Sheathing Membranes: Air Barrier Building Code Requirements
In a previous article, we discussed the North American building code requirements for sheathing membranes and their function as water-resistive barriers (WRB). This third and last article of the blog series will uncover the additional, critical role of sheathing membranes as air barriers. Sheathing membranes that act as air barriers are typically of the polymeric type, and can be separated into two categories: mechanically fastened and self-adhered. This article will help familiarize you with the North American building code requirements and testing standards in relation to air barrier sheathing membranes. It will also help clarify the functional properties that are sought by building codes and trade associations, as well as explain how these properties relate to whole-building performance.
Air Barrier Code Requirements
North American building codes typically include instructions related to the proper installation of sheathing membranes for use as air barriers, including requirements to maintain continuity (i.e., sheathing laps and taping transitions), ensuring structural integrity, installing products as per manufacturer directions, and taking steps to provide integrity for the lifecycle of the enclosure. While important, these requirements are generally qualitative and cannot be measured or tested (except by testing whole-building airtightness). Two other important requirements, however, can be directly tested: material air permeance and assembly air leakage. These measures are required by both the National Building Code of Canada (NBC) and the I-Codes.
Table 1 provides a summary of prescriptive compliance for air barrier materials and/or assemblies for the International Residential Code (IRC), International Building Code (IBC) and the NBC.
Under both the NBC and the I-Codes, the path to satisfy air barrier material requirements is more straightforward than the path for WRB requirements. Prescriptive performance and specification requirements for air barriers are clearly defined, without allowing for equivalency or other approved materials. Referenced standards are generally technically up to date and are applicable to modern sheathing membranes as well.
Air Barrier Material Standards
Material standards used in prescriptive requirements assess critical air barrier properties, such as structural integrity, durability, continuity and, of course, resistance to airflow. A short summary of the prescriptive testing standards is provided in Table 2.
These standards, among others, are also used by the primary North American air barrier associations for certification of sheathing membranes. The US-based Air Barrier Association of America (ABAA) and the Canadian-based National Air Barrier Association (NABA) are trade associations that provide guidance to the construction industry on air barrier materials, installation procedures, and other related information. These organizations, although not strictly concerned with vapor-permeable sheathing membranes, do list sheathing membrane products that are evaluated by ABAA. Manufacturers, in addition to testing reports, must provide further information such as sample details, proving that they support holistic airtightness goals. Interestingly, ABAA also evaluates sheathing membranes as WRBs, evaluating them to ASTM E2556 or ICC-ES AC38, as discussed in part two of this blog series.
Beyond Prescriptive Requirements and Material Standards
Meeting prescriptive requirements does not guarantee the ultimate objective of whole-building airtightness. Realizing the benefits of airtight enclosures, including energy efficiency, increased building durability, and increased occupancy comfort, requires a certain level of whole-building airtightness. Sheathing membranes act together with many components to create an air barrier system. With the majority of air leakage occurring at junctions, transitions, and penetrations, installation and detailing are vitally important. While limited airtightness testing of standard penetrations and junctions is required, the true airtightness for these locations is highly project specific. Read the article on whole-building airtightness for further information.
What prescriptive requirements do offer is assurance that the sheathing membrane and accessories provide an adequate level of resistance to air flow and other important properties. Meeting minimum requirements can, with any luck, translate to an airtight building, as well as avoid costly and time-consuming whole-building airtightness testing. In the future, as we move towards whole-building airtightness, some of these prescriptive requirements may change to eliminate redundancy in testing. Material-centric prescriptive testing (UV resistance, temperature resistance, etc.) should always be required as whole-building airtightness testing cannot provide this information.
Sheathing membranes can be a good choice for the primary air barrier, provided proper detailing and installation take place. The selection of the sheathing membrane system type can also impact a project’s success in achieving airtightness (i.e., mechanical vs. self-adhered). Self-adhered sheathing membrane systems typically achieve greater success due to the simplicity of the system. On the other hand, mechanically fastened sheathing membranes can achieve airtightness, but require additional consideration for controlling air leakage through unadhered seams and through fastener penetrations. The long-term implications of the mechanical fastening forces on the membrane must also be taken into account. The airtightness and durability of mechanically fastened systems can be improved by installing continuous exterior insulation and/or continuous furring strips that provide additional support.
Regardless of the system, sheathing membranes must demonstrate a minimum level of performance to meet North American building code prescriptive requirements. The minimum requirements aim to ensure an airtight building throughout its lifecycle. However, achieving whole-building airtightness requires proper design, installation, and detailing of the complete air barrier system. The industry appears to be recognizing the importance of whole-building airtightness and is moving toward mandating testing as reflected in the IECC/IBC optional testing for commercial buildings, along with several other high-performance building standards. Designers should be aware of this and the other factors that impact sheathing membranes’ capacity to perform as an air barrier (i.e., elemental degradation, structural integrity, etc.). By understanding what is mandated in different jurisdictions, which properties are tested, which technical resources exist, and what is additionally required to achieve whole-building airtightness, the industry can make smart choices to help create energy-efficient, durable, and comfortable buildings.
Graham Finch, Principal and Senior Building Science Specialist, is a building science engineer who specializes in research and investigation work. His work experience includes a wide range of projects including building enclosure condition assessments, forensic investigations, research studies, energy assessments, building monitoring programs, field review, and testing services for new and existing buildings across North America. He has worked with numerous building product manufacturers on product research and development, performance monitoring, and field testing.
Graham has authored and contributed to many publications, including industry guideline documents related to durable and energy-efficient building enclosures. He is regularly invited by building industry organizations and clients to speak on practical and technical issues related to a broad range of building science topics, and actively presents technical papers and presentations at local and international conferences. Examples of his work can be found on the RDH Building Science Laboratories website.