November 1, 2012
Providing adequate rainwater management and control for air, vapor, and heat flow through the building enclosure is critical to long-term performance and durability. In contemporary wall assemblies with layered materials, the complexity of coordinating these various demands can be overwhelming.
Historical developments of building enclosure design and construction evolved through the centuries from monolithic, load-bearing mass walls through transitional masonry to today’s modern building enclosure systems. Thanks to technological advances over the last several decades, emerging construction materials, methods, and techniques have led to thinner, lighter, and more complex exterior walls consisting of multiple material layers.
Contemporary building enclosures perform a multi-faceted role in managing what is typically referred to as ‘environmental loads.’ In addition to transferring dead and live loads, the enclosure must manage moisture in both liquid and vapor states—transported through air leakage and diffusion—as well as the flow of thermal energy. In design practice, these elements are controlled by using water, air, vapor, and thermal barriers.
Each barrier function may be achieved with a dedicated material, but one can serve more than a single prescribed function. In one example, fiberglass-batt insulation functions as a thermal barrier in resisting the transfer of thermal energy across the wall assembly—thus providing the thermal barrier. Above-grade waterproofing membrane, on the other hand, often serves multiple functions and can be used as drainage plane or vapor barrier and can comprise an integral part of the air barrier system (provided certain physical properties are met).
Selecting the appropriate materials and their arrangement within the wall assembly may not always be a straightforward process. Nevertheless, it is essential in preventing moisture-related performance problems. A case in point being the use of low-permeable interior wall finishes in hotels in southern climates during the 1990s, which led to widespread moisture-related problems in exterior walls.
Another example during the 1990s was the use of substandard exterior sheathing materials and improper detailing and installation of the waterproofing layer. This led to moisture-related failures in exterior insulation and finish systems (EIFS) in coastal regions of Canada and warm coastal climates in the United States. Such failures brought about extreme hardship to the owners, insurance companies, and financial institutions involved.
The examples of past failures emphasize two critical aspects in ensuring properly functioning building enclosures:
Building enclosures consisting of layered materials—such as board stock insulation, rubberized asphalt membrane, or fluid-applied waterproofing/air barrier and exterior-grade sheathing and porous claddings—require a certain level of building science insight to ensure moisture does not condense and accumulate within the assembly.
Specifying waterproofing/air barrier products with an adequate range of water vapor permeance often necessitates use of advanced engineering analysis to verify the assembly’s hygrothermal response. The general rule of thumb is to locate the vapor barrier toward the interior side of the insulation layer in cold climates and toward the exterior side in warm and humid regions.
In moderate climate zones (where the net water vapor transport across the building enclosure is more balanced), a vapor barrier with low permeance may not be needed. Even with design requirements met on paper, proper field installation is critical to ensure the building enclosure performs well for years to come. With multiple trades involved in construction of a multi-layered wall system, the opportunity exists for mistakes and deficient installation.
Using insulated composite metal panels (IMPs) in building enclosures can simplify the design of the barriers and their installation—since all barriers are integrated into a single unitized component with flashings and sealants to provide a continuous waterproofing, air, vapor, and thermal barrier. From the installation standpoint, using IMPs eliminates the need for multiple trades as the systems are erected by a single trade.
Two questions arise in relation to weather protection, as well as controlling the flow of air, water vapor, and thermal energy:
Impact of the codes
Most new commercial buildings must be designed and constructed to comply with regulatory requirements in model codes. From the standpoint of environmental load control, the building enclosure must meet demands related to weather protection, energy efficiency, air leakage, and vapor diffusion control.
Weather protection (more specifically, defense against rainwater penetration) is critical to a properly functioning enclosure. Performance requirements are listed in Section 1403 of the 2012/2009 International Building Code (IBC). The provisions in this section require exterior walls to be designed and constructed to:
The provisions for vapor flow control listed in IBC Section 1405.3 are highlighted in subsequent sections. The code provides exceptions to the requirements for means of drainage with WRBs in Section 1404.2 and flashings in Section 1405.4. Compliance to the exceptions can be met when the exterior wall assembly resists water penetration 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.
Energy performance requirements for new commercial buildings may be found in the model code, such as the International Energy Conservation Code (IECC) referenced in Chapter 13 of IBC. IECC establishes minimum levels for energy efficiency using prescriptive and performance-based provisions.
A project need only comply with one method. The prescriptive approach is the easiest to apply and the most commonly used. The requirements for opaque thermal enclosure components are provided in a tabulated format and listed separately for each component and climate zone in terms of a maximum U-factor or a minimum R-value. The latter becomes applicable in cases when insulation materials are used in the assembly, such as board stock insulation in combination with cavity insulation. In instances when the insulation material is an integral part of a building enclosure component, the use of maximum U-factor criteria for the assembly is the correct approach in representing energy efficiency for the product. In Section C402 of the 2012/2009 IECC, the code is clear—the maximum U-factor can be used in lieu of the minimum R-value criteria:
C402.1.2 U-factor alternative. An assembly with a U-factor, C-factor, or F-factor equal to or less than specified in Table C402.1.2 shall be permitted as an alternative to the R-value in Table C402.2.
The control of unintended airflow is of primary concern within an energy efficiency context. Air leakage typically occurs through any cracks, gaps, discontinuities between adjacent materials, and openings. It can have a negative impact on energy efficiency, as well as the envelope’s long-term moisture performance and durability.
Some consequences stemming from air leakage include degradation of materials resulting from interstitial condensation and moisture accumulation, thermal discomfort, and increased energy use. An understanding of air leakage’s effect on the building’s energy efficiency is growing in the design/construction community. The codes followed suit and, during the last two cycles, IECC expanded its air barrier requirements. The current 2012 code defines the air barrier, along with construction requirements, compliance options, and applicable testing.
Water vapor diffusion control
As indicated, controlling air leakage is critical in achieving an optimal building enclosure. Unlike air leakage, which can transport a greater quantity of moisture, the transfer of water vapor via diffusion occurs through the material surface and across its porous matrix. The property that relates the quantity of water vapor diffusing through the material in question is called ‘permeance.’ In Section 1405, IBC list three types of vapor retarders:
Additional language in this section requires Class I or II vapor retarders on the interior side of frame walls in Zones 5, 6, 7, 8, and Marine 4—states positioned north of the Mason-Dixon Line.
Achieving barriers with IMPs
Insulated composite metal panels are typically manufactured with metal facers on both sides of a closed-cell polyisocyanurate (polyiso) or polyurethane core. The formed facer edges connect the panels structurally and achieve panel-to-panel seals, leading to performance barrier continuity without thermal conductivity through the panel thickness.
All the constituent materials of the IMPs are ‘tight,’ relative to moisture absorption and vapor diffusion. The metal is impervious to vapor diffusion, and the other components—including a closed-cell foam core and non-curing butyl sealant—have a permeance of less than 0.3 perms, as per ASTM E96-95, Standard Test Methods for Water Vapor Transmission of Materials. The moisture absorption of the foam is a 0.3 percent weight change when tested in accordance with ASTM C209-98, Standard Test Methods for Cellulosic Fiber Insulating Board.
When detailed and installed with a seal plane on the liner side, the product houses the air, water, and vapor seal on the liner side, with the thermal barrier to the exterior. In cold climates, the vapor barrier is on the wall’s liner, or heated side, and easily resists the inside-to-outside vapor drive. This liner-side seal detail also functions in hot and humid climates—where the vapor drive is reversed. Should condensation occur, it would take place on the exterior of the vapor barrier (liner) and drain to the outside. Since the foam exposed within the joint is closed-cell, it is resistant to holding water or vapor diffusion. Hence, the same product installation details can be used in all climates without a concern for moisture entrapment within the wall.
IMP assemblies can achieve high performance levels for air infiltration and water penetration when they are tested in accordance with:
Air infiltration levels that are less than 0.15 L/s/m2 at 75 Pa (0.03 cfm/sf at 6.24 psf) can be achieved, as well as passing a static or dynamic water test at 718 Pa (15 psf).
IMP products are typically tested in accordance with ASTM C1363, Standard Test Method for Thermal Performance of Building Materials and Envelope Assemblies by Means of a Hot Box Apparatus, as per the assembly U-factors listed in Section A9.3.2 of American Society of Heating, Refrigerating, and Air-conditioning Engineers (ASHRAE) 90.1-2010, Energy Standard for Buildings Except Low-rise Residential Buildings. This documents the efficiency of the panel-side joint. Subsequent modeling can be done to depict actual job conditions or assemblies as required.
Insulated metal panels can achieve the high performance standards for water, air, vapor, and thermal barriers as required in codes today. Due to the non-absorbing properties of the panel materials, the same details using liner-side seals can be used in any climate, thus facilitating the complexities involved with multi-component wall systems, barrier selection, and location within the assembly.
Keith Boyer, PE, is the director of architectural wall technology for Centria. He is involved with long-term product development for architectural wall products and participates with industry councils and professional organizations. Boyer has been with the company for more than 36 years. Extensively involved in the industry, he has served on several ASTM committees for standing-seam metal roofs, has recently been involved with an American Architectural Manufacturers Association (AAMA) task force to develop testing protocols for rainscreen wall systems, and holds numerous patents on wall and roof systems. Boyer can be reached at firstname.lastname@example.org.
Marcin Pazera, PhD, is a senior research and development associate with Centria. He has worked as a consultant on building science issues related to building enclosure design, performance, and durability. Pazera is a member of the editorial board for the Journal of Building Physics and has authored more than 30 publications in peer-reviewed journals, industry publications, and conference proceedings. He is a member of several committees in ASTM International, and chairs the Western Pennsylvania Building Envelope Council (BEC) Chapter. Pazera can be contacted via e-mail
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