If you aren’t expecting it, Allianz Field looks like an alien mother ship strategically landed at an area known as “Midway” between Minneapolis and St. Paul. Technically this unearthly framework has risen from the grounds of the old St. Paul bus barn. Populous, formerly known as HOK Sport Venue Event, was tasked with the design of the stadium. Never failing in creating a memorable aesthetic that becomes a destination in and of itself; Populous created a venue that features undulating “driver tubes” to create the facade and a translucent polytetrafluoroethylene (PTFE) skin that will be backlit to change colors at night, for a kind of “close encounter” of the soccer kind.
“That’s what has made this installation unique,” says Jeff Manick, general superintendent for Olympic Companies Inc., charged with doing the exterior stucco on the project. “From the outside you won’t even see our work.”
That didn’t keep the contract from being any less complicated. As originally specified, the framed walls in parts of the assembly were supposed to get a drainage medium between the sheathing and 2 to 3 inches of continuous insulation before the installation of the lath and stucco. All of which led to a bit of a conundrum according to Kevin Larson, Olympic’s senior project manager. “The code required that the project meet NFPA 285 requirements,” he explains.
Yellow Card Caution
To put it in context, the full title of NFPA 285 is Standard Fire Test Method for Evaluation of Fire Propagation Characteristics of Exterior Non-load-bearing Wall Assemblies Containing Combustible Components. The key word that must be taken from that mouthful is “assemblies,” because NFPA 285 does not test single components, but rather the entire wall assembly as a whole. So any deviation from the assembly detail profiled in the contract specification (other than verifiable like-kind product replacements) requires a separate NFPA 285 test. This is of course significant in terms of cost because the NFPA 285 test must be performed on a two-story wall assembly.
As Olympic’s project manager and an NFPA Member, Larson was acutely aware of how NFPA 285 imposed restrictions on the assembly. Reviewing the materials specified he passed on his concerns to MA Mortenson, the general contractor. This soon led to a scramble to find verifiable like-kind materials, that when assembled, had been tested in accordance with NFPA 285.
So why didn’t they just use EIFS with drainage? “That was out of the question,” responds Larson. It would seem EIFS would be a solid choice, considering it has always had to pass the NFPA 285 hurdle to get an Evaluation Services Report as an alternative material in the building code. The reality is that EIFS has always been in the crosshairs of other cladding materials in response to its increasing market share. Over the years, the large scale fire tests among other measures imposed on EIFS have presented costly roadblocks to its use.
However, with EIFS now officially recognized by the building code, it seems that the tables have turned, and as a result, other materials must meet the same fire criteria. “The contract didn’t allow us to deviate much from the specified materials,” confides Larson. “We knew that we had to find alternatives to meet the intent of the specification, but fast.”
Larson’s hunt for acceptable materials began with a Technical Services Evaluation Report with a table of like kind NFPA 285 approved assemblies, which included:
- 20 gauge steel studs (min.). Marino\WARE Materials, 16 and 18 Gauge
- One layer–5/8-inch thick Type X gypsum wallboard on interior. National Gypsum Gold Bond XP 5/8-inch board.
- Cavity Insulation - Urethane QS 112, 2 pcf spray foam partial fill with a maximum 2½-inch air gap. BASF Spraytite #81206
- Half inch or thicker exterior type gypsum sheathing. National Gypsum Gold Bond eXP Sheathing.
- Fluid applied air and water barrier. WR Meadows Air-Shield LMP.
- Exterior insulation that has been tested per ASTM E1354 (at a minimum of 20 kW/m2 heat flux) and shown by analysis to be less flammable (improved Tign, Pk. HRR) 4 inch (max). Atlas EnergyShield Pro.
- Water-Resistive Barrier over the Exterior Insulation. VaproShield WrapShield SA.
- (Lath) Portland Cement Plaster – Minimum 3/4 inch-thick. Marino\WARE Lath, SPEC MIX Stucco Mix, Dryvit Acrylic Finish.
It seemed as if Olympic was down by two scores with little time remaining. So what of the drainage mat? “The one we were looking for was not among the NFPA 285 Approved Wall Assemblies,” says Larson. “And sourcing of the WRB over the Exterior Insulation took a little more research.”
A data sheet on VaproShield WrapShield SA verified testing compliance with NFPA 285 and ASTM E1354, for a non-combustible self-adhering water-resistive barrier.
With those circumstances addressed, now came the execution. Olympic built and sheathed the walls. EBS was charged with applying the Air-Shield LMP fluid applied membrane. After the dry film thickness was tested and cured, it was once again Olympic’s turn to install the foam board and the self-adhering membrane. “Did you have any problems with primers, fish mouths, or bubbles in installing the WrapShield over the foam insulation?” I asked. “No, not really,” says Manick. “WrapShield is installed without primer and it certainly helps to have the right number of personnel on hand to place and roller it out.”
Next came the lath and accessories. Casing beads around the “driver tubes” that penetrate the walls were especially tedious. These had to be expressly “pie cut” to be able to wrap the tubes, terminate the stucco and allow for backer rod and sealant all while allowing for anticipated movement.
“The lath attachment had to be particularly exact,” says Manick. “The screws had to be long enough to go through 2 to 3 inches of foam board and sheathing and penetrate the stud flange.”
Precise measurements had to be taken to ensure that happened but inevitably it was expected that a screw would miss the framing here and there.
“We were directed to take out the screws that did not hit the studs,” says Manick. “We had to come up with a protocol that could seal the errant screw penetrations that were few, but did occasionally happen.”
This generally consisted of a syringe type squirt of elastomeric sealant through the WRB and foam board to the face of the sheathing membrane rather than a complete removal and patch. “How do you know if you actually repaired the hole?” I ask. “You don’t,” says Manick. “However, with all the redundancies built into the design we don’t expect any problems going forward.”
“For my own assurance, this patch method was actually mocked up and successfully water tested by a third-party inspector,” Larson adds
Compared with all of the drama imposed on the installation, applying the Portland cement plaster was like a spot-kick in front of the net. A SPEC MIX Silo was sited adjacent to the scaffolding. The stucco was then mixed, pumped, and placed in lifts, about five stories up. Remarkably, by the time I made it up the scaffolding, Brad Larson and Mark Ellefson had already covered and scratched about fifty lineal feet of area.
With less than a year remaining on the construction schedule to finish the approximately 20,000 seat stadium, no “timeouts” will be taken by Olympic Companies in completing their work. “We get this part completed,” says Larson, “then we move on to approximately 20,000 square feet of DAFS on the ceilings of private suites, mezzanines and concourses.”
Congratulations Olympic Companies on a World Cup team effort (and oh yeah, to France).