Cavity Wall Design, Air Barrier/Moisture Retarder Design, and Workmanship Part 2

Words: Paul Potts

Words: Paul Potts
Photo: Warchi

Introduction to Part 2: Part 1 presented an overview of cavity wall design and the function of air barriers and moisture retarders. Part 2 covers the importance of the selection of brick and mortar materials and how workmanship is fundamental to weatherproofing the exterior of brick masonry. The movement of the brick and mortar in the hands of the mason will determine the reliability of the mortar to stick and spread completely along the bed and head joints. We will discuss other materials are optimized by the selection of masonry materials and collaboration of designer and craftsmen.  


Wind that penetrates mortar joints accounts for 200 times as much moisture in the cavity as does water vapor penetrating the brick itself. It is possible to minimize this moisture movement by improving material choices and masonry practices that will be discussed below. The mason must perform their work consistent with these recommendations. (1) The bed joint mortar must be placed in a beveled fashion from back to front to prevent mortar from bulging out the back side of the joint. (2) The mason must fully butter the head joints and fill the entire joint with the mortar. Tooling joints improves the imperviousness of mortar joints to moisture movement. (3) continuously monitoring the mortar to match the moisture characteristics of the mortar with the brick. All design efforts to keep the building dry and at peak energy efficiency are wasted if the masonry cavity and weep openings are not kept free of mortar and the bricks are not laid in full contact with the mortar. 


Face brick is specified by type and grade, type FBX and FBS and grade SW and MW. Type is an indication of a unit's color range, shape and dimensional tolerances. FBX brick allows only minimal size and shape variation, which improves the full contact of the brick with the mortar. Face brick are also graded as SW (extreme exposure) and MW (moderate exposure) referring to its weather resistance. Type FBX brick graded as SW is the best choice for repelling rainwater.  


The most important function of mortar in face brick is to form a watertight bond with the brick to minimize the passage of water and water vapor. The moisture compatibility of the two materials – brick and mortar, the workability of the mortar (the ease with which the mason can get the mortar to spread and stick to the unit), and the craftsmanship of the mason are basic to making watertight joints.  


There are four common mortar types M, S, N and O. Type N.  While Type N is the weakest of the mortar types, Type N is recommended in most masonry design manuals for face brick and type S for supporting CMU. The building code does not allow type N mortar to be used in any application in Seismic Design Categories D, E or F. 

Type N mortar is weaker in compression than type S due to its higher water/cement ratio; but the water improves workability making it stickier and thus easier for the mason to spread across the entire head joint of the brick.  Much rainwater and water vapor that gets into the cavity is there because the mason did not completely butter the head joints. 


Suction of the brick, measured as the Initial Rate of Absorption (IRA), produces the brick-mortar bond when a certain amount of the cement/lime paste is absorbed into the pores of the brick. If the brick is too dry, measured as having an IRA above 30 g/30 sq. in. (30 g/194 sq. cm), the suction prematurely draws too much water from the mortar leaving a dry paste behind -  too wet, below 30, and the brick produces too little suction weakening the bond strength and reducing the weather tightness of the joint.  


There are two important characteristics of the bond between brick and mortar – the extent of bond and strength of bond.  The extent of bond is directly related to the water tightness of the mortar joints; and depends on how completely the mortar is spread in intimate contact with the brick across the entire surface of the joint. The strength of bond is not important to our discussion.  

The extent of bond is dependent on the compatibility of the moisture characteristics of brick and mortar and the craftmanship of the mason to make corrective changes to the workability of the mortar on the fly.  Workability is a description of the ease and stickiness the mason experiences when spreading the mortar on the brick. An experienced mason can feel if the mortar is sticking to the brick properly and adjust by wetting the brick or retempering mortar that has gotten too dry on the board.   

When brick and mortar are brought together a tug of war begins between the suction of the brick to draw water out of the mortar and the water retention qualities of the mortar to keep its water to itself. The objective is to keep the water and cement together and be absorbed into the pores of the brick as paste and letting the brick absorb as much paste as possible. Water retention is a manufactured quality of mortar and can vary between manufacturers and manufactured mixes - whether it is Portland cement-lime mortar or masonry cement.  

Tooling the masonry joints presses the mortar into the brick and produces a glaze on the surface of the mortar that is more water repellent than mortar that has not been struck 


Workability, a measure of how easily the mortar spreads and sticks to the brick, is directly related to the amount of moisture remaining in the mortar. Retempering, by adding a small amount of water to mortar on the board, replaces water lost to evaporation.  

Many architect’s and engineer’s specifications prohibit retempering of mortar; however, the rule predicated by ASTM and the Portland Cement association is that mortar can be retempered as necessary but should be discarded after 2 hours from the time it left the mixer. Colored mortar should not be retempered. 


Architectural detailing and craftsmanship of the through wall flashing at the bottom of the cavity and the connection of the air barrier to the roof membrane or parapet flashing is of importance to making the air barrier continuous. If the through wall flashing forming part of the drainage system is not correct, water will be ponded at the base of the cavity compartment. The overall design of cavity wall masonry is well understood, but architectural details, requirements and how they are expressed in the documents and inspected to provide a continuous air barrier are weaknesses.    


The most critical product mentioned in this article is the air barrier. While there are many good choices on the market, design detailing and workmanship are the most critical factors. The construction of the cavity must be approached with skill and knowledge of the function of the cavity construction to purge the void of moisture, water and water vapor.   It is also unimaginable that construction drawings for a large commercial building could anticipate all the potential breeches in the air barrier that would require special treatment to make an airtight barrier. The owner, designer and construction manager should expect a considerable number of change orders to cover unanticipated openings and thermal bridges requiring special treatment.  

Throughout this article I have avoided discussing the stack effect. While stack effect is an important issue in multistory buildings, it does not add much to a basic understanding of the importance of masonry air permeance and water vapor permeance. 

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. 

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 or visit his website at or at LinkedIn 

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