Performance Standards For Metal Rainscreen Wall Systems
By Frank Resso, IMETCO
In recent years, rainscreen wall systems utilizing metal cladding panels have become an increasingly popular choice for architectural exteriors. Currently, there are few metal panel manufacturers that offer a weather tightness warranty on a complete rainscreen assembly, and water resistive barrier warranties are generally limited to replacement of defective materials only. So, the general contractor and subcontractors assume much of the liability for the weather tightness of these systems. The performance testing and requirements applicable to rainscreen wall systems may be less familiar to industry professionals than those for metal roofing systems. Principal among the testing standards for rainscreen wall panel systems are the American Architectural Manufacturers Association (AAMA) 508 and 509 testing protocols. These two standards aim to validate and quantify the weather resistance of a rainscreen wall system.
It is important to keep in mind that the rainscreen design principle relies on the metal cladding panel to resist wind loads, protect the wall system from UV exposure, limit water intrusion into the wall system, and allow trapped moisture to escape to the exterior of the wall system. As such, the two test standards that will be discussed evaluate the capability of the metal panel system to minimize water infiltration and maximize drainage and ventilation to the exterior.
The AAMA 508 standard, titled “Voluntary Test and Classification Method for Pressure Equalized Rain Screen Wall Cladding Systems”, is intended to demonstrate weather resistance by testing the panel system for static and dynamic water infiltration, and documenting achievement of pressure equalization between the interior leaf of the rainscreen assembly and the exterior of the wall.
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Dynamic water penetration test required by AAMA 508 and 509 standards.
Before the tests are conducted, the sample wall assembly is intentionally made to be “leaky” with respect to air tightness. Holes are drilled in the interior surface of the assembly (where the air barrier is located), until approximately 0.12 cfm/sf of air leakage is experienced (as per ASTM E283). This is to simulate an imperfect air barrier installation.
Next, the ASTM E1233 pressure cycling test is performed. As a cyclical pressure is applied to the assembly, the internal air cavity within the rainscreen assembly must reach equilibrium with the pressure chamber. The time allowed to achieve pressure equalization at each of the 100 cycles may not lag by more than 0.08 seconds to pass this portion of the standard.
Lastly, water infiltration rates are tested as per ASTM E331 (static pressure) and AAMA 501.1 (dynamic pressure). For each of these tests, water droplets observed over more than 5 percent of the air barrier area constitutes failure of the system.
The AAMA 509 standard, titled “Voluntary Test and Classification Method for Drained and Back Ventilated Rain Screen Wall Cladding Systems” provides quantitative measurements and performance classification values for rainscreen wall systems. In addition to measuring water infiltration through the metal cladding, this standard accounts for water drainage out of the system, and air ventilation through the system.
Again, a simulated “real-world” imperfect air barrier is calibrated prior to system testing. Next, a series of ASTM E283 air infiltration tests are conducted, isolating each of the assembly head, sill, and panel transverse and longitudinal joints. Leakage through these joints constitutes beneficial air ventilation which can remove trapped moisture from the rainscreen assembly. The tabulated air ventilation value is expressed as a rating from V1 (minimal ventilation) to V11 (extreme ventilation).
Finally, a series of static (ASTM E331) and dynamic (AAMA 501.1) water penetration tests are performed. For these tests, water that drains off, or penetrates through, the air barrier layer is collected and quantified. Again, a rating is applied from W1 (no or minimal water penetration) to W11 (excessive water penetration).
Both the AAMA 508 and 509 standards for rainscreen panel systems provide valuable performance testing criteria. AAMA 508, a pass-fail standard, can demonstrate that a system uses the principle of pressure equalization to limit water intrusion into the assembly. However, it does not demonstrate how the water that does penetrate into the system (or originates from leaks and diffusion elsewhere in the structure) is evacuated from the assembly.
The AAMA 509 standard does provide quantitative data regarding the amount of ventilation available to dry the interior of the assembly, and the amount of water that air barrier layer will experience. The standard can be used to compare different systems. However, it does not provide guidelines for what rating levels might be applicable for the desired performance level. There are generally trade-offs between ventilation and water resistance. Highly vented systems tend be less watertight, and extremely watertight systems tend to have limited ventilation capacity. In practice, high-performance metal panel systems tend to have AAMA 509 water penetration ratings of W1 to W2, with corresponding ventilation ratings of V2 to V4.
While each standard provides a measure of performance assurance in its own right, each standard is also limited in scope. It is recommended to specify and supply a system that has been tested, with excellent results, to both standards. This helps to achieve the primary function of metal cladding as a component of a rainscreen wall system, which is to limit the quantity of water intrusion from the exterior, and provide effective means of draining and drying trapped moisture from within the wall system.
The author, Frank Resso, PE, has been with IMETCO in a variety of technical and operational position for 17 years. Currently, he serves as Director of R&D and Business Development.
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