Masonry Cavity Walls: Systems and Construction

When architects and contractors wantto build the most durable, watertightbuilding possible, they rely on masonrycavity walls. The top performing of allmasonry wall types, cavity walls providea balance of beauty, design versatility,durability and economy. As with anymasonry wall, the skills of the bricklay-ers and masonry contractors areparamount to the long life and successof the wall. Cavity wall systems are composed oftwo wythes of masonry, usually brickand concrete block, secured together bymetal ties with an open space, orcavity, between the two wythes fordrainage. Flashing and weep holes, tocollect moisture and drain the space, arealso part of a cavity wall system. A cavity wall addresses moisturepenetration by realizing that, during thelife of a building, it is likely that waterwill penetrate the exterior wythe of thewall. It can enter through top caps,copings, sills, at windows, doors andmany other locations. Wind-driven rainmay find its way through hairline fissuresbetween the mortar and the masonryunits, and larger cracks may occur overtime, allowing moisture to penetrate.The cavity wall system recognizes thatwater will get through the exterior wytheat some time in the building’s life, so itis designed to drain that water throughthe cavity and direct it to the exteriorthrough the flashing and weep system. Four essential components for propercavity wall performance, in addition tothe masonry wythes, are:

� Drainage Cavity (also calledair space)

� Flashing� Weeps� Workmanship

Bricklayers maintain a two-inch clear airspace drainage cavity between the rigid insulationand the back of the face brick.

Drainage Cavity The drainage cavity should be a nominal2" clear to allow for proper constructionof the wall and to minimize mortar fins,droppings and bridging. The purpose ofthe cavity is to drain any water before itcan penetrate to the interior. A cavitycan do this when it is clean, but it doesnot have to be pristine. Some mortardroppings and protrusions are to beexpected, but the key point is theperformance of the cavity: Is it drainingwater to the flashing? Cavities can be any width greater thanthe 2-inch nominal (1 5/8" actual) dimen-sion. However, beyond 4 1/2" in width,a wall tie analysis must be done to makesure the ties can transfer the loads tothe interior wythe over the longercavity span. Insulation and vapor retarders are

also likely to be installed in the cavity,depending on project design criteria.Insulation should not inhibit the freeflow of water draining through thecavity. It also needs to allow for roomto place the units - masons need to havea finger hold on the back of the unit forproper installation. The vapor retarderis frequently, but not always, placed onthe cavity side of the backup wall. It alsocan serve as a dampproofing barrier - akind of secondary level of defense forany moisture that does bridge thecavity. The architect specifies the typeand location of the vapor retarder basedon a condensation analysis. Cavity drainage inserts have beenintroduced into cavity wall constructionto assist with keeping the flashing andweeps relatively clear of mortar drop-pings. There are many types of cavity

RESOURCE INFORMATION FROM THE INTERNATIONAL MASONRY INSTITUTE

TECHNOLOGY BRIEF October 2000

Section 2.12.4

TM

drainage inserts on the market, andwhile they do add cost to the wall, mostare effective at helping to keep theweeps clear and the cavity draining. Notall drainage inserts perform equally, soexercise care when specifying. Cavityinserts can provide extra assurance ofclean cavities, but they should never bea substitute for good workmanship.Union bricklayers are trained duringtheir apprenticeship to keep cavitiesclean, and they are known for theirsuperior workmanship.

Flashing Flashing collects water thatpenetrates the exterior masonry andprevents it from entering the buildinginterior. It also can provide a slip planebetween dissimilar masonry materials.Three general categories of flashing areused in masonry cavity walls: metals,membranes and composites. Each typehas distinct advantages and disadvan-tages. The designer considers theproject requirements and selects a flash-ing material that is appropriate for thatproject. There are several importantpractices that should be followed,regardless of flashing material selection:

� Flashing must be located at thebase of all cavity walls, over

shelf angles, lintels, openings,under sills, copings and caps, atwall/roof intersections, parapetsand any other location wherewater may collect or enter thecavity.

� The flashing must be installedproperly lapped and sealed sowater can not penetrate under it,or the lap.

� It should be run vertically up thebackup wall approximately 6" to8" in most cases, and returnedinto the backup wythe.

� It must be continued, at a mini-mum, to the face of the exteriorwythe and not terminated in thewall itself. Generally, drip edgesare good details to include withany flashing system, as theydrain the water away from the wallrather than risking it reenteringthe exterior units. Since someflashing materials degrade underexposure to ultraviolet rays,separate metal drip edge piecescan be used with those materials.

� When flashing terminateshorizontally, such as at the end ofa sill or lintel, the end of theflashing must be turned up toform an end dam, which directsthe water back to the exterior.

Bituminous copper flashing pieces at the pier base are sealed for watertightness.

The stainless steel flashing above thewindow returns up to form an end dam.

Weeps

If you think of flashing as the guttersystem of the cavity wall, then the weepsare the downspouts. They allow thewater collected on the flashing to drainout and escape to the exterior. Depend-ing on the type, they can also facilitatea flow of air through the cavity, allow-ing the wall to breathe, which canresult in evaporation of moisture withinthe cavity. There are many choices for weeps.Some of the most effective are weepvents and open head joints. Theseallow air to circulate through the wall.Vents come in many different sizes,materials and colors to match mortarjoints. They must be properly installedand maintained throughout the life ofthe building, as should all weeps. Openhead joints offer the largest open areafor drainage and airflow, but canproduce undesired shadow lines. Oiledrods removed to leave a small hole, andcotton sash cords provide only smalldrainage areas, and may be easilyplugged by small mortar droppings inthe cavity or by debris on the outside ofthe building. For those reasons, if theyare chosen as the weep system, it isrecommended that they be installed atcloser intervals than weep vents or openhead joints. Plastic weep tubes are not

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recommended because they are easilyplugged with mortar, it is difficult to keepthem flat in the cavity, and if they turnup, the water has to rise to the level ofthe tube inside the cavity before it willdrain. Vents or open head joints shouldbe spaced 24" o.c. max., and otherweeps, if used, should be spaced 16"o.c. max.

Workmanship The skills of the bricklayers areessential to the overall performance ofany masonry wall, especially cavitywalls. Union contractors and bricklay-ers have the training and experience toprovide expert installation of flashings,weeps and other components of thecavity wall system. Keeping the cavity relatively free andclear of mortar droppings is an impor-tant consideration. The purpose of thecavity is to drain any water penetratingthe exterior. If the cavity is clean enoughto do this, it is an acceptable cavity.Generally, mortar droppings and projec-tions should be kept to a minimum, butsome should be expected. Techniques to keep the cavity cleanvary. Beveling the mortar bed backslightly from the cavity side of the unitworks well, as does parging excess finswith the tip of the trowel. Inserting aboard into the cavity to catch mortardroppings and then drawing it up as wallconstruction progresses is a practice thatdoes not work well in the field. Theboard can impact the newly constructedmasonry, breaking the bond, and it isdifficult to raise the board while keep-ing it level. In cases, this has resulted inthe mortar droppings on the boardfalling back into the cavity. The masoncontractor should be instructed simplyto keep the cavity clean so it drains, withthe specific technique worked out onthe job site sample panel to thesatisfaction of the designer and thecontractor. Proper tooling of the mortar joints onthe exterior wythe plays a major role inreducing the water penetration into thecavity. Concave or V-shaped joints thatare tooled when thumbprint-hard andcompressed slightly into the units forma better weathering surface. Other

This cavity insert will assure that the weep holes willbe unobstructed by mortar falling into the cavity.

joint profiles can be used, especially indry or moderate climates where waterinfiltration and freeze/thaw concerns arenot as severe as in the northern regions. Full head joints and bed joints, whenusing solid units, are also importantworkmanship issues, again to keep thewater from entering the cavity. The term“full,” however, is hard to quantify. Theentire masonry wall is laid one unit at atime by hand, in all weather, under alljobsite conditions. It is not possible, ornecessary, to have every single head andbed joint on the building 100% full, frontto back. It is possible to make sure thatevery attempt is made to try to reachthat goal. The industry is attempting toquantify what is an acceptable level of“full,” but to date, no consensus hasbeen reached. The job site sample panelcan be used as a communication tool toreach an agreement between thedesigner and the mason contractor asto what is acceptable on this job. Thegoal of a cavity wall is to minimize waterthat penetrates the exterior wythe; adrainage system is in place to collectand direct any moisture that does enterback to the outside.

Other Design Considerations The metal wall tie system that con-nects the two wythes of masonry shouldbe an adjustable system, allowing thetwo wythes to expand and contractindependently. It also must transferloads from the exterior wythe to theinterior backup wall. The tie systemmust have the ability to adjust and move,while being rigid enough to transfer theloads structurally. The exterior masonry experiences awider variation of temperature andthermal stresses than the interiormasonry wythe because the interior wallis only exposed to the conditioned spaceon one side and isolated from some ofthe impact of the outside temperaturesby the cavity insulation on the other side. Frequently, cavity walls are built ofdifferent types of masonry units - claybrick on the exterior and concretemasonry units (CMU) on the interior,for example. Since brick expands andconcrete block shrinks, differentialmovement between the two wythesresults. If the two wythes are tiedtogether too tightly by the tie system,this movement cannot be accommo-dated and cracks result.

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Expansion and/or control joints arealso used in cavity walls, as well as inother masonry wall systems. Masonryexpansion joints are provided in claybrick walls to accommodate moisturegrowth and thermal movement. Controljoints are provided in concrete masonrywalls to accommodate unit shrinkageand provide for thermal movement.These movement joints should be clearlylocated and detailed on the projectdrawings.

Summary The purpose of this article is tooutline the function and major compo-nents of the cavity wall system. It is notall-inclusive, but gives an outline of themajor components of the system and theway they function. A cavity wall isforgiving by nature, with several layersof built-in insurance to minimizeinterior moisture penetration, character-istics not found in most other wallsystems. Cavity walls are considered thebest performing masonry wall systembecause they offer this excellent resis-tance to interior moisture penetrationwhen properly designed, specified andconstructed by union mason contractorsand bricklayers.

Beveling the bed joint minimizes mortar droppings into the cavity, a technique often usedby trained, union bricklayers.

Providing full head joints, as shown here, is one line of defense against moisture gettinginto the masonry wall.

Photos feature St. Vincent de PaulCenter, Daughters of Charity,Chicago, Illinois.

Architect: VOA Associates, Inc.

General Contractor: Walsh Construction

Mason Contractor: A.L.L. Masonry

Bricklayers: BAC Local 21 Illinois

This document is intended for the use of industry professionals who are competent to evaluate the significance andlimitations of the information provided herein. This publication should not be used as the sole guide for masonry design andconstruction, and IMI disclaims any and all legal responsibility for the consequences of applying the information.

International Masonry Institute � 42 East Street � Annapolis, MD 21401 � 1-800-IMI-0988 � www.imiweb.org

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