In an effort to achieve low first costs, low lifecycle costs, and an accelerated building schedule for a college residence hall project in Vermont, KieranTimberlake explored the use of metal stud framing in the construction of the project's exterior wall system. In so doing, the firm hoped to identify a cost-effective means to achieve a wall system appropriate for a building project with an expected 100+ year lifespan.
KieranTimberlake
Kevin Rasmussen
Introduction
In an effort to achieve low first costs, low lifecycle costs, and an accelerated building schedule for a college residence hall project in Vermont, KieranTimberlake explored the use of metal stud framing in the construction of the project's exterior wall system. In so doing, the firm hoped to identify a cost-effective means to achieve a wall system appropriate for a building project with an expected 100+ year lifespan.
The project consists of two separate residence halls housing a total of 154 students in single bedrooms arranged in three, four, and five-bedroom suites with private living spaces, kitchen, and bathroom facilities. Each building has an approximately 10,000 SF footprint; the first hall is five stories tall, the second is four stories. The structural system, developed to be independent of the exterior wall system, consists of pre-cast concrete plank floors supported by a combination moment-resisting and braced steel frame. The exterior wall, and specifically the exterior stone veneer, is entirely self-supporting. The backup wall system resists only horizontal loads transferred from the veneer.
Early in the design process, the client established the goal of achieving compatibility with and longevity equal to the buildings located in the project's immediate context. The two halls sit at the edge of a campus that displays remarkable consistency in the use of local stone in its buildings. Gray and buff-toned granites, marbles, and limestone from Vermont and Canada predominate the campus facades, including those of the earliest group of core buildings built in 1800.
At the same time, the client established a fixed construction budget and the goal of occupying the buildings approximately 18 months after the start of construction - an accelerated schedule constrained by both a short construction season and a need to coordinate completion with the arrival of students for the fall semester. After schematic design, the project's local cost estimator identified changing the exterior backup wall from CMU to metal stud as a primary means to cut the construction schedule by a crucial 4-5 weeks. This relatively small reduction in time meant the difference between opening the buildings on time for the fall semester or the building standing idle for an entire school year. The cost estimator also established savings of around 25% when comparing costs of metal stud construction to those of CMU.
All components of the exterior wall system required careful analysis before our firm and the client committed to the metal stud backup wall. Each of the of the project's consulting engineers (CVM Engineers (www.cvmengineers.com) for structural, LKPB Consulting Engineers (www.lkpb.com) for mechanical) provided background data and calculations critical to predicting the structural and thermal performance of the wall. In addition, KT engaged an independent testing laboratory (Architectural Testing Inc. (www.archtest.com)) to computer-simulate the thermal dynamics of the wall and Ove Arup & Partners Consulting Engineers PC (www.arup.com) to computer-simulate interior humidity levels of a typical suite.
To meet the client's expectations for building longevity, our research needed to establish with certainty that the wall for this project would match the reputed longevity of a CMU wall system and address the following specific shortfalls associated with conventional metal stud framed walls:
Before delving into the above issues, we asked our structural engineer to determine if a metal stud wall could be designed to achieve a stiffness comparable to CMU and great enough to prevent cracking in the stone veneer. Limiting maximum horizontal deflection to less than L/600, CVM answered the challenge by developing a preliminary backup wall using 6" 14-gauge studs placed 16" on center, braced back to structure at every floor. These early determinations provided us with a suitably rigid structure for our other wall components and a solid base from which to begin the rest of our studies.
 Metal Stud Backup Components of exterior wall, from outside to inside
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 CMU Backup Components of exterior wall, from outside to inside
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Stone Tie Anchorage
The connection between the backup wall and the masonry veneer constitutes the 'weak link' in conventional metal stud/masonry veneer wall assemblies. Conventional metal stud veneer anchors are fastened to the flange of the metal stud from the outside of the wall. While easy to install and adjust (many can be applied directly through the wall's insulation, vapor barrier, and sheathing layers), their connection to the stud is subject to deterioration due to corrosion or improper installation. Anchors mounted to stud flanges transfer horizontal load from the veneer to the backup wall via tension and compression; they rely on the bearing surface between a single thread of a fastener and the face of the flange to resist lateral forces. If the fastener is overtightened, or the connection is exposed to water and corrodes, the hole in the flange can enlarge, disengaging the fastener and causing the connection to fail.
To address this issue, KT researched alternative masonry anchor systems capable of developing a superior connection to the stud. A local stone supplier recommended specifying a web-mounted anchor made by Canadian manufacturer FERO Corporation (www.ferocorp.com). Unlike the flange-mounted anchors described above, the FERO anchor is attached to the web of the stud; its fasteners are loaded in shear (rather than tension) when resisting horizontal loads. The shear loading provides more bearing area and less overall stress on the fastener and provides a structural connection that is less susceptible to failure if corroded.
The use of a web-mounted anchor requires careful coordination between the stud layout, the installation of the exterior sheathing, and the application of the vapor/air barrier. Rigid exterior sheathings such as gypsum/glass mat boards require slots to be made and precisely aligned with stud edges before the anchors can be installed. Each anchor also creates a penetration through the wall's air/vapor barrier that must be carefully sealed with a compatible mastic.
Resistance To Water Infiltration
The durability of any exterior wall system depends upon careful control of moisture entering the wall from the outside environment. Moisture control is especially critical in metal stud construction where structural connections and anchorages are steel and susceptible to corrosion if their galvanized coatings are damaged and water is present.
In conventional masonry exterior walls (backed by either CMU or metal stud), water infiltration occurs largely through differentials between interior and exterior air pressures. When a relatively low interior air pressure exists (caused by relatively high outside wind or atmospheric pressure, or by improperly balanced interior mechanical systems), moisture present on the outside face of the wall is drawn by air toward the building interior wherever a crack in the masonry or a gap in the building's air/vapor barrier occurs.
To provide a first line of defense against infiltration, KT looked to a pressure-equalized rain screen wall system. Documented by the Brick Institute of America and used successfully in a number of the firm's past masonry veneer walls, this system creates a buffer zone between differing interior and exterior air pressures. Open joints in the masonry vent the wall's airspace to the outside and equalize the air pressure on each side of the veneer. Because varying outside air pressures are expected to occur simultaneously along different sections of the wall, neoprene sponge baffles subdivide the airspace into multiple chambers, each capable of maintaining separate air pressures. A minimum of 2" clear is maintained between the back of the masonry veneer and the face of insulation - a depth great enough to prevent clogging of the airspace by mortar droppings.
The effectiveness of the pressure-equalized rain screen wall system relies on the proper selection, detailing, and installation of the wall's air/vapor barrier. The tighter the barrier, the less likely interior air pressure leaks are to 'contaminate' the buffering action of the chambered air space. In conventional metal stud construction, the vapor barrier is often a polyethylene sheet applied to the inside face of the studs. At this location in the wall, it is prone to leakage at numerous points, including fastener locations, power and data receptacle penetrations, window openings, and floor/ceiling interruptions. In the residence halls' wall, the air/vapor barrier is a 40 mil, self-adhered modified bitumen membrane applied to the outside face of the metal stud backup wall. It runs continuous from grade to eave, consistently covering floor interruptions and sheathing fasteners. Penetrations at the veneer anchors and windows remain accessible after the membrane is applied. They are individually inspected and sealed with mastics and flashings compatible with the air/vapor barrier.
Resistance To Condensation
The durability of a metal stud framed backup wall also depends upon careful design for the control of condensation buildup inside the stud cavity during cold-weather seasons. Without a vapor barrier applied to the inside face of the studs, water vapor borne by relatively humid interior air migrates into the stud cavity through the wall's interior finishes (in this case, painted gypsum drywall). This action is normal and not problematic in itself; it becomes problematic only if surface temperatures inside the stud cavity, especially on the studs and anchors, fall below dew point temperature. Below dew point, the interior air looses some of its ability to carry water vapor, releasing moisture upon relatively cold surfaces.
Dew point temperature - the first variable of condensation - changes constantly and is a function of inside air temperature and relative humidity. During design development, the project's mechanical engineer and the client established interior air temperature goals ranging from as low as 55 F during unoccupied winter vacation conditions to as high as 75 F when occupied; 70 F was determined to be the anticipated average. Computer-simulated maximum interior humidity levels (assuming showers, cooking, and normal occupant activity) proved to range from 8% to 20%, depending on the relative humidity of the makeup air drawn from the exterior. (The makeup air is not mechanically humidified; its relative humidity is a function of exterior air temperature. The colder the air, the less moisture it holds.) Given ranges for interior temperature and relative humidity, the anticipated dew point temperature was calculated to range from 10 F to 30 F. (See Figures A-2.1 and A-2.2.)
Stud cavity temperature - the second variable - also varies and is a function of indoor air temperature, exterior air temperature, the general thermal resistance of the wall, the location and thickness of the wall's thermal insulation, and the treatment of the masonry veneer anchors where they protrude into the wall's airspace.
Interior temperatures
The heating system is designed to maintain temperatures ranging from 55 F to 75 F (see paragraph above).
Exterior temperatures
Regional climate data indicates exterior temperatures consistently reach season lows of -20 F each winter; the average design temperature is 6 F.
Thermal resistance
To prevent heat loss due to wintertime cold weather extremes, the wall complies with client and state energy code requirements for an overall thermal resistance of R-19 - R-20 (U-values of approximately 0.05).
Insulation
The interruption of insulation in the stud cavity at every stud creates the primary thermal short circuit present in conventionally framed walls. A table of 'correction factors' provided by Owens-Corning shows that for a 6" stud cavity, an R-19 rated bat insulation provides an actual resistance of R-7.1 when used between 6" metal studs framed 16" on center. The interrupted insulation fails to protect the outside flanges of the studs from temperatures in the wall's airspace that are cold enough to make the framing fall below dew point temperature and collect condensation.
In conventionally framed walls, locating the insulation within the stud cavity also pushes the wall's condensation point toward the interior of the building. If insulation is provided between studs, the temperature in the stud cavity tends to stay close to the average of the interior and exterior temperatures and in danger of falling below dew point temperature. If insulation is provided to the outside of the studs, the temperature in the stud cavity tends to stay closer to that of the interior and safely above dew point temperature.
To address the shortfalls inherent in the conventional use of bat insulation, the insulation in the residence halls' wall runs continuous along the outside face of the metal studs and sheathing. Uninterrupted by studs, 3" of R-5 per inch rigid polystyrene is water resistant, protects the wall's air/vapor barrier, and efficiently provides the bulk of the wall's thermal performance. It protects the outside flange of the studs from extreme colds, and effectively keeps the entire stud cavity above dew point temperature.
Masonry veneer anchor treatment
With rigid insulation running outside the studs and sheathing, only the penetrations made by the masonry veneer anchors have the potential to create a thermal short circuit. To analyze their effect on temperatures inside the stud cavity, Architectural Testing Inc. simulated combinations of interior and exterior temperatures in a computer model of a typical anchor penetration.
Results indicated under worst case wintertime weather conditions (VT low of -20 F) and typical suite interior air conditions ( 70 F temp and 8% relative humidity), temperatures at a veneer anchor will occur 15 -30 F above the anticipated dew point if the projecting anchor end is left exposed. However, if a one-inch application of R-5 spray polyurethane foam is applied to the projecting end, stud cavity temperatures at the penetration will occur up to 40 - 45 F above the anticipated dew point. Under atypical suite interior air conditions (winter setback temp of 55 F), temperatures at the unprotected veneer anchor occur 10 - 20 F above dew point; temperatures at the protected anchor occur 30 - 35 F above dew point. For both typical and atypical conditions, condensation is not expected to occur - even at an unanticipated extreme high of 40% interior relative humidity. (See Figures 2.1A and 2.1 below, and figures A-2.3 - A-2.6 for additional information.)
Exterior Wall Thermal Analysis at 70F Interior Temperature
Exterior Wall Thermal Analysis at 55F Interior Temperature
Conclusion
With research completed by the end of design development, we compiled our test results and presented an Exterior Wall Analysis to the client recommending the use of the metal stud framed walls for the project. Built to address the issues discussed above, a metal stud frame wall will achieve the client's short-term goals of cost and time savings and its long-term goals of providing durable campus housing.
Appendix
Uninsulated Anchor Thermal Analysis
Insulated Anchor Thermal Analysis
Steam Anchor Diagram
Stone Anchor Thermal Analysis
Stone Anchor Thermal Analysis
© 1997– KieranTimberlake
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