Metal Composite Material (MCM) rainscreen cladding systems have been popular for decades due to their versatility in various cladding applications. They often perform well for many years when designed, fabricated, and installed appropriately. However, improperly executed systems can cause issues for building owners and project teams. This article will explore key aspects of MCM cladding, lessons learned from past projects, and best practices for future applications.
Sheet MCM panel material generally consists of two thin sheet-metal “skins” with an inner non-metal core. The metal skins, typically aluminum (though sometimes other metals), are approximately 0.5 mm (0.020 in.) thick and cover both sides of a plastic-based or a mineral-plastic core, which is approximately 3 to 5 mm (0.12 to 0.2 in.) thick. Unlike sheet metal, MCM panels cannot be easily bent, so grooves are routed into the backside of the material to allow for bending, leaving only the outer sheet metal skin at back-routed conditions. Stiffeners, usually aluminum extrusions, are often used to reinforce panels and panel edges to help increase rigidity, limit distortion and deflection, and facilitate attachment.
Thermal movement and attachment
The aluminum skins of these panels have a high coefficient of thermal expansion, making them prone to distortion (oil-canning) if not properly designed and installed. To mitigate this effect, attachment systems typically include slotted holes, sliding clips, or other provisions that allow for thermal movement. Insufficient stiffening, especially on larger panels, can lead to excessive deflection and distortion, underscoring the need for both thoughtful panel composition/fabrication and attachment strategies.
Subframing
MCM panels are normally installed over subframing systems that support the panels, create a drainage plane and allow for exterior insulation. Traditional subframing consists of a layer (or grid pattern) of horizontal and/or vertical metal girts, though thermally-broken girt systems are growing in popularity to reduce thermal bridging effects and meet more stringent energy codes. Common “thermal” metal girt systems include bracket-and-rail systems (with only occasional discrete brackets interrupting insulation), perforated metal girts, and fiberglass girts.
Cladding support and attachment systems must be carefully designed and installed to account for the added flexibility of these less stiff thermal girt systems, and for the increased insulation depths (and increased cantilevering effect) required by some modern energy codes.
Design, engineering, and industry literature (or lack thereof)
Despite the long-standing use of MCM panels and known challenges with thermal movement, distortion, and excessive deflection, there is limited industry literature to guide their design and fabrication, with one of the first manuals published by the NRCA in 2022. However, in general, design guidance in industry manuals and manufacturer’s literature is still limited. Many architects lack experience in the detailed design of MCM panels and subframing systems, and the responsibility for the detailed design is often delegated to the contractor.
While seasoned panel contractors often have many years of practical experience with fabricating and installing MCM panels, there seems to be a general lack of experience or interest in the industry with respect to performing any engineering design or analysis for these systems. Calculation submittals are sometimes “required” as part of the architectural specifications but often are never performed. The strategies used for stiffening and attachment of panels on real-life projects are often based more on a contractor’s past practices rather than actual engineering calculations. This lack of rigorous engineering can lead to issues, particularly on tall buildings, in high-wind areas and at uniquely shaped panels, where improper stiffening and attachment strategies may lead to failure.
Lessons learned from past failures
Investigative work and research of past failures, such as panel blow-offs, reveal common causes and offer insights for future projects:
Insufficient clip engagement: Panels may disengage from their attachment clips if there is not enough interlock to accommodate thermal movement and fabrication/installation tolerances. Clips that disengage along one panel edge can result in eventual disengagement of an entire panel from a building. Improved engagement length and secondary “safety clips,” which can act as a backup measure, can provide added security.
Inadequate stiffening: Large, inadequately stiffened panels can be prone to deflection and eventual disengagement of clips if not properly designed or if poorly fabricated. Aluminum extrusions (stiffeners) should be installed along the backside of large panels at regular intervals, commonly 610 mm (24 in.) on center, and adhered with reliable methods, such as structural silicone or a tape/adhesive combination, to help prevent this problem. Proper stiffening also helps minimize risk of visual distortion (oil-canning).
Poor reinforcement of L-shaped corner panels and deep returns: L-shaped corner panels and panels with deep returns are particularly susceptible to disengagement of clips. It is common practice to bond stiffeners to the backside of large flat panels. However, experience shows there is no consistency in the industry on how to treat deep return edges on panels or 90-degree corner panels. Traditional perimeter extrusions are normally only ~25.4 mm (1 in.) deep and do not reliably reinforce panel returns that are greater than 25.4 to 50.8 mm (1 to 2 in.). Gusset plates, made from solid aluminum plate or MCM material, can help reinforce deeper edges, but their use is not widespread at this time. Without reinforcement, these panels may be prone to excessive flexure and displacement. It is best to carefully reinforce these panel shapes or avoid them altogether.
Flexible subframing: Subframing should provide adequate support to prevent excessive flexibility. Subframing systems should be designed for each specific project, with consideration given to panel shapes and sizes, wind loads, clip/attachment design, thermal performance requirements, and other factors. A grid of horizontal and vertical metal girts is often most effective, and cantilevered girt conditions around window openings should be avoided.
Ultimately, success with MCM rainscreen systems hinges on a proactive, detail-oriented approach—one that prioritizes engineering, coordination, and a clear understanding of the system’s limitations.
Greg Sassaman is a project manager for Simpson Gumpertz & Heger (SGH) with a decade of experience consulting on building enclosures of historic and contemporary structures. He specializes in the investigation, repair/restoration, and replacement of existing facade, glazing, roofing, and waterproofing systems. Greg also has experience with new construction projects, failure analysis, material selection, and performance testing.
Derek McGowan is a principal with Simpson Gumpertz & Heger (SGH) and has over 20 years of experience consulting on facade systems, glazing, roofing and waterproofing. He frequently consults on large and complex projects and has experience with various alternative project delivery methods. He has particular interest in metal panel cladding and glazing systems, he writes and presents frequently on facade topics and is an officer of ASTM Committee C24 Building Seals and Sealants.