For 20 years, there’s been a push to change building codes and safety requirements toward higher standards for durability, energy efficiency and weather protection. Currently, Metal Composite Material (MCM) rainscreen cladding systems are rarely installed in High Velocity Hurricane Zones (HVHZ) in multistory buildings. This is because the standard exterior sheathings being used in combination with MCMs do not allow the system, as a whole, to meet the high design wind loads and missile impact requirements of HVHZs. MCM rainscreen claddings, attachment methods and sheathing must evolve if these systems are to be installed in HVHZs.
The code section, 1609-Wind Loads, has recently changed. The increased design wind loading presents a challenge for the sheathing beneath pressure equalized rainscreens, such as MCM. Another area that causes multiple design considerations is the requirement of Continuous Insulation (CI). These code changes, along with existing requirements, have three significant challenges for pressure-equalized MCM rainscreens to overcome: (1) new energy codes and the use of continuous insulation; (2) wall assembly fire performance over 40 feet in elevation, and (3) developing a system incorporating an insulating sheathing that can meet the first two requirements and also meet the new wind design pressures in HVHZs required by the 2010 Florida Building Code.
The first challenge is the incorporation of CI. The 2012 International Energy Conservation Code (IECC) divides the country into eight climate zones and into Marine, Dry and Moist areas. The 2012 IECC uses the various climate zones to specify cavity and continuous insulation requirements depending on the location and the type of assembly within the building envelope. To meet the CI requirement, a material needs to be continuous across all structural members without thermal bridges other than fasteners and service openings. A number of products meet the CI requirements, however, these insulative sheathings must also meet fire requirements such as NFPA 285.
This leads to the second challenge. The NFPA 285 intermediate scale multistory apparatus fire test requires the entire exterior wall assembly be tested, and the International Building Code requires that MCM claddings be included in the NFPA 285 testing. This test determines the performance of the exterior wall assembly with regard to flame propagation up the wall while measuring wall surface temperature increases. In general terms, any cladding system installed over 40 feet over grade, or any wall assembly using CI above grade, must comply with the standard.
The critical materials in determining a wall assembly’s ability to pass this requirement are those with significant energy content. To meet this challenge, MCM cladding and CI material, when tested together, must demonstrate the code-required fire resistance.
Rainscreen applications in HVHZ areas also have a third challenge. While maintaining the CI and fire-resistance characteristics, the wall assembly needs to be designed so that the sheathing can withstand the high pressures being transferred to it. With any type of “true” rainscreen system, the loads are on the sheathing. AAMA 508, the nationally recognized voluntary standard that addresses pressure equalized rainscreens, requires that 50 percent of the exterior load be equalized in the wall cavity within 0.08 seconds. Many of the AAMA 508 compliant walls transfer a significant percentage, if not all of the load, to the sheathing within 0.01 seconds. Unfortunately, many of the traditional fire-resistant sheathing products used in NFPA 285 complaint wall systems fracture as they cannot withstand these high loads. This challenge has been made even more difficult by the design load increases seen in the 2010 Florida Building Code.
Looking at these three challenges and their interrelationships, it quickly becomes evident that the solution is a high-performance wall system that has been carefully designed and tested extensively. The process and requirements to achieve this solution are complex and costly. Such a system has these five characteristics:
- Identify a material that complies with the CI requirements. Preferably one that can also act as the sheathing and air and water barrier for cost effectiveness.
- Partner that CI/sheathing/air and water barrier material with an MCM panel that has a demonstrated ability to perform well under the NFPA 285 test protocol.
- Using ASTM E 330, load test the CI/sheathing/air and water barrier material with the proper wall construction (studs) and determine the required fastener type and placement that will, without the MCM cladding system, accommodate the full load design requirements.
- Identify a high-performance rainscreen attachment system that has successfully been tested to the AAMA 508 standard. Merge all of these components into the final wall assembly, perform and pass the Florida impact and load cycling tests, TAS 201, 202, 203, as well as the ASTM equivalents E1886 and E 1996.
- Develop and have a Florida listing published that provides both the assurance of performance as well as a design guide for the optimal wall assembly construction balancing cost with performance-to-project specific load requirements.
Three companies, Altech Panel Systems, Cartersville, Ga.; Rmax, Dallas; and Mitsubishi Plastics Composites America Inc., Chesapeake, Va.; developed the solution: the R-TRAC HVHZ Pressure Equalized Rainscreen System.
This pressure-equalized system has been tested and listed with the Florida Building Commission to demonstrate its ability to meet and exceed all requirements for CI, fire, air, water and wind loading. The Florida listing report identifies various design pressures and construction requirements needed to properly install the R-TRAC HVHZ system in any area of the country. Details regarding this report can be found on the Florida Building Departments website.
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Bill Yannetti is the director of operations at Mitsubishi Plastics Composites America Inc., Chesapeake, Va.; Doug McIntyre is the project director at Altech Panel Systems Inc., Cartersville, Ga.; and Jay Saldaña, PE, is a research and building science engineer at Rmax Operating LLC, Dallas.