Field mockups for exterior wall systems: Will the design pass the test?

August 10, 2018

All images courtesy Simpson Gumpertz & Heger[1]
All images courtesy Simpson Gumpertz & Heger

by Daniel Clark and Eric Olson, PE
What are the risks involved with enclosure design and how can owners and designers help mitigate them? How does one verify compliance with specified performance requirements, especially for critical elements intended to protect the building from the outdoor environment? Is the design constructible? How can owners, designers, and contractors know they have the correct materials, a functioning assembly, and proper workmanship? When is testing appropriate and cost effective? When should tests be conducted? What is the difference between mockup and compliance tests? The enclosure performance mockup process is a fundamental part of answering these questions.

Mockups can be constructed offsite, onsite, or in-situ, and depending on the scope, specifiers may include more than one of these options. Traditionally, large-scale, offsite mockups are used in new designs when employing systems not previously tested or when combining multiple components together to form an assembly. Onsite mockups are typically smaller, centered around visual approvals and verifying constructability, which, in turn, will be used as a benchmark through the construction phase. Performance testing can be incorporated into an onsite mockup, though traditionally, it is limited to compliance testing. In-situ mockups are similar to onsite with the benefit of being incorporated into the permanent construction. However, in-situ mockups are primarily useful for trade coordination, as major design changes cannot be incorporated at this stage. This article focuses on considerations for testing related to a performance mockup.

A simple concept, a performance mockup is essentially a prototype of the building façade design—a small section of the façade, constructed in a laboratory, as a freestanding wall section on the jobsite, or on the building itself, and subjected to inspection and testing to verify constructability and important performance characteristics—such as structural performance, air and water intrusion resistance, material adhesion and compatibility—and construction detailing and conflict avoidance. Construction of the mockup should be collaborative, with the designer, contractor, manufacturers, and trades working together to arrive at an optimal and functional solution. Once completed, the mockup serves to establish the technical, quality, and aesthetic standards for the project.

Specifying a mockup and suitable testing program can be confusing, with numerous available tests and approaches. The American Architectural Manufacturers Association (AAMA) 501, Methods of Test for Exterior Wall, is the standard guide for most performance mockup testing. Designers, contractors, and owners are not always clear on (or often cannot agree on) what testing to include, why it is applicable, the appropriate performance criteria, or whether the testing is worth the cost and schedule impacts. As a result, testing programs may be misguided or fall short of expectations, causing frustration and schedule delays.

Figure 1: Stand-alone wall assembly mockup prior to cladding installation.[2]
Figure 1: Stand-alone wall assembly mockup prior to cladding installation.

Preconstruction considerations
Mockups are an essential part of any construction project, whether a complicated innovative design utilizing new materials or a simple renovation replacing materials in-kind. Even with today’s photorealistic renderings generated through building information modeling (BIM), people still want to touch, feel, and see the real thing. However, and more importantly, BIM or computer-aided design (CAD) detailing cannot resolve all of the field-dependent variables discussed above. Constructing and testing a mockup represents the last opportunity to refine the design approach before the actual fabrication of components begins and before any unresolved or unknown issues are constructed into the building.

However, while the value of the mockup process cannot be overstated, it comes with cost and schedule impacts. This means getting the most value out of a mockup is critical. Performance parameters should be further verified during the course of the project with a program of field testing, which is outside the scope of this article.

When incorporating mockups into a project, the design documents and specifications should outline the intent and requirements for the mockup. Historically, aesthetic mockups have been utilized early in the construction process for architects and owners or authorities having jurisdiction (AHJ) to approve design appearance, finish materials, and color palette. However, a project team will also include performance testing of the façade mockup components and assemblies if it is determined to be necessary. Many believe projects do not require performance mockups when the systems have previously been tested. However, even if the individual systems have been tested, a mockup may still be beneficial to establish the methods of integration between these systems and to establish the finished quality and performance of these integrations, as this is where water leakage and performance problems are frequently encountered. It is critical for team members to communicate and understand the intent of the mockup, whether aesthetic, performance, or both, early in the process to avoid differing expectations.

The designer or specifier should include requirements for the mockup, defining all tests, performance requirements, testing conditions, and qualifications for the testing agency and installers, as well as the pass/fail criteria of individual components or the assembly. Tests and pass/fail criteria may be based on previous laboratory certification testing for the various systems, building code requirements, and special project considerations such as history of severe weather at the site. Where more than one system is involved, detailing of system-specific testing at individual areas of the mockup may be warranted. The specifier should also identify the parties responsible for paying for tests and retests in case of failure. Finally, methods of resolving defects in failed assemblies may be specified if they can be anticipated.

The complexity of the enclosure design is an important consideration when developing and specifying the mockup process, as it correlates to the level of risk inherent in the design. For example, a simple rectangular building clad with standard systems and components should present minimal risk and may require less extensive field tests during installation. Similarly, the risk tolerance of the end user is an important factor. A storage warehouse may be less affected by leakage than a hospital. As a designer, it is critical to understand how to balance risk mitigation and cost when implementing performance mockup criteria.

Costs for a mockup, including design, construction, and testing costs, can vary greatly depending on the size, complexity, and testing requirements. A simple single-story metal stud, veneer construction mockup with a single water infiltration test built at the jobsite could cost as little as $25,000, while a multistory mockup in a laboratory, with multiple systems and transitions undergoing several performance tests, may cost $250,000 or more and take several months to plan, construct, and test. Both cost and time are major factors to be considered, especially in today’s fast-track construction environment. However, the mockup tests can reveal system weaknesses that can be costly and affect project performance and the schedule if not caught early, far outweighing the initial cost. Therefore, mockup construction and testing can provide assurance a custom-designed system will match the performance of other standard systems with established track records of success.

Constructing the mockup
Enclosure performance mockup testing can include structural loading, thermal cycling, and condensation resistance, but the most common tests the authors have encountered are air and water infiltration testing of fenestration and air infiltration testing of air barrier assemblies.

Figure 2: Secondary sealant joint cut open at head of curtain wall mockup to test primary seal and drainage capacity.[3]
Figure 2: Secondary sealant joint cut open at head of curtain wall mockup to test primary seal and drainage capacity.

Air infiltration testing of fenestration products is conducted in accordance with ASTM E283, Standard Test Method for Determining Rate of Air Leakage Through Exterior Windows, Curtain Walls, and Doors Under Specified Pressure Differences Across the Specimen, or ASTM E783, Standard Test Method for Field Measurement of Air Leakage Through Installed Exterior Windows and Doors, depending on whether testing is performed in the laboratory or field.

Water infiltration testing of fenestration products is conducted in accordance with ASTM E331, Standard Test Method for Water Penetration of Exterior Windows, Skylights, Doors, and Curtain Walls by Uniform Static Air Pressure Difference, or ASTM E1105, Standard Test Method for Field Determination of Water Penetration of Installed Exterior Windows, Skylights, Doors, and Curtain Walls by Uniform or Cyclic Static Air Pressure Difference, depending on the laboratory or field conditions respectively. While all of the above-mentioned tests were developed for test of fenestrations, they are also used for opaque walls today.

For both air and water infiltration testing it is important the specifier not only include the applicable ASTM test, but also the appropriate performance and procedure requirements, and whether to test surrounding flashing conditions. A common mistake is to simply reference the product performance requirements for field testing. This can cause confusion between the specifier, manufacturer, installer, and testing agency, as both air and water infiltration laboratory testing criteria are modified to be less stringent for use in the field in accordance with AAMA 502, Voluntary Specification for Field Testing of Newly Installed Fenestration Products. For air infiltration the test pressure is the same between lab and field conditions, but AAMA 502 allows a 50 percent increase in air leakage rates from lab to field performance. Water infiltration tests conducted in the field use a one-third reduction in test differential pressure from the lab-applied pressure in accordance with AAMA 502. Both changes from lab to field conditions are required within AAMA 502 unless the specifier states otherwise. In the absence of either option being clearly defined within the specifications, the agency will typically default to the less stringent testing requirements, which include the reduced test pressure and not testing surrounding flashing conditions.

Additionally, there are two different test types for water penetration resistance. ASTM 1105 provides two test methods; procedure A is a uniform pressure test while procedure B is a cyclical test. A uniform test applies to AW class windows and cyclical tests apply to R, LC, and CW performance classes as defined by AAMA 101, Voluntary Performance Specification for Windows, Skylights, and Glass Doors. A uniform test is a one-test cycle, consisting of water spray being applied continuously for 15 minutes with the specified pressure. Cyclic tests include applications of air pressures, with each cycle consisting of constant pressure for five minutes followed by zero pressure for one minute, and then repeated four times, during which the water spray is continuously applied.

Air infiltration testing of air barrier assemblies is conducted in accordance with ASTM E2357, Standard Test Method for Determining Air Leakage of Air Barrier Assemblies, and includes testing two wall assemblies: an opaque wall and a wall with various penetrations. The opaque wall assembly is a simple stud frame wall panel, 2286 mm (90 in.) square, which includes fasteners through the sheathing, panel joints, and treatment of the joints according to the air barrier manufacturer’s instructions. The penetrations assembly includes junction boxes, pipe and duct penetrations, masonry ties, and a window in addition to the opaque panel requirements (Figure 1). Air testing of the two wall assemblies is then conducted in accordance with ASTM E283. It is important to note ASTM E2357 and ASTM E283 are both laboratory test standards not intended to be used in the field. However, in some instances, this testing has been attempted onsite. One of the major challenges in performing this test in the field is the difficulty of constructing a sufficiently airtight test chamber to exclude extraneous air leakage during the test, which may affect its validity. Therefore, this test may not be practical outside of a laboratory setting.

Figure 3: Modified American Architectural Manufacturers Association (AAMA) nozzle test at curtain wall mockup.[4]
Figure 3: Modified American Architectural Manufacturers Association (AAMA) nozzle test at curtain wall mockup.

Case studies
The following two cases studies illustrate the benefits of mockup testing and lessons learned.The first case study includes laboratory, field, and in-situ mockups for an elaborate custom faceted curtain wall system, and illustrates a mockup’s usefulness in resolving performance issues before they become manifest within the construction. The curtain wall system consisted of structural steel mullions, which supported aluminum carrier and cassette framing onto which the insulated glass unit (IGU) was structurally glazed. The system included field-applied primary and secondary weather seals. Given the complexity and nonstandard use of the curtain wall system, the authors determined a full-scale laboratory mockup would be required in addition to a visual field mockup.

The laboratory performance testing included the standard testing protocol outlined in AAMA 501. The authors also incorporated AAMA 501.2, Quality Assurance and Diagnostic Water Leakage Field Check of Installed Storefronts, Curtain Walls and Sloped Glazing Systems, which involves a calibrated spray nozzle, for testing the primary and secondary weather seals after both are installed for water infiltration resistance. Additionally, the project team incorporated a modified AAMA nozzle test to evaluate the effectiveness of the drainage plane between the primary and secondary seals. The AAMA nozzle test was conducted at a modified water pressure; water was applied to the openings made in the primary weather seal to confirm adequate drainage from the weeps within the system.

During nozzle testing, holes were cut in the secondary weather seal at the head of the mockup (Figure 2) and the nozzle was used at a low pressure 34 kPa (5 psi) and volume to test the drainage plane between the sealant joints (Figure 3). Within minutes, water leakage was observed on the interior side of the curtain wall mullions. However, water was not draining from the weeps. Portions of the primary seals were then removed to discover primary and secondary seals had bonded together at certain locations, thus preventing drainage (Figure 4). It was determined the leakage was a result of tooling marks during the application of the secondary weather seal around the cassettes and glazing chairs (Figure 5).

Given the laboratory mockup was offsite at a testing facility, the project team elected to use the onsite visual mockup (Figure 6) as an opportunity to correct the observed defects in the sealant installation. The visual mockup did not have either the primary or secondary seals installed previously, which allowed for further review of the sealant detailing around the cassettes and chairs as these seals were installed. The project team modified the backer rod sizes to better accommodate the required spacing needed to adequately install the sealant while allowing drainage. Testing was then performed in two phases. First, the AAMA nozzle testing of the primary seal without the secondary seal installed (Figure 7) was performed, and then the AAMA nozzle test was repeated with the secondary seal installed. The modified AAMA nozzle test to verify drainage capability was also conducted.

The nozzle testing allowed the design team to evaluate all of the sealant joints installed within the curtain wall assembly, which would have been cost prohibitive with a chamber test. Without the modified AAMA nozzle testing procedure, the project team would likely not have identified the potential water leakage problems that could have occurred after the building was occupied and in service. This was a fitting example of the entire team working together to ensure a successful outcome.

The second case study is an example of why it is critical to engage all parties involved in the mockup as early as possible to establish requirements for mockup construction, including elements for supporting it. The mockup was a combination of a glass/metal curtain wall and fiber cement panel rainscreen wall system. It consisted of a full-scale, two-story laboratory mockup. The performance testing included structural performance, static air and water infiltration, and deflection and dynamic water testing. The structural testing was conducted in two phases: an initial 100 percent design load test with subsequent air and water infiltration tests, followed by a second structural test at 150 percent of design load (Figure 8). During the second structural test the authors observed a failure in the ad-hoc stud wall supporting the mockup because the stud framing installed had not been designed for the applied structural load. In this case, the mockup was constructed and tested before a framing subcontractor was hired for the project and before the wall framing was engineered. Fortunately, the observed failure occurred at the end of the testing protocol after all of the other testing was successfully conducted. The project team could design the framing system and incorporate the changes into the final construction documents.

The integrity of a building enclosure is dependent on both the performance of its individual components and systems, and the performance of the transitions between these systems.

Mockups are useful to help verify the enclosure design is constructible, integrates properly, and meets required performance expectations. When properly used, mockup construction and performance testing allow the project team to collaboratively work out detailing and compatibility issues arising during the course of construction, verify proper function of the intended construction, and apply the results from the process to the project, thereby avoiding costly, repeated defects in the completed building.

Daniel Clark joined Simpson Gumpertz & Heger Inc.’s building technology group in 2009. He has worked on multiple projects involving investigation, survey, rehabilitation of historic buildings, peer reviews, and design of building enclosure systems, and construction administration services for repairs or new construction of building enclosures and roofing/waterproofing systems. Clark can be reached at[10].

Eric Olson, PE, is an associate principal in Simpson Gumpertz & Heger Inc.’s building technology group. He has 20 years of experience in the investigation and repair design of building enclosure systems, including windows, curtain walls, masonry, exterior insulation finish system (EIFS), and stucco, roofing, and plaza and below-grade waterproofing. Olson can be reached via e-mail at[11].

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