Designing an Adhered Masonry Veneer – Part 2

Words: Steven Fechino

In last month’s issue, we covered several components of the structural substrate. We focused on stud wall construction, Oriented Strand Board (OSB), gypsum and bituminous sheathing, and plywood. We discussed the performance of each product and how it relates to the wall.  

In part two of this series on wall design for adhered masonry veneers we will discuss weather-resistant barriers commonly found on light commercial and residential construction projects. This discussion will give you insight on how you can properly select the correct material for the application that you are building.  

House Wrap, Tar Paper, Grade D Paper 

The perm rating should be part of your consideration when selecting the house wrap material as the vapor that passes into the wall must be able to escape to the outside.  

House Wrap 

Weather-resistant barriers (WRB) are an important consideration that many seem to take for granted.  It seems that the contractors typically just pick up a roll at a local supplier or big box and staple it into place without considering the pros and cons of each product. 

House wraps are a (WRB),  are generally made from lightweight synthetic materials.  They compete with other roll forms of building wrap material such has Grade D paper, number 15 building felt and number 30 building felt.  The current use of synthetic house wraps have widely replaced other types of WRB’s during the past 10 to 15 years. House wraps are a commonly used product, known for the ease in installation due to conveniently dimensioned roll sizes (typically three or nine-foot-wide rolls) and light weight. Most framed walls are initially wrapped by the framing crews, the second layer is typically installed by the veneer crew. Local code regulations will guide the builder on how many layers are required in each geographical area. 

House wraps are designed to shed liquid weather events  (rain) during construction and to prevent substrate saturation as well as protecting the air space from direct moisture transmission during the life of the building.  The house wraps are not completely waterproof and can allow moisture to penetrate the substrate from time to time. A WRB, whether it’s a house wrap, or a number 15 building felt, is designed to allow the drying of the substrate and this function is critical to the overall performance of the wall.  As with all WRB’s, it is important that all vertical and horizontal joints be lapped a minimum of 6 inches and fastened every 12 to 16 inches vertically at the stud locations. Taping all joints will complete the wrap application’s durability and overall performance. 

WRB can be purchased perforated or non-perforated.  

Discussions surrounding the need for perforated or non-perforated WRB are because house wrap is a non-directional material, when water or water-vapor passes through, it must have the ability to quickly dry to prevent the substrate from becoming saturated. Some critics feel that perforated house wrap products offer a faster drying cycle while other take the opinion that moisture will not be as likely to get behind the house wrap without holes. The decision to use one product instead of another is based on many geographical and project specific details. The different types are for vapor transmission and the hole will allow for vapor transmission, something that is beneficial in many parts of the country.  

Drainage Planes  

Thin adhered veneers typically are installed where the expanded wire mesh is fastened directly to the substrate just prior to the application of the scratch coat of the thin adhered veneer.  The absence of an air space or drainage plane allows for direct moisture contact between the inside face of the veneer and the outside face of the substrate.  The direct contact between the veneer and the substrate will allow for moisture to penetrate the substrate over time creating sheathing deterioration and, in some cases, direct leaks.  Building scientists have published documents that state the use of a drainage plane between the thin adhered veneer and the substrate have positive effects on the performance of the veneer and the overall life of the building.  The drainage planes we typically see are either a polyester or polypropylene high loft or entangled mesh, that can offer approximately 90 percent air passage through the mesh. 

The drainage plane can eliminate one layer of weather resistant barrier (WRB). 

LathNet, by Mortar Net Solutions, is a new product on the market that has been successful as a labor-saving alternative to installing the second layer of weather resistant barrier and an extruded metal lath in two individual steps.  LathNet combines the two products into one easy-to-install product that overlaps and shingles as it is applied up the wall with code approved overlaps to ensure proper installation.  LathNet can also be successfully installed over rigid insulation with taped joints to offer a code compliant system. 

Number 15 Felt  

In the old days, cannot believe I am old enough to really say this, 15 and 30 pound felt was exactly that, 15 and 30 pounds per square (square is 100 square feet).  Today they felt designations are slightly different, but the weights are very different.  Number 15 and number 30 felt are still sold in 36-inch rolls, the weight per square for number 15 felt can be in a range of 7 ½ to 13 pounds per square. Typically, Number 15 building felt can meet ASTM D 4869 "Standard Specification for Asphalt-Saturated Organic Felt Shingle Underlayment Used in Roofing". However, it rarely meets the stricter requirements listed under ASTM D226 "Standard Specification for Asphalt-Saturated Organic Felt Used in Roofing and Waterproofing".  Type I as discussed in the specifications for ASTM D 226 refers to the number 15 or the 15-pound product, type II refers to the number 30 or 30-pound material.  

Felt has a perm rating that is very low, approximately 5.  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. Number 15 and number 30 low perm ratings means it can take a long time to dry, so keeping it dry is an important construction consideration.  A perm rating of 5 is the lowest code approved rating, however, when it is wet the perm rating can increase to 60. The increase wet perm rating does not correlate to linear increased drying times.   

Number 15 felt will wrinkle and shrink slightly once saturated.  After completing a drying cycle, the remaining wrinkles do add a small inconsistent air space between the layers of the felt, but do not create a functioning drainage plane.  Number 15 and number 30 felts are not widely used today under thin adhered veneers, but have been widely used under the previous thickness for over 50 years with very good overall results. 

Grade D Paper 

The use of Grade D paper across the country is growing in vast numbers due to outstanding performance in both the testing labs and in real world construction.  The strength of Grade D paper is in the western states. Sold in different thicknesses, you can purchase 10, 20, 30- or 60-minute material.  The numbers refer to the boat method as it is referred in the industry, where the paper is basically folded into a shape that can be related to a boat, placed in a dish of water and a desiccant on the paper will change color from a light shade to a darker shade representing saturation.  The test is timed, and ratings are published.  Grade D paper when properly selected as a WRB will perform well for many years. 

Grade D paper is a system of an asphalt-saturated virgin paper with a tight, uniform matrix. Usually found to be lighter weight than a number 15 felt and easy to crease when wrapped around corners. Grade D paper has many advantages.  When used under adhered masonry veneers, it can be used with one or two layers when applied to the substrate, depending on the requirements of the individual project.  Many local codes require a second layer only to shed direct water and can perform better with the added material.  Continuous wetting of the grade D paper must be controlled because it is can deteriorate faster than other types of house wraps, mainly due to the selection of the virgin paper as the base paper.