Air Barriers – They Might Be Continuous, but Still in Danger of Allowing Building Component Damage Ryan Krug, BECxP, CxA+BE Pie Consulting & Engineering 10250 Valley View Road, Suite 149, Eden Prairie, MN 55344 612-284-7080 • rkrug@pieglobal.com Building Enclosure Symposium • NovembeBEr 11-12, 2019 Krug • 113 Abstract Uncontrolled airflow within a building enclosure can cause damage to building components. Due to our harsh winter conditions in Minnesota and the services our firm offers, we have had the unique opportunity—through our forensic investigations, monitoring of existing buildings, and extensive design consultation—to see areas of concern to designers, contractors, and building owners. Remediation and prevention of problems will be outlined. Speaker Ryan Krug RYAN KRUG provides building enclosure consulting, building enclosure commissioning (BECx), and forensic engineering services for a variety of clients in both the private and government sectors. His primary responsibilities include design development assistance, technical plan and specification peer review, and quality assurance field observations related to exterior enclosure systems (air, water, thermal, and vapor control layers). In addition, Krug has extensive experience in building enclosure field performance testing, including fenestrations, curtainwalls, claddings, and whole-building air barrier testing. 114 • Krug Building Enclosure Symposium • NovembeBEr 11-12, 2019 AIR BARRIERS Airtight buildings are a goal of building design and construction in order to decrease building operational costs (energy usage) and to minimize moisture damage to the building. The 2018 International Energy Conservation Code (IECC) identifies the definition of an air barrier and provides air leakage rate requirements for materials, assemblies, and the building. In addition, it provides guidelines for the integration of the air barrier between assemblies, at penetrations, and for the installation to resist failure over time. In practice, construction material product manufacturer installation instructions typically follow the installation guidelines of the IECC. Generally speaking, when air barriers are installed to code, whole-building leakage rates are within acceptable ranges, and occur primarily through fenestrations. Through air barrier testing experience, if installed with the code intent, whole-building air barrier leakage rates generally do not exceed code air leakage rates, and the observed air leakage generally occurs through fenestrations. The IECC accepts a quantifiable amount of air leakage through components. However, if a building exceeds the maximum allowable leakage rate identified by the IECC, it is typically due to a systemic transitional failure between differing assemblies—whether that be between below- and above-grade conditions, on exterior walls between cladding types, or at transitions from exterior walls to roof assemblies. It is worth noting that whole-building air barrier testing is not required by code but is a path to code compliance. So, if using tested assemblies that meet code requirements, systemic transitional failures will not be observed until the building is put into service; obvious clues to air barrier deficiencies are higher than expected energy usage and, potentially, observed moisture damage. But what happens within a building and wall assemblies under normal operation when you have built to or exceeded code requirements for whole-building air leakage? There are still locations and environmental scenarios that can lead to moisture accumulation and assembly damage. THE LOCATION DILEMMA The IECC does not identify where within the assembly the air barrier is to be installed. That is left up to the design professional and is based on the project type, geographic location, and the designer’s experience. So where should the air barrier be placed within an assembly? From a constructability and cost standpoint, it makes sense to control air with the same product that acts as the moisture barrier since the goal is the same with both: prevent water from entering the assembly and prevent air movement through the assembly. In many climates, this is a sufficient solution to prevent moisture damage to assemblies. But in colder climates, to control water vapor migration from occupancy usage or building function from entering the assembly, the vapor retarder must be interior of the insulation in order to minimize the potential for condensation to occur within the assembly. In cold-climate commercial construction, this means typically insulation is installed to the exterior of the stud framing, and the weather, air, and vapor barrier on the exterior sheathing performs triple duty. In this configuration, insulation is moved outboard of the vapor retarder to sufficiently elevate the sheathing temperature above the dew point to prevent moisture accumulation and damage. In cold-climate residential construction, insulation is typically installed within the stud cavity. During winter months, the dew point occurs within the stud cavity and a vapor retarder is required on the interior of the stud framing. Since air movement within assemblies carries with it much more moisture than that which occurs through diffusion, professionals strive to utilize the vapor retarder as an air barrier as well by ensuring continuity at electrical boxes, wall-to-ceiling transitions, bearing walls predetailed with a vapor retarder, common walls, elevator walls, and other penetrations. Regardless of the construction type, if designed correctly, some moisture accumulation can occur during the most extreme conditions, which can be accommodated for if the duration of saturation is small and the assembly allows for drying in at least one direction. However, an interior air barrier is not required per the IECC. Therefore, moisture accumulation can occur within the façade assembly, outpacing the assembly’s drying capacity due to uncontrolled airflow within the assembly and thermal bridging or distance from the heat source causing assembly components to drop below the dew point temperature. In these conditions, if incorporating mechanical heating to ensure assembly components do not drop below the dew point is not an option, it is important to minimize the severity of moisture accumulation. Over time, design professionals, consultants, contractors, and installers have developed best practices for phenomena inherent to building conditions susceptible to condensation formation. CONDITIONS SUSCEPTIBLE TO CONDENSATION FORMATION Wall assembly components inherently have localized areas where condensation formation can occur due to thermal bridging. Typically, conditions prone to condensation formation are the structural connection of cladding to building framing that occurs from girt attachment to stud framing, masonry tie-backs, masonry support angles, and other similar structural conditions. The energy loss through localized thermal bridges can result in condensation within the wall assembly if sufficient heat is not supplied to ensure the surface temperature is above the dew point temperature. By utilizing connection materials with Air Barriers – They Might Be Continuous, but Still in Danger of Allowing Building Component Damage Building Enclosure Symposium • NovembeBEr 11-12, 2019 Krug • 115 less conductance than metals or by placing thermal breaks at the attachment-to-sheathing interfaces, the localized condition is thermally improved and the potential for condensation is minimized. Making an analogy to the air barrier, thermal bridges are air leaks, and adding thermal breaks patches those leaks. Such a strategy works well within the field of an assembly where the conditioned space is as close as possible to the exterior, and constant temperature lines (isotherms) are more or less parallel to the sheathing planes. Moisture accumulation issues can occur when the dew point temperature inverts or moves interior of the air barrier, within the air in the cavity, and condenses. Locations where the dew point inversion routinely occurs are within parapets, soffits, wood-framed roof assemblies, and glazing assemblies. PARAPETS – STEEL-STUD FRAMED Let’s consider commercial construction assemblies; ideally, the vapor retarder is installed on the warm side of the roof assembly and extends continuously to shingle-lap the exterior sheathing weather barrier. A parapet would then be installed on top of the vapor barrier. This rarely happens, though. Typically, steel studs extend in front of the roof deck to form the building parapets, wall insulation extends to the top of the wall assembly, and insulation is installed on the backside of the parapet, prior to waterproofing membrane installation. Thermal modeling of 99.6% heating dry-bulb temperatures for Minnesota, as outlined in Chapter 14 of the 2017 ASHRAE Handbook – Fundamentals, has shown that parapet components are typically the 99.6% heating DB temperature at approximately 12 inches above the roof waterproofing membrane elevation. Therefore, condensation, moisture accumulation, and deterioration can occur particularly quickly due to the extremely cold temperatures within the parapet. Figure 1 shows a thermal model of a steel-stud parapet condition with fiberglass batt insulation within the stud cavity, and Figures 2 and 3 illustrate deterioration of a similar condition after a few years of in-service life in Minnesota. Installing an internal air stop is most effective as close as possible to the conditioned space. So, in the case of a parapet, it should be installed in line with the roof insulation. Figure 4 illustrates such an installation on the roof side of a curtainwall that extends above deck to form the parapet, similar to a steel-stud condition. In this installation, mineral wool was placed between the back of the glazing assembly and the roof slab, a liquid-applied air barrier was installed, and spray foam was installed to the roof insulation depth as shown in Figure 5; the roof assembly has yet to be installed in this example. 116 • Krug Building Enclosure Symposium • NovembeBEr 11-12, 2019 Figure 1 – Thermal model of a parapet condition with fiberglass batt insulation within the stud cavity. Figure 2 – Destructive test opening of a steel-stud-framed parapet with moisture damage. Figure 3 – Closeup of steel-stud-framed parapet shown in Figure 2. PARAPETS – WOOD FRAMED In a steep-slope wood-framed roof, the joists typically sit on the exterior framed walls. In this application, the interior vapor retarder typically follows the interior drywall line in the ceiling condition with blown-in insulation installed in the joist cavity, and typically, no issues arise. However, low-slope roof assemblies are being used with wood-framed multifamily structures more frequently. Since the roof membrane is a weather, air, and vapor retarder, this leads to dual vapor retarders in the roof assembly. Therefore, cavities within the assembly, such as the one that forms between the underside of the deck due to blown-in insulation that does not completely fill the cavity and/or settles over time, can condense. Unfortunately, in a wood-framed, low-sloped roof assembly, drying potential to the exterior is reduced, since the air space above the insulation is typically not vented. When the membrane is attached directly to the deck, (or even if there is insulation above the deck), we have seen accelerated assembly deterioration of the decking within a few years of installation if the interior vapor retarder is not functioning as an air barrier to prevent interior moisture from completely saturating the roof assembly. Figures 6 and 7 show damage that occurred Figure 4 – Air barrier installed at the roof slab to back of glazing, creating an air barrier into the parapet cavity. Figure 5 – Spray foam being installed above air barrier shown in Figure 4 to the height of the roof insulation. Figure 7 – Joist damage observed under RTU curb where condensation deteriorated curb and joist. Figure 6 – RTU curb damage observed on wood-framed roof deck from condensation. Figure 8 – Parapet damage observed due to air migration into the parapet and subsequent condensation damage. Building Enclosure Symposium • NovembeBEr 11-12, 2019 Krug • 117 due to condensation on the underside of deck/curbs. These failures are similar to parapet conditions in that they rise above the insulation line, and as shown in Figure 1, become very cold, causing condensation. Even with insulation above the deck, moisture damage can occur if a vapor retarder is not installed on the deck and tied into the interior wall vapor retarder. Figure 8 shows parapet damage after two years due to air migration into the parapet from the joist cavity that resulted in substantial condensation formation, saturation of framing components, and subsequent deterioration. To prevent such deterioration, a commercial roof assembly should typically include a fully adhered vapor retarder on the deck, adhered insulation, and adhered exterior membrane similar to that shown in Figure 9. The installation should also include (prior to the joist and roof installation) the interior vapor retarder terminating on top of the framed wall, and spray foam in the joist cavity above the framed wall to ensure continuity between the wall and roof assemblies. SOFFITS Soffits could be considered inverted parapets. However, in most cases, based on mechanical, structural, plumbing, and other systems utilizing the underside of the deck for distribution, the weather, air, vapor, and insulation layers are not installed tightly to the underside of the deck as they would be on the roof deck. The soffit is typically dropped to the bottom of the exterior wall overhang to allow for continuity of the weather barrier at approximately right angles from the exterior wall assembly to the soffit, and to the wall assembly below the soffit condition. Therefore, an unconditioned cavity is formed. In extreme cold conditions, the heat supplied to this space occurs from transfer through the floor above, and the conditioned space adjacent to the cavity (which can be up to 15 feet or more), is insufficient to heat the cavity. Active heating is the preferred method of ensuring surface temperatures are maintained above the dew point. However, typical project budgets do not allow for such an approach, and the cavities are left unconditioned. In the more typical scenario where mechanical heating is not employed, it is recommended to extend the inset wall to the underside of the deck to create an internal air barrier at the exterior sheathing line and integrate the weather/air/vapor barrier installed on the soffit sheathing with the exterior wall weather/air/vapor barrier to create a sealed enclosure; insulation follows the exposed weather barrier line. By compartmentalizing the soffit space from the interior, air contacting cold assembly components where it could frost and result in moisture accumulation is reduced. Figures 10 and 11 show such an installation where an air stop at the exterior wall to the underside of a deck was implemented with framing and mineral wool—in this instance to conform around structural steel components and limit air migration into the soffit cavity. Then the membrane 118 • Krug Building Enclosure Symposium • NovembeBEr 11-12, 2019 Figure 9 – Parapet detail illustrating roof and exterior wall vapor retarder continuity. Figure 10 – Soffit condition with air stop installed at exterior wall line. Figure 11 – Recess created for installation of a light fixture in a soffit; insulation is the next layer of the assembly. in the soffit framing was sealed to the exterior wall membrane. In the rare instances where the weather, air, vapor, and thermal barriers can be installed to the underside of a deck, maintaining continuity of the air and vapor barriers becomes challenging and continuity of the thermal barrier is typically impossible as studs or curtainwall framing flying by the slab are not thermally broken at the soffit insulation line. However, thermal bridging, while increasing energy usage, keeps the dew point outside of the enclosure. But especially with glazing, maintaining the air barrier requires its installation onto the glazing framing and capping/sealing the vertical mullions to prevent free flow of air through the mullions. Figure 12 shows such a condition; however, mullion plugs have not yet been installed. GLAZING ASSEMBLIES Due to the relatively high thermal transmittance through glazing assemblies, spandrel conditions within the glazing assembly are routinely utilized to increase the thermal resistance of the assembly with the addition of insulation in the space behind the spandrel glass. While beneficial from an internally exposed surface temperature and energy performance standpoint, it presents the opportunity for condensation formation on the interior face of the insulated glazing unit (IGU) and moisture damage to interior finishes. Most thermal improvement solutions used to decrease the glazing spandrel area U-value include sealing an impermeable material to the inside face of the glazing frame and installing insulation to the exterior of the material, interior to the IGU. However, by adding internal insulation, the probability of condensation formation on the interior face of glazing within the spandrel cavity increases. To ensure that air movement into the cavity and subsequent frost/condensation formation does not occur, the horizontal mullions must be fully sealed to the vertical mullions to ensure an airtight spandrel cavity. This step is routinely missed, and air migrates into the cavity to form frost/condensation on the interior surface of the IGU. Significant moisture can be released due to rising exterior air temperatures or solar loading when the frost quickly melts. Figure 13 shows such a condition where condensation and frost formation were observed, which melted and was perceived as a glazing leak. An additional caveat to this phenomenon is that the internal impermeable material sealed to the glazing frame matters. Foil-faced mineral wool is routinely used in the spandrel cavity and is an acceptable product if installed correctly. However, in practice, it often provides inadequate performance due to poor adhesion of the foil-faced tape used to seal it to the frame from inadequate overlap, oils from workers’ hands during installation, and dust/debris accumulation on the frame during shop or site storage prior to installation. In addition, the foil facing is susceptible to damage during installation by the glaziers, adjacent construction-related activities, and—the most problematic—inadequate installation/sealing due to insufficient clearance at floor lines and behind columns. Figure 14 shows a slab line area where the impermeable material could not be sealed to the glazing assembly. Figure 12 – Soffit condition where the glazing forms the soffit exterior wall. Figure 13 – Glazing spandrel location with condensation/frost accumulation. Figure 14 – Spandrel insulation installation where slab accessibility prevented sealing of the foil-faced mineral wool to the glazing assembly. Building Enclosure Symposium • NovembeBEr 11-12, 2019 Krug • 119 The most successful solution to mitigate condensation within the glazing assembly is to use a sheet metal back pan that is mechanically attached to the frame, fully sealed at the pan-to-mullion interface, and installed with a fillet bead of sealant at the horizontal-to-vertical mullion interfaces as shown in Figure 15. This creates the condition where air cannot transfer into or out of the spandrel cavity/assembly. Therefore, air into the cavity/assembly is limited as a source of moisture, and should minor condensation occur within the sealed cavity, it cannot transfer onto surrounding finishes; it will be reabsorbed by the air upon warming of the cavity. If maintaining a 1-inch airspace between the insulating glazing unit and spandrel insulation, glazing manufacturers typically do not take issue with this type of assembly. However, if utilizing such a solution in southern climates where solar loading could elevate temperatures, venting of the cavity to the exterior would be an acceptable means to mitigate pressurizing the cavity and excessive temperatures, which could cause IGU failure. Careful placement of vents is necessary to ensure that should moisture weeping occur, it enters the glazing assembly weep system. CONCLUSIONS Evolution of the IEEC has addressed many of the building damage issues arising from condensation formation in extreme cold climates, but there is room for improvement through revisions. Distance from heat sources, air cavities, air movement within assemblies through uncontrolled flow, and lack of interior air barriers in commercial construction should all be considered when designing exterior wall assemblies and transitional details, especially between plane changes. When professional experience is absent or solutions are not included due to budget constraints, moisture damage still occurs in code-compliant extreme-cold-climate buildings, as illustrated in the examples presented. Through experienced design by an architect, upfront review and analysis from a consultant, and knowledgeable construction by a contractor, proactive measures to prevent building damage can be incorporated into construction. 120 • Krug Building Enclosure Symposium • NovembeBEr 11-12, 2019 Figure 15 – Fully sealed back pan at the spandrel location prior to mineral wool installation.
