May 3, 2016
by Sarah K. Flock, CDT, AIA and Carole Ceja, NCARB, RRC
Many designers and specifiers understand controlling air, vapor, and thermal transfer helps mitigate moisture problems within the building envelope. Moisture accumulation is a performance adversary that can lead to structural deterioration, finish damage, organic growth, and reduced building longevity (Figure 1). However, navigating fundamentals, code requirements, and industry trends related to these transfer mechanisms can be complex.
The 2015 International Codes (I-Codes) were recently released; as of this writing, six states have adopted the 2015 International Energy Conservation Code (IECC) and International Building Code (IBC), with many more expected to join in the months to come. Even when designs meeting current codes are achieved, moisture issues can result. This article explores the impact of recent code changes, highlights various provisions’ potential limitations, and presents examples of ‘gaps’ between codes and real-world performance as they relate to the topics of air, vapor, and thermal control.
To incorporate the concept of thermal control in design, the fundamentals of heat transfer must be understood. Heat can transfer in various ways, such as conduction, convection, or radiation; it moves from areas of high to low temperatures independent of orientation.
Conduction is the most familiar mode and is the flow of heat through solid materials, such as window frames or metal studs. The amount of heat transfer via conduction depends on the material’s conductivity, mass, and configuration.
Convection is the transfer of heat through a gas or liquid, such as air, and can occur either naturally or by force. Natural convection occurs in the wake of differing densities, exemplified in the old adage of hot air rising. Forced convection is based on similar principles, but may deal with increased rates of air movement generated by forces from outside (e.g. wind) or inside (e.g. HVAC systems).
Thermal radiation is the movement of heat energy from a warm object to a cooler item through space. A common example involves an occupant standing in front of a window and experiencing cold during the winter. In this instance, one’s body is radiating heat to the cooler window surface.
The importance of limiting heat transfer is not only to promote energy efficiency and occupant comfort, but also to reduce the potential for condensation within building assemblies. When moist air comes into contact with nonporous surfaces below the dewpoint, water can condense and create frost or water droplets (Figure 2).
Insulating materials are categorized by their resistance to heat flow, otherwise known as R-value. The higher the R-value, the greater the resistance to heat transfer. Another measure associated with heat transfer is the U-factor, typically associated with opaque assemblies or fenestration products. The smaller the U-factor, the better the product limits heat transfer.
The new codes include major changes regarding air transfer—not only because of a desire for increased energy efficiency, but also because air movement is often a large contributor to moisture migration.
Air has a natural tendency to move from high to low pressure (Figure 3). Sources of pressure differentials at a building envelope are diverse. For example:
Air carries moisture in the form of vapor, which can then encounter surfaces below the dewpoint and cause condensation. For air transfer to occur, there must be a path or a hole between two areas of differing pressure—either exterior to interior, or between varying interior conditions.
Air permeance is defined as the rate of airflow through a unit area of material under a given pressure difference. The performance of the air barrier materials is measured in L/(s•m2) (cfm/sf) of air leakage. Air barrier systems require complete continuity as any gap, hole, or crack allows for air transfer.
While often considered a less impactful transfer mechanism due to the quantity of moisture moved, vapor transfer should not be ignored. Small amounts of moisture in the form of water vapor can pass directly through the exterior enclosure materials by a process called diffusion. The amount of vapor diffusion occurring in a building is partly determined by the force that pushes it, commonly known as the vapor pressure differential, as well as the material’s vapor permeance. The lower the vapor permeance (measured in perms), the less diffusion occurs through the material.
Similar to air transfer, vapor migration also moves from high to low vapor pressure. High vapor pressure can be caused by numerous things, including an increase in relative temperature or mechanical pressurization. Most materials cannot eliminate vapor diffusion, but there are options to significantly slow the process.
Codes and design
With respect to air, vapor, and thermal control for the exterior enclosure, many changes and updates have been incorporated into not only the 2015 I-Codes, but also the 2013 American Society of Heating, Refrigeration, and Air-conditioning Engineers (ASHRAE) 90.1, Energy Standard for Buildings Except Low-rise Residential Buildings. These standards and codes contain both prescriptive and performance paths for compliance. This article primarily focuses on the updates and additions associated with the latest prescriptive requirements, which can be more universally implemented. This discussion provides an overview of provisions typically referenced for enclosure design, but not all changes are included—one should also consult the codes for any changes potentially affecting the specific design.
Updated thermal control provisions are provided in the 2015 IECC and 2013 ASHRAE 90.1. Each document requires all envelope surfaces, both opaque and fenestration products, meet specific thermal values based on location and structure composition. Compliance for opaque thermal envelopes is shown prescriptively through R-values for insulation products or U-factors for assemblies, which were made more stringent in some instances in the recent code updates.
Some U-factors and Solar Heat Gain Coefficients (SHGC) for fenestration products were also tightened in recent editions. A few other notable changes in the 2015 IECC for the opaque wall thermal requirements include a method to determine the effective R-value for steel stud wall assemblies, as well as new criteria for mass walls.
While not new to the recent code updates, it is important to note a few limitations to R-value and U-factor testing. Opaque walls can be assigned R-values derived in accordance with ASTM C518, Standard Test Method for Steady-state Thermal Transmission Properties by Means of the Heat Flow Meter Apparatus, or U-factors determined through ASTM C1363, Standard Test Method for Thermal Performance of Building Materials and Envelope Assemblies by Means of Hot-box Apparatus.
While both test methods involve measuring thermal transfer between cold and hot planes, ASTM C518 measures transfer through materials, whereas ASTM C1363 measures transfer through the assembly, including components that may produce thermal bridging. However, there might be deviations between test conditions and proposed built details, which the project team may need
For example, whether for opaque walls or for fenestration products, test methods are conducted with a prescribed set of boundary conditions—such as −18 C (0 F) exterior and 21 C (70 F) interior—that may or may not align with the conditions experienced by a given building. The project team must understand the test conditions to evaluate the results’ applicability to individual designs.
Both ASHRAE 90.1 and 2015 IECC have similar air control provisions calling for a continuous air barrier throughout the building envelope. The 2015 IECC also stipulates the air barrier can be placed on the inside or the outside, as further explored later in this article.
Beyond the presence of a continuous air barrier, there are further prescriptive provisions mandating the air barrier materials not exceed an air permeance of 0.02 L/(s.m2) @ 75 Pa (0.004 cfm/sf @ 1.57 lb/sf) when tested in accordance with ASTM E2178, Standard Test Method for Air Permeance of Building Materials, and assemblies not exceed 0.2 L/(s•m2) @ 75 Pa (0.04 cfm/sf @ 1.57 lb/sf) per ASTM E2357, Standard Test Method for Determining Air Leakage of Air Barrier Assemblies.
In lieu of compliance with the materials and assemblies air leakage rates, IECC stipulates the air barrier system, or whole building, be tested in accordance with ASTM E779, Standard Test Method for Determining Air Leakage Rate by Fan Pressurization, with a not-to-exceed rate of 2 L/(s•m2) @ 75 Pa (0.4 cfm/sf @ 1.57 lb/sf). The whole-building air performance is determined not only by the materials selected, but also by the constructed assembly or collection of one or more of those materials. A material itself may limit air transfer to extremely small amounts, but once this component is assembled with other materials, the acceptable air leakage rate is increased. Other industry standards, such as the U.S. Army Corps of Engineers (USACE) or the 2012 International Green Construction Code (IgCC), provide varying and more stringent thresholds for recommended whole building air leakage.
It is important to note all these code-referenced test methods quantify the amount of air leakage, but do not identify the location or specific source(s) of leakage. Therefore, if air leakage testing fails to meet requirements, then other standards (e.g. ASTM E1186, Standard Practices for Air Leakage Site Detection in Building Envelopes and Air Barrier Systems) would need to be utilized to identify deficiencies in the air barrier system.
The 2015 IBC also addresses air transfer in specific building assemblies in colder zones with certain operating conditions. For example, it limits use of air-permeable insulation materials in unvented cathedral ceilings and provides placement information for such materials.
As of 2009, the code provisions related to vapor transfer were moved from IECC to IBC. During this time, there were more detailed vapor retarder classifications also added. Historically, a vapor retarder had a permeance of 1 perm or less—a common example was a polyethylene sheet. In the 2015 IBC, the classifications for vapor retarders include:
Therefore, the traditional polyethylene sheet is now a Class I vapor retarder.
Common examples of a Class II vapor retarder are unfaced polystyrene or plywood, whereas a Class III example is gypsum board or some water-resistive barriers (WRBs).
To determine a material’s vapor permeance and subsequent compliance with code, there are two test methods within ASTM E96, Standard Test Methods for Water Vapor Transmission of Materials. It is important to understand which method may be applicable to the specific design because the results can vary. In the water method, the material sample is adhered to a test dish containing water so water vapor ‘flows’ from the wet side through the test specimen and into the dry chamber—a condition similar to winter with humidified interior space. The desiccant method is similar to the performance of vapor transfer in a heated, dry structure during rain; it measures the inward drive into the building.
Placement of a vapor retarder within an assembly becomes particularly important given that more than one material within an assembly may now qualify as some class of vapor retarder. Caution must be dedicated to these situations to not trap or allow unwanted moisture to migrate between layers of vapor retarders within a given assembly.
The 2015 IBC definition of a roof assembly hints at the intended location of the vapor retarder inboard of thermal insulation, but other provisions within that code are more specific. For exterior walls, the vapor retarder location is specified to be on the interior side of frame walls in Zones 5 through 8 and Marine 4. The code does permit an exception with accepted engineering practice for hygrothermal analysis. Special case assemblies, such as unvented cathedral ceilings, include provisions that actually limit the use of Class I vapor retarders, but require a Class II material with airtight insulation.
Codes and performance
Despite meeting all the provisions discussed in this article, compliance may not guarantee performance, and subsequently creates ‘gaps’ between design and the built world. However, many tools and upfront services are available to the project team to better understand and help anticipate performance prior to construction.
Continuous insulation (ci) is often required to meet prescriptive code requirements. However, current codes and standards recognize insulation penetrated by fasteners as ‘continuous.’ ASHRAE 90.1 identifies continuous insulation as:
Insulation that is uncompressed and continuous across all structural members without thermal bridges other than fasteners (i.e. screws and nails) and service openings.
Therefore, it may be important for the project team to understand any potential overall or localized effects fasteners or service openings may have on the overall thermal performance of an assembly.
One method of upfront analysis is to utilize various computer simulations to estimate thermal transfer. It is important to note two- and three-dimensional thermal models exist and may yield varying results. Utilizing a two-dimensional analysis, Figure 4 depicts a thermal model of a roof assembly where the insulation is continuous per the code and the fasteners are not included. The surface of the metal decking is well above dewpoint for most typical interior operating conditions, and the U-factor of the assembly meets code.
However, once metal fasteners that penetrate the decking are modeled, the temperature of the metal decking is altered locally at the fasteners, as well as the assembly’s overall U-factor.
The fasteners direct heat away from the metal decking as they bridge to cold exterior conditions. This increases the risk for localized condensation on the metal deck, depending on interior operating conditions. Therefore, the designer may wish to evaluate the impact of thermal bridging in the early stages of the design process to understand whether or not changes or specific detailing to eliminate thermal bridging is warranted.
Air barrier placement
According to the 2015 IECC, the air barrier can be placed at the interior, exterior, or within the wall/roof assembly. However, the mechanical pressurization can significantly complicate air control strategies and should have an impact on the barrier’s placement for optimal performance.
In this example, a typical low-slope roof assembly was designed to include (from exterior to interior) medium-weight ballast, fully adhered roofing membrane, and polyisocyanurate (polyiso) insulation over metal decking. The building was located in a heating climate, and the interior space was to be humidified to approximately 30 percent, with positive pressurization at the interior spaces (Figure 5, page 36). The metal decking was specified to function as the vapor retarder, and the fully adhered roofing membrane (toward the exterior) was identified as the air barrier. The splices, joints, and ends of the metal decking were not detailed for continuity.
A computer simulation evaluated the proposed roofing assembly’s hygrothermal performance with only heat and moisture considered. An additional evaluation was then undertaken to include the effects of an air change source. By comparing the two models, the project team could grasp the potential effects of uncontrolled airflow within the roofing assembly.
In this instance, the applied air change source was from the interior to mimic the positive pressurization specified for the mechanical system. Simulating the assembly without air transfer (Figure 6), the model indicated the roof assembly would not accumulate moisture or result in a sustained relative humidity (RH) above 80 percent—the lower boundary for organic growth—for extended periods. However, once the air change source was applied to simulate the building mechanicals pushing air into the enclosure, the model output indicated the assembly’s moisture content increased in comparison to the model without the air change. The surface temperature reached the dewpoint at the underside of the roofing membrane (Figure 7), and the RH was maintained above 80 percent for longer than 30 days during the heating months.
Therefore, conditions in the model with air changes were conducive to organic growth and other potential damage to materials and components. As a result, the designer revised the project criteria to include a continuous air barrier at the interior and developed specific termination details to ensure continuity with adjacent assemblies.
The 2015 IECC stipulates interior design conditions used for calculations to be a maximum of 22 C (72 F) for heating. Therefore, whenever thermal simulations are undertaken for the exterior enclosure, a temperature of approximately 22 C is often applied as the interior boundary condition. However, does this temperature correlate with conditions adjacent to the exterior wall? The authors’ research and experience has yielded interesting findings regarding microclimates in operation versus design criteria.
Thermal simulations can provide valuable information prior to construction, but data collection after construction can also be an important tool for validation. In this study, a thermal simulation, as shown in Figure 8, was performed with an interior temperature in concert with 2015 IECC. Given the proposed interior operating conditions of 22 C and 25 percent RH, the surface temperatures were above the dewpoint of 1 C (34 F).
However, instrumentation was also enlisted to monitor the conditions following substantial completion. The frame surface temperatures from the instrumented data were compared to thermal simulations and found to be considerably lower than those predicted, prompting a closer look at the adjacent microclimate. The results indicated the temperature at the exterior wall can be significantly different (i.e. 5.5 to 8 C [10 to 15 F]) than the adjacent room temperature, as shown in Figure 9, and sometimes greater depending on the project details.
Once the temperature of the microclimate was applied as the interior boundary condition in
the thermal simulation, the frame temperatures were indicative of the instrumented data. Further study indicated the microclimate’s behavior could be impacted by the placement of the window, air leakage, and adjacent materials. Therefore, these may be important considerations for the project team when trying to estimate built behavior during the design phase.
Parapets are another source of envelope moisture issues. The code does not define the parapet as part of the roofing or of the exterior wall, therefore the designer is left without clear direction. Consequently, the parapet is often overlooked when it comes to code-compliance and is not fully detailed as a part of either. The parapet is located where two envelope systems meet, often with dissimilar materials, structural interference, and construction sequence challenges. Failure to maintain continuity of air and thermal control planes can lead to moisture issues.
Parapets are often supported on projecting or upturned structural slab elements. These structures commonly define the edge of the thermal barrier of either the wall or the roofing. A thermal discontinuity between the roof and wall can be readily noted in Figure 10. However, this connection is easily overlooked during the design process. Peer review by various building system specialists can bring new perspective and expertise to assist with identification and resolution.
Air barrier continuity can be equally challenging at parapets where dissimilar materials, competing manufacturers, and different construction trades meet and attempt to integrate. Therefore, material compatibility must be considered where walls and roofs meet. While many common roofing materials have been in use for decades, new products are continually developed and proprietary air barrier systems are being marketed to keep pace with the new code mandates. The chemical compatibility between varying materials can be identified sooner with manufacturer involvement during design and resolved through additional specified testing or detailed language for warranties.
Even when continuity is specifically checked during design and dissimilar materials are resolved, geometries where vertical and horizontal elements meet can create hidden locations for moisture issues to propagate. As seen in Figure 11, a quick three-dimensional model of a simple parapet in two dimensions can highlight the need to further evaluate in-plane detailing. The typical building section detail is inadequate to show the special consideration needed at joints between parapet panels. Given this, the project team may wish to consider peer review and three-dimensional modeling early in the design process to assist with identifying and resolving performance gaps before they are constructed.
As codes advance and higher performance is expected from building assemblies, the control of moisture within and through the exterior envelope is critical to the success of any project. Good design anticipates the transfer of air, vapor, and heat under various conditions. However, understanding the fundamental nature of these properties is frequently insufficient to achieve success when considering the code-required performance or construction practices. To complicate matters, even when the code is satisfied, optimal performance may not always be achieved.
It is important to consider options and activities that can be done early in the design process to better understand and estimate the potential performance issues. Due diligence can also continue beyond the design process and provide a benefit to the overall project performance. In other words, every effort to identify potential performance risks or issues prior to completion is a benefit to project success.
Sarah K. Flock, CDT, AIA, is a consulting architect with Raths, Raths, and Johnson (RRJ). Specializing in building component diagnostics and repair design, she has participated in projects involving various types of building materials and systems. Flock is a graduate of advanced WUFI and ASHRAE training courses, a certified simulator through the National Fenestration Rating Council (NFRC), and an accredited commissioning authority + building enclosure (CxA+BE). She can be contacted at firstname.lastname@example.org.
Carole M Ceja, NCARB, RRC, is an architect II with RRJ. Her practice areas include roofing and waterproofing, fasade and curtain wall, water leakage and condensation, and structural components of buildings. She develops repair designs and construction documents to correct building deficiencies and performs construction observation services of these projects to help ensure repairs comply with plans and specifications. Ceja is a Registered Roof Consultant certified by RCI. She can be contacted at email@example.com.
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