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Air Barriers and Insulation
Fluid Applied Air and Water Barriers:
Air and water barriers serve an important role in the functionality of high-performance building veneers. Many factors go into the engineering of a building’s exterior envelope, not the least of these is the design, selection, construction and the installation of the air and water barrier and any finishes that are applied over the membrane.
Air barrier materials are defined by the air permeance. To qualify as an air barrier product, the air permeance must be equal to or less than 0.02L/(s•m2) @ 75Pa (0.004 cfm/ft2 @ 1.57 psf) when tested in accordance with ASTM E2178.
Air permeance is the amount of air that migrates through an air and water barrier material. For more information on air barriers, visit the Air Barrier Association of America (ABAA) at www.airbarrier.org.
In addition, the ability for an air barrier to manage moisture migration through a buildings veneer is just as critical. Typically, water barriers are vapor permeable and are designed to meet a range of permeance levels. The criterion used to qualify these membrane characteristics may require specific project testing.
Several composition types of air and water barriers are available in the market (e.g. liquid applied, sheet types, etc.). Fluid applied air and water barriers have specific advantages, in that they can be applied easily using high-production techniques (e.g. airless sprayer, power rollers, etc.) and provide a seamless installation. Fluid applied products are more prone to seal up difficult-to-reach sections of a building envelope and will seal up tighter around penetrations.
In addition, fluid applied systems are fully adhered to a substrate versus sheet types that rely on either limited adhesion or on mechanical attachment, which can defeat the continuity of the product. These products are scientifically engineered to achieve performance characteristics that can protect a building’s envelope and improve its efficiency.
We will consider four factors that can help a specifier select an appropriate air and water barrier product for the project:
The selection process begins with an understanding of which relevant test methods and specifications are required for the specific project/application. In addition, local building codes also play an important role in identifying which air and water barrier type is ultimately selected, and which performance specifications are required for a given project/application.
The construct and substrate type of the building envelope will factor into what performance values are ultimately required and will help to narrow down which product can be specified. The method of attaching/adhering the finish type also bears on which product can be used. Simply put, if a veneer type is scheduled to be directly adhered to the air and water barrier, then that product must have the ability to accept an adhesive for this purpose. Let’s consider these four factors in more depth.
Generally, the reported test values are the result of testing an actual assembly in its entirety. The following test methods/standards generally apply to fluid applied air and water barriers (including but not limited to):
The values achieved by a specific product in any of the above categories are important factors when tailoring the product for a project. For example, if the engineering study for a project requires that the air and water barrier have a rating of 1.0 perms or less per ASTM E-96 (procedure B as called for in ICC ES AC212); that requirement serves as a filter for products that are being considered for the project.
The engineering study should determine which characteristics carry the most weight for the intended design.
Compliance to codes and regulatory mandates requires the use of air and water barriers to improve energy efficiency. For example, the U.S. Department of Energy requires them on federally funded building projects. Certain states have mandated that air and water barriers be used on new building constructs as part of energy conservation codes and building codes. The U.S. Department of Energy has instituted a program with a goal to reduce building energy consumption by 50 percent by the year 2020. Simply put, tighter buildings use less energy.
It’s important to note that the entire building envelope assembly be evaluated in its entirety as requirements for additional vapor barriers and thermal protection also may be required.
Note that commercial building constructs have different requirements when compared to residential constructs:
The following is a list of building codes that includes references and requirements for air and water barriers assemblies and products:
The third and fourth factors for air and water barrier product selection go hand in hand. Typical backup wall constructs consist of concrete, concrete masonry units, wood or steel framed assemblies with fastened lath and plaster systems, wood or steel framed assemblies with exterior rated sheathing or properly designed cement backer board substrates.
Design and placement location of air and water barriers can vary depending on the construct type, climate and type of building envelope (see Figure 2).
Typically, fluid applied air and water barriers are placed on the underlying substrate or sheathing. In some cases, the location of the membrane may of necessity be placed behind the building’s exterior envelope. In addition, specify a product that includes a complete line of complementary flashing tapes and sealants that will create a complete building envelope system (see Figure 5).
These are design considerations that must be evaluated for specific projects.
Veneer type and attachment methods
The fourth factor can be viewed in three main categories and can bear on the selection of a product type and placement location. Critical application modeling and detailing of the air and water barrier installation are important features and benefits of the ultimate products selected and utilized. The veneer type and attachment categories that we will consider are as follows:
Cavity wall veneers
Further prescriptive requirements of exterior anchored veneers fall under the auspices of Chapter 6 of the Building Code Requirements for Masonry Structures TMS 402/ACI 530/ASCE 5.
Direct adhered veneers
Further prescriptive requirements of exterior adhered veneer fall under the auspices of Chapter 6 of the Building Code Requirements for Masonry Structures TMS 402/ACI 530/ASCE 5; namely section 6.3 – Adhered Veneer.
Cement backer board applications installed over steel framing in accord with Tile Council of North America Method W-244 (E) is a commonly specified installation system for adhered veneers (see Figure 5) that incorporate an air and water barrier membrane.
Veneers that require penetration through the air and water barrier
In some cases, mechanical anchoring of veneers applies to the category of finishes. In these cases, follow the prescriptive requirements of exterior anchored veneers, which fall under the auspices of Chapter 6 of the Building Code Requirements for Masonry Structures TMS 402/ACI 530/ASCE 5.
Therefore, the selection of an appropriate air and water barrier must be made by considering which factors and performance criteria are the most appropriate and relevant to that specific project design. Considering the four factors in this discussion can assist property owners, their design teams and specifiers to make the proper air and water barrier selection.
Arthur Mintie is director, technical services for LATICRETE International Inc. For more information, visit www.laticrete.com.
|Last Updated on Thursday, 26 June 2014 15:19|