early 1960s research was conducted in Norway on rain penetration of windows and walls, and Øivind Birkeland published a treatise referring to a “rain barrier.” In 1963 the Canadian National Research Council published a pamphlet titled “Rain Penetration and its Control” using the term “open rain screen.”}

[15]Paul Potts is a technical writer, owner’s representative, and construction administrator. He has worked in the construction industry as an independent contractor and administrator for architects, engineers, and owners in Michigan since 1980. Potts can be contacted at paulpotts1@comcast.net[16].

Source URL: https://www.constructioncanada.net/designing-masonry-cavity-walls/

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