Designing an Adhered Masonry Veneer

Words: Steven Fechino  
Photos: Steven Fechino, Mortar Net Solutions 

The improvements in wall section technology today far exceeds the performance from just a few years back.  Thin adhered veneers, though they have been constructed for many years on the west coast, have seen the most dramatic improvements with polymers that enhance the performance of the mortar, materials that have a more durable and attractive appearance and drainage planes that can increase the long-term performance of the walls.  This article will look at the wall section and discuss the performance, need and options that can be found in today’s designs. 

Adhered Masonry Veneers 

The aesthetic beauty of the thin adhered veneers of today far exceed even just a few years ago, with patterns that offer five times the number of shapes, natural stone sawn veneers and thin brick, the walls of today can be difficult to separate from thin veneer or full depth materials. 

Part One 

This month we will look at the differences between a cavity wall and an adhered masonry veneer.  Our discussion will cover basic materials used in residential and light commercial construction of a stud constructed structural substrate. 

Parts and Pieces of the Adhered Veneer Wall 

The design weight of thin adhered veneers typically ranges from 3 to 25 pounds per square foot (psf). Accurate weight calculations will consider the veneer, scratch coat, expanded wire and Thin Veneer units, with allowable thicknesses currently at 1 ¾ inches under current ASTM standards.  The calculation of weight per square foot is critical in sizing the proper stud system for each project.  Thin adhered veneer manufacturers can offer the designer accurate psf estimated based on the style that has been chosen.  Cavity wall masonry veneer places the load on a lintel, shelf angle of brick shelf, thin adhered veneers and stuccos are supported by attachment. Or bond between the lath system and the structural substrate (scratch) and the bond to the thin veneer material.  Adhered masonry veneers do not place the load on a foundation or weep screed. 

Starting at the structural Substrate wall and working outward  

When designing a thin adhered veneer system, the selection of the expanded wire weight is important.  Available in 1.75, 2.5 and 3.4 pounds per square yard.  The weights that should be considered for thin adhered veneers are the 2.5 and the 3.4 pounds per square yard products.  The 1.75 product is for interior use only and if scraps are left over on a project, they could be incorrectly used in the wrong application.  Knowing the weight of the veneer, if rigid insulation is being used and the gauge of the structural studs will all be useful in selecting the weight of the expanded wire material required for your project.  The second consideration is the anchoring that is selected for the best performance.  Typically, the use of screws and galvanized caps are used to secure the veneer to the structural substrate on the project.  It is important to note that when rigid insulation is used on the outside of the substrate that the length of the anchoring screws be adjusted in order to obtain the proper embedment for the screws. 

Comparing the Function of a Cavity Wall Veneer to an Adhered Masonry Veneer 

Wall sections in a majority of residential and light construction included, from the interior to the exterior; drywall, polyethylene plastic; fiberglass batt insulation installed between studs; a sheathing choice of, bituminous board sheathing, plywood, gypsum board or more recently Oriented Strand Board (OSB); thin layer of rigid insulation, an air space; and a brick veneer.  The wall section performed well in most geographical locations within North America in part due to the air space or cavity between the veneer and the structural wall. 

When cavity wall construction is used, the cavity or as commonly described as the air space serves many functions, the most important of which is that it allows air, water and water vapor pass through the veneer and into the cavity without direct contact to the substrate.  This is important because the moist air within the cavity can drain to the weeps and exit the section without damaging the building structure When weeps are placed under the shelf angles or at soffits, then the ability for air to enter the cavity and enhance the drying process becomes an outstanding way to increase the drying and drying time that is required. 

Thin adhered veneer wall sections can benefit from understanding the science of the cavity wall and applying it to the thin veneer systems.  Though the drainage planes found in the thin adhered veneers are small in comparison to the standard cavities found today, the ability to drain, restrict direct moisture contact between the veneer and the substrate can demonstrate the common-sense approach to building a sustainable thin adhered veneer. 

Components of a Substrate Wall 

Polyethylene 

Polyethylene is commonly found when renovating homes built during the 1970’s through the 1990’s.  The polyethylene was used to prevent condensation from saturating the drywall. This could occur when the exterior air and vapor moved from the cooler temperature air to the warmer temperature air, typically found on the interior of the structure during many seasons during the year. This polyethylene created many issues over the years and is seldom used today due to the moisture-related problems with this use of polyethylene, in part because cooler air will always find its way into a wall, and once met by the vapor barrier, the colder air will condense, allowing water droplets to saturate the adjacent batt insulation.  The saturated insulation will cause the moisture to remain in the walls much longer than planned. This condition occurs mainly in climate zones that exhibit higher humidity and cooler temperatures.   In the Southwestern part of the United States the polyethylene will not create the same reaction as describes within this paragraph because of the low humidity’s that exist. 

Roll or Batt Insulation Placed Within a Stud Wall 

Batt insulation is still commonly used in many projects across the country, it is easy to install, easy to obtain and easy to see what the published R-value is for the product.  Heat always passes from a higher temperature to a lower temperature and R- values refer to the thermal resistance of heat passing through an insulating material.  Since R-values are based on a set temperature, they can vary within the same materials when the temperature variance differs due to a wider difference in outside and inside temperatures. R-values are used as part of a calculation that computes British Thermal Units (BTU) required for sizing your heating and ventilating system.  Insulation rated values ranging from a common R-11 to R-38 are common in products from multiple manufacturers with similar thicknesses of materials ranging from 3 ½ inches to 12 inches of fiberglass.  The discussion regarding R-values is not something that will interest many masonry contractors. However, understanding the R-value that is designed into the detail of the section can quickly affect labor productions when additional exterior rigid insulations are required when the interior batts were insufficient to meet the design. 

R-values can diminish when batts are improperly installed.  Batts can be split over wire found within the walls, placed directly against outlets and junction boxes and around all openings within a wall.  Scraps of insulation can be used; however, it is important to note that batt insulation that is tightly compressed and jammed into the wall will see a reduction in efficiency. 

OSB, Plywood, Gypsum and Bituminous Sheathing  

Oriented Strand Board (OSB) was invented in 1963 as a competitor to plywood. It is made by compressing wood chips, moisture resistant wax and resin-based adhesives into a dimensionally stable engineered wood component.  Beginning as a competitor to commonly known plywood, OSB is now found in many of the larger structural wood framing members available today at many of the commercial and residential lumberyards that serve the commercial and residential markets.  OSB has a low perm rating. A “perm” is a term used to describe the moisture vapor permeance of a material, or the ability of a material to allow moisture vapor to pass through it. The higher the perm rating, the faster moisture will pass through it from one humidity level to another. OSB’s low perm rating means it can take a long time to dry, so keeping it dry is an important construction consideration.    

Plywood initially was invented in the early 1900s, but became more functional after World War II when the adhesive resins used to laminate the different grains of wood were improved to the point where it performed to the satisfaction of the building industry. OSB and plywood are equivalent in performance and are typically described in the codes with similar verbiage. Plywood or OSB for exterior wall sheathing can generally be used interchangeably, but the amount and specific locations of each sheathing can depend on local codes. Other competitive sheathing products include gypsum-based or bituminous wallboard sheathing.  

To simplify this discussion, let’s assume that the substrate walls are completely framed in plywood or OSB. Then the focus becomes keeping the sheathing dry during construction as well as for the life of the structure, which is performed in several ways. First, during the framing of the walls, the carpenter installs one layer of Weather Resistant Barrier (WRB) or an air and/or vapor permeable or impermeable membrane on the sheathing. The WRB is found on the exposed sheathing to shed water off the walls until the veneer is installed. Since the water vapor from humidity, wind driven rain and incomplete construction allows moisture to directly come in contact with the sheathing it is important to understand what it will take for the sheathing to perform a drying cycle. Here is where the WRB or air barrier selection can provide important information.  If the sheathing shows indications of moisture after the veneer is installed or was damaged prior to the installation of the veneer, as the installer of the adhered masonry veneer, you are the one that will be blamed.  

Moisture can sometimes be observed from the inside of the wall before the batt insulation and drywall are installed. With the knowledge that the perm rating of the sheathing is a minimum of 5 and the WRB layer, for example is number 15 felt which also has 5 perm rating, you can better defend your work by understanding a full wall design with a reference to how each component is effected by the others.  For example, a building wrap that continuously and repeatedly becomes saturated will deteriorate, reducing its main functions and ability to shed water from the substrate walls. 5 is a low perm rating and will dry slowly, there is not a way to determine how fast a wall will dry based on perm ratings alone since there are so many different temperatures and humidity levels within the United States, but you can use this knowledge to base a decision to “hold off” on installing of the veneer over wet sheathing.  This step to wait it out can eliminate blame that later falls into your lap because someone sees a wet substrate that they blame on the mason for a leaky masonry installation. 

Gypsum board stock sheathing is fastened directly to the exterior side of the substrate framing using typical means such as galvanized screws, nails or staples. Standard thicknesses for residential construction is ½-inch and 5/8-inch is commonly used for commercial applications. Some brands of 5/8-inch material can meet fire resistance standards depending on individual manufacturer. The most common board stock has an exterior sheathing surface of fiberglass facing or wax-treated water repellant paper. Exterior sheathing offers a longer exposure time than many other products, in some cases up to 12 months.   

A new 5/8” sheathing product with the air and vapor barrier made within the board has entered the market. The manufacturer claims that the new product eliminates the need for a surface-applied air and vapor barrier, which reduces labor times and the chances for workmanship error.  Perm ratings for exterior sheathing typically range in the 20’s, which allows a rapid movement of vapor through the product.