Wind load and air barrier performance levels

by Molly Doyle | June 25, 2014 4:11 pm

[1]

by Maria Spinu, PhD, LEED AP, Ben Meyer, RA, LEED AP, and Andrew Miles
Continuous air barriers have become mandatory for the building envelope, with energy codes recognizing the importance of air leakage control. However, simple inclusion of an air barrier requirement does not guarantee the desired performance in the field. These systems must be properly installed, meet the building envelope structural wind loads, and maintain their function over time.

There are two accepted performance levels for commercial air barrier systems, determined by the structural design parameters for the building envelope:

• ASTM E1677, Standard Specification for Air Barrier Material or System for Low-rise Framed Building Walls, applicable to envelope design specifications of up to 105-km/h (65-mph) equivalent structural loads; and
• ASTM 2357, Standard Test Method for Determining Air Leakage of Air Barrier Assemblies, for buildings designed to withstand structural loads beyond that level.

This article describes the air barrier performance requirements for the desired wind load design specifications. The performance level is not determined by the type of air barrier material, but by the installation details. Examples of how these details can impact the performance level for a given air barrier system will be provided, with special emphasis on mechanically fastened air barriers.

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Air leakage control and air barrier materials
Air leakage control is achieved through a continuous air barrier. Any material with an air permeance less than 0.02 L/(s • m²) @ 75 Pa pressure differential (0.004 cfm/sf @ 0.3 in. w.c. or 1.56 psf pressure differential), when tested in accordance with ASTM E 2178, Standard Test Method for Air Permeance of Building Materials, qualifies as an air barrier. Even though many common building products are air barrier materials (e.g. metal sheets, glass, oriented strandboard [OSB], and gypsum board), a continuous air barrier requires many compatible components to achieve a plane of airtightness. In practice, most air barrier materials are specifically designed membranes effectively integrated into a continuous air barrier system.

[3]Common air barrier materials include mechanically fastened (i.e. building wraps), fluid-applied, and self-adhered membranes. The choice depends on many factors, such as the substrate, desired performance level, installed cost, personal preference, local practices, and regional availability.

For example, in framed construction where air barriers are applied over exterior sheathing, building wraps are the most cost-effective. For masonry or concrete backup walls, fluid-applied membranes are the common choice. Self-adhered membranes can be used with either substrate, but most are vapor-impermeable and their use should be limited to specific climates and wall design options.

In the case of vapor-impermeable air barriers, the membrane plays a dual role: air and vapor barrier. While air barriers could be installed anywhere in the building envelope, vapor barrier location and use is climate- and design-specific. For example, vapor barriers are required only in cold climates, and must be installed at the ‘warm in winter’ side of the envelope. In warm-humid climates, a vapor barrier could still be acceptable to the outside of the envelope (where the air barrier is generally installed) when design options for drying pathways are available—such as when vapor-permeable materials must be used at least in one direction (in this example, everything to the inboard from vapor barrier must be vapor-permeable). Exulation wall design (i.e. exterior insulation only, no insulation in the stud cavity) can also use vapor-impermeable air barriers. Building physics must always be considered when an unintended vapor barrier is used in a wall assembly.

There are four essential performance requirements for air barriers:

• air infiltration resistance;
• continuity;
• structural integrity; and
• durability.¹[4]

Another critical property is vapor permeability, which could impact moisture management in wall assemblies. However, the codes do not specify the air barriers’ vapor permeance—the decision is left to the building envelope designer.²[5]

[6]Air infiltration resistance is an inherent material property for air barrier materials. Other requirements depend not only on material properties, but also on the performance of the installed system determined by the integration of air barrier components into a continuous system, as well as the durability under use conditions. In addition to the primary air barrier membrane, an air barrier system includes installation and continuity accessories, such as primers, mechanical fasteners, seam tapes, flashing, adhesives, and sealants.

This article mainly focuses on structural integrity requirement, which is the ability of an air barrier system to withstand wind loads experienced during the building’s use after construction is complete. There are two accepted performance levels based on building envelope design parameters with regard to wind loads and wind-driven rain. To establish the performance level of an installed air barrier system, air barrier wall assemblies must be tested in accordance with the respective ASTM standards.

Installed air barrier performance and wall assembly testing
Testing is essential for demonstrating performance of installed air barrier assemblies. This process is critical for developing robust installation guidelines for achieving air barrier performance levels consistent with structural design specifications.

As mentioned, ASTM E1677-11 applies to air barrier performance levels for building envelope design requiring up to 105-km/h (65-mph) equivalent structural loads, and up to 24-km/h (15-mph) equivalent wind-driven rain water infiltration resistance. This level is generally adequate for buildings of up to four or five stories, but higher performance is typically required on some low-rise structures like medical facilities and military buildings. ASTM E2357-11, on the other hand, applies to air barrier performance levels for building envelope design structural loads beyond this—such a performance level is generally necessary for buildings taller than five stories.

[7]Both test methods are performed on 2.4 x 2.4-m (8 x 8-ft) wall assemblies. ASTM E1677 requires testing of a single, opaque wall assembly (i.e. no penetrations except for the fasteners), while ASTM E2357 involves two specimen—an opaque wall and a penetrated wall that includes standard penetrations such as window openings, external junction boxes, and galvanized duct.

Both test methods require pressurization and depressurization testing, but use different pressure loads and schedules. The major differences between the two test methods are summarized in Figure 1, and consist of the pressure loads, schedule, and requirement for water infiltration resistance testing.

As shown, ASTM E1677 requires five test pressures:

• ± 75-Pa pressure differential (1.56 psf, 25 mph);
• two pressures below 75 Pa; and
• two pressures above 75 Pa.

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The pressure loading schedule includes sustained loads of up to ±500 Pa (10.4 psf, 65 mph). This standard requires testing for water infiltration resistance per ASTM E331, Standard Test Method for Water Penetration of Exterior Windows, Skylights, Doors, and Curtain Walls by Uniform Static Air Pressure Difference. Air barriers or air retarders (as they are referred to in ASTM E 331) are classified as either Type I or Type II. Type I air barriers, which can also perform as water-resistive barriers (WRBs), must exhibit no water penetration when tested at 27 Pa (11 in. water pressure difference)—equivalent wind speed of approximately 24 km/h (15 mph)—during a 15-minute test period. Type II air barriers are not required to be tested in accordance with ASTM E 331.

ASTM E2357 requires a minimum of seven test pressures, from ±25 Pa (0.56 psf, 15 mph) to ±300 Pa (6.24 psf, 50 mph). The pressure loading schedule includes sustained, cyclic, and gust winds up to ±160-km/h (100-mph) equivalent wind speed. This standard does not require ASTM E331 testing for water infiltration resistance, which is a significant limitation since many air barriers are commonly required to also perform the WRB function and are exposed to pressure loads above 105-km/h (65-mph) wind.³[9]

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Air leakage results are reported at 75 Pa for both methods. Current codes require the average air leakage rate for air barrier assemblies must not exceed 0.2 L/(s•m²) @ 75 Pa pressure differential (0.04 cfm/sf under a pressure differential of 0.3 in. w.g. or 1.57 psf) when tested in accordance with ASTM E2357 or ASTM E1677.

Since a continuous air barrier experiences both positive and negative pressures during its use, it is important assemblies be tested under both positive and negative pressures. The negative load (under suction) is typically the most severe, as it tries to pull the air barrier off the wall. Different air barrier types have different susceptibility to negative pressure loads.⁴[11]

For fluid-applied air barriers, wind loads are transferred to the substrate underneath. When the substrate is masonry or concrete, a fully adhered fluid-applied air barrier has excellent structural performance under suction, as the pressure it typically takes to separate it from the substrate far exceeds the actual pressure it must withstand.

However, for framed wall construction, the structural performance of fully adhered fluid-applied air barriers under negative wind loads depends on how well the sheathing is fastened to the building structure. When the exterior sheathing is not installed to withstand the design wind loads, this could reduce the air barrier system’s structural performance. In this case, the typical mode of failure for fluid-applied air barrier is the sheathing pulling over the screws.

[12]In comparison, when building wraps are installed over exterior sheathing, the air barrier membrane is supporting the entire load. Consequently, this type of air barrier is more susceptible to wind. The suction forces are transferred through the air barrier membrane to the mechanical fasteners, and then back to the structural supports (i.e. steel or wood studs). As a result, for a mechanically fastened air barrier, the wind load performance is determined by the type of fasteners and the fastener schedule.

The photos in Figure 2 show an example of high-pressure performance testing of commercial building wraps and exemplify the extreme forces experienced by the air barrier wall assemblies under negative pressure loads. The steel studs actually buckle under the pressure differentials used for high performance testing of building wraps (left), but a properly fastened building wrap withstands this pressure and maintains the system’s structural integrity (right).⁵[13]

These pictures demonstrate the importance of proper fastening of building wraps to withstand high suction loads and maintain the air barrier structural integrity during use. A common mistake with building wraps installation is use of staples for fastening the building wrap into the exterior sheathing (a practice often employed for WRBs in residential construction), rather than employing recommended screws with washers to fasten the membrane into the structural members (wood or steel studs).

Building wrap manufacturers usually provide guidelines on the type of fasteners and the fastening schedule recommended for meeting the desired performance level. Figure 3 provides an example of fastening type and schedule guidelines and the maximum wind loads allowable.

Alternate fasteners are also allowed, when applicable. Examples include standard brick tie base plates and metal plates, metal channels, horizontal z-girts, and wood furring strips mounted vertically. They can be used in conjunction with the manufacturer-recommended fasteners to meet and/or satisfy the desired design performance.

In addition to fastener selection and spacing, other installation details are critical when designing for a specific performance level. Some building wrap manufacturers provide different installation details for ASTM E1677 and ASTM E2357. These include details on sealing of penetrations, transitions, and interfaces. For example, no additional fastener sealing is necessary for building envelope design requiring up to 105-km/h (65-mph) equivalent structural loads (i.e. ASTM E1677), when recommended fasteners and schedules are used. However, if higher air infiltration resistance is desired (i.e. ASTM E2357), self-adhered flashing must be used under the fasteners.

Figure 4 shows examples of alternate fasteners, as well as the use of self-adhered flashing under the fasteners for ASTM E2357 performance level. The same recommendations are also for fluid-applied membrane fasteners.

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Among the most critical details determining the air barrier structural performance level are windows and doors integration into the continuous system. Most manufacturers provide step-by-step window installation guidelines. Changes in the provider’s detailing and sequencing could change the performance level (i.e. ASTM E1677 or ASTM E2357). Figures 5 and 6 show examples of specific details for achieving the desired wind load design specifications.

The detail in Figure 5 shows how the window rough openings are treated with self-adhered flashing, for the high-performance level required by ASTM E2357. For example, when the building has non-flanged, storefront, and/or curtain wall windows, the air barrier membrane is typically cut flush with the edge or the rough opening. Then, the self-adhered flashing is installed to protect the rough opening and provide a positive termination of the air barrier membrane. The pictures on the right show examples of high-performance flashing for non-flanged and/or curtain wall windows that may be bumped out from the wall plane.

Figure 6 shows an example of window flashing for ASTM E1677 performance level. The picture captures the alternate head detail, which is generally allowed for building structures with building envelope design requirements not exceeding ASTM E1677. After the air and water barrier is wrapped into the window rough opening, a top hat is created with sealant to divert water away from the window opening (if the air barrier is also intended to serve as the WRB). WRB cut pieces are then installed (I) by wrapping in and around the studs at the jamb and the head, and stapling to inside framing to secure (A). The next steps include (II): (A) apply a continuous sealant bead along jambs and head, (B) install flanged window, (C) install jamb flashing, and (D) install head flashing.

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The recommended installation guidelines are based on many wall assembly tests, and changing the installation details in the field could affect the performance level for the installed air barrier assembly. Engaging the air barrier manufacturers in early design stages is critical to understanding the installation details requirement and the optimal installation sequence to achieve the desired performance level. Additionally, it helps avoid unnecessary delays during the construction phase.

The difference between the two performance levels for air barriers is not always understood by industry professionals and installers, and not clearly stated by codes. For example, American Society of Heating, Refrigerating, and Air-conditioning Engineers (ASHRAE) 90.1-2013, Energy Standard for Buildings Except Low-rise Residential Buildings, Section 5.4.3.1.3.b, defines code-compliant air barrier assemblies as:

Assemblies of materials and components (sealants, tapes, etc.) that have an average air leakage not to exceed 0.04 cfm/sf under a pressure differential of 0.3 in. w.g. (1.57 psf) when tested in accordance with ASTME 2357, ASTM E1677.

As evident from this article, performance levels for ASTM E1677 and ASTM E2357 are not equivalent—nevertheless, ASHRAE 90.1-2013 provides the two options as equals. It is little wonder there is confusion in the industry, and the potential impact of changes to the manufacturer’s installation guidelines is not always appreciated.

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Fortunately, Air Barrier Association of America (ABAA) recently introduced an evaluation process for air barriers, in order to apply consistent standards across the industry. On its website, the group lists the air barriers that have been evaluated and demonstrated to meet ASTM E2357 performance level. (For more, see “ABAA-evaluated Air Barrier Assemblies[17].”)

Current limitations of air barrier standards
Code requirements on air leakage control have led to a large increase in the number of materials claiming to perform as air barriers. The challenge with some airtight materials is in achieving a continuous and durable air barrier system. For example, many materials designed to perform other functions (e.g. thermal insulation or exterior sheathing) that are also resistant to air infiltration have been promoted as air barriers. While such products are adequate air barrier ‘materials,’ the long term continuity and durability of these air barriers as ‘systems’ is still an open question.

Some such materials have passed the current air infiltration resistance requirements for ‘as-installed’ air barrier assemblies per ASTM E2357 and are listed at the ABAA website. However, these systems may fall short of long-term durability under the use conditions. Additional testing, such as thermal cycling and water resistance, would be necessary to assess the long-term durability of these systems. This discussion is beyond the scope of this paper, but it should be of concern to the industry.

Conclusion
Building energy codes mandate a continuous air barrier for leakage control. The air barrier system must withstand the conditions a building is exposed to during its use. There are two acceptable performance levels for air barrier wall assemblies—ASTM E1677 and ASTM E2357—that are determined by the structural design parameters for the building envelope. Some air barrier manufacturers have developed two-tier installation guidelines for the desired level, and altering the guidelines could change the performance.

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A major limitation of ASTM E2357 is the lack of water infiltration resistance requirement. Very few manufacturers integrate testing for ASTM E2357 air infiltration resistance with ASTM E331 water infiltration resistance of installed wall assemblies.

Current test methods are effective in measuring performance of newly installed air barrier assemblies under pressure differentials experienced by above-grade exterior walls and represent a huge step forward from relying solely on materials properties. However, current standards do not provide information about the long-term performance under field use conditions experienced by the buildings, which include seasonal and daily temperature variations.

The air barrier system performance is only as good as the weakest link, and differential expansion and contraction of multicomponent air barrier systems can compromise its continuity. Integration of rigorous structural integrity testing of air barrier wall assemblies with thermal cycling and water infiltration resistance will provide valuable information on the long-term durability of these systems.

Notes
¹ These requirements have been described in various articles, including 2006’s “Air Barriers: Walls Meet Roofs,” by Wagdy Anis and William Waterston (www.shepleybulfinch.com/pdf/Air_Barriers_wall_meets_roof_final.pdf[19]) and 2004’s “Air Barriers, Research Report,” by Joseph Lstiburek (www.buildingscience.com/documents/reports/rr-0403-air-barriers[20]). Additional references can be found at the Air Barrier Association of America (ABAA) web site at www.airbarrier.org[21]. (back to top[22])
² The impact of air barriers’ vapor permeance on moisture management has been discussed by co-author Spinu in two other articles that were published in The Construction Specifier: April 2007’s “To Be or Not to Be Vapor-Permeable[23],” and November 2012’s “Designing without Compromise: Balancing Durability and Energy Efficiency in Buildings[24].” (back to top[25])
³ The wind loads and schedule considered in these tests have been developed by the ASTM standard committee. While the authors are not part of this committee, it is possible one of the reasons for developing multiple pressure loads is to extrapolate the data at low pressures through linear regression. At low pressure loads, the errors are larger than at high pressures, so it is important to have multiple data points. (back to top[26])
⁴ The standards assume the air barrier plane will take the full wind loads (even though this would only be true for pressure-equalized façades). (back to top[27])
⁵ The air barrier structural loading is based on the assumption the air barrier plane: (1) takes the full wind loads (even though this only occurs for pressure-equalized façade systems), (2) experiences thousands of cycles of high positive and negative pressure loads during its service life, and (3) experiences two severe storms in the first 15 years of service. The steel studs shown buckling in the picture are at the very high end of the pressure loads. The point is when proper fasteners and spacing are used, air barriers can perform under wind gust conditions. These tests are quite stringent, but air barriers must perform for the life of the building envelope and such conditions could be occasionally experienced. (back to top[28])

Maria Spinu, PhD, LEED AP, is a building scientist with DuPont Building Knowledge Center, where she has led global building science and sustainability initiatives for the commercial market for the past decade. She is the author of 16 patents and has been a speaker at many regional, national, and international conferences on building science and sustainability topics. Spinu can be contacted via e-mail at maria.spinu-1@dupont.com[29].

Benjamin Meyer, RA, LEED AP, is a building science architect with DuPont Building Knowledge Center, where he works with customers and industry associations to answer questions on commercial building envelope design. Meyer is on the board of the Air Barrier Association of America (ABAA), a member of the Materials and Resources Technical Advisory Group of LEED, and also a consultant of the ASHRAE 90.1 Envelope Subcommittee. He can be reached benjamin.meyer@dupont.com[30].

Andrew Miles is a forensics engineer, providing technical support as part of the DuPont Tyvek Specialist network. His responsibilities include mock wall testing and field investigations related to use, performance, and customer concerns. Miles can be e-mailed at andrew.s.miles@dupont.com[31].

Source URL: https://www.constructionspecifier.com/wind-load-and-air-barrier-performance-levels/

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