Rainscreens: Engineering and Installation Issues
by Paul Deffenbaugh | 1 May 2023 12:00 am
Comprised of sub-assemblies and components, learn rainscreens’ many engineering and installation issues
With rainscreens, more care and detailing is needed at the joinery at the perimeter of the panel materials. The fewer the joints, the more efficient the design. The National Museum of the Army is one such example, where 60-square-foot panels were used efficiently.
Photo courtesy of Metalwërks
Their sub-assemblies and components are typically comprised of an air barrier system and an exoskeletal system commonly called a cladding support system. “The modern cladding support systems serve three primary purposes: to create a cavity for the use of exterior insulation; transfer the loads from the cladding through the layer of insulation to the back-up wall; and support the cladding in service,” says Brian Nelson, general manager, Knight Wall Systems, Deer Park, Wash. “The final, and arguably most significant, sub-assembly of a modern rainscreen system is the cladding. Engineering, coordinating and installing all these parts and pieces can present many issues or problems.” Because of this, there are many engineering and installation issues that should be adhered to.
Correctly Designed and Tested
Correctly designed and tested rainscreens must meet industry standard project wind loads, structural and flatness requirements as well as AAMA 508. The Fenestration and Glazing Industry Alliance issues testing methods for rainscreen wall cladding systems referred to as AAMA standards. “AAMA 508 measures the performance of pressure-equalized rainscreen (PER) systems on pass/fail criteria,” says Michael Bowie, technical support specialist, Mitsubishi Chemical America Inc.–ALPOLIC Division, Chesapeake, Va. “AAMA 509 measures the performance of drained/back ventilated (DB/V) rainscreen systems on a classification system that measures moisture accumulation down the air water barrier (AWB) as well as the capacity for air to circulate.”
Photo courtesy of ALPOLIC Division of of Mitsubishi Chemical America
With all rainscreen products, an important variable is identifying how efficiently and how quickly pressure is equalized. In AAMA 508, the pass/fail testing protocol for pressure-equalized wall cladding systems, the time it takes for the pressure in the space between the air barrier and outer plane is measured. “The pressure in the joinery must equalize to the applied pressure within 0.08 seconds after each pressure change, and peak pressure differentials must be less than 50%,” says Stephen Scharr, president, director of business development at Metalwërks, Kennett Square, Pa. “This is almost simultaneous for a reason. The differential pressures between the two spaces create a moisture-drive condition, which will force laden air and rain into the cavity.”
Mitigating capillary action between the inner and outer layers is achieved by reducing the pressure differences rapidly. This requires design considerations that block wind-driven rain by providing a barrier to entry. Scharr explains that one way to do this is to install panels that have an upturned leg on the top portion of a lower panel. “That upturned leg is mounted behind the panel above it as a moisture barrier in a shiplap fashion. This is one of the most common characteristics in metal rainscreen wall products across the industry. Ideally, open spaces in the joinery where such panels meet, as well as baffled weeps, will provide circulation through this area, and thus, immediate pressure equalization.” In D/BV rainscreens, there’s less worry about permitting moisture and wind-driven rain through the rainscreen joints.
Photo courtesy of Knight Wall Systems
Scott Samson, national sales manager, American Metalcraft Inc., Atlanta, explains that part of achieving a successful rainscreen installation is to “make sure the building has the proper ice/water shield, proper field measurements and attachment/sub-girt system. Make sure that whatever system you choose can be designed to meet size of panel and wind load criteria specific to your project. Choosing the correct sub-girt system, if applicable, and proper layout is also an important decision that deserves proper consideration.”
Rainscreen Weight Considerations
The weight of a rainscreen is an important consideration because it is the bridge back to structure. A heavier cladding material, double-wall terra cotta for instance, will typically require more attachments when compared to a lighter material such as single-skin aluminum panels. Michael Moorhouse, partner and design consultant at ISA architectural, Grasonville, Md., explains that while terra cotta is viewed as a premium rainscreen option from an aesthetic and durability standpoint, the added weight is harder to handle during installation. “It will require more attachment points and lengthen the installation time/project schedule equaling higher overall project costs.”
Lightweight rainscreens will apply less dead load onto the surface of the wall structure. With heavier cladding systems, Scharr explains that “more internal reinforcement is required to resist dead loads and live loads on cantilevered surfaces like cornices, slab edge covers, and projecting sunshades or fins. Resin-laden plastics like fiberglass reinforced plastic, glass fiber-reinforced concrete, and other solid, surfacing materials tend to be much heavier on a square-footage basis than an aluminum-based products.”
Photo courtesy of Laminators Inc.
Bowie agrees that more complex cladding assemblies may add more load. A heavier rainscreen requires greater structural supports. “The further away materials are from the wall structure, stronger support structures are required. Lightweight materials, such as metal composite material, help mitigate weight concerns.” Larger rainscreen cavities will increase the loads on the connections and heavier cladding components will too. These two scenarios will result in more connections or stronger connections, being required to secure the system to the back-up wall.
In addition to weight, panel size is a concern and must be reviewed during the design phase. Moorhouse contends whether it’s selecting the correct gauge, alloy type and panel measurements for single-skin metal panels to reduce risk of oil canning, or understanding panel yield for open-joint cladding based on standard sizes: high-density fiber cement, phenolic, etc., early design help from architects is critical. “To me, education and communication is key to obfuscating potential engineering or installation issues with rainscreens.”
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