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MCMs and Rainscreens—A Dynamic, Durable Duo

Mcn  Special Feature  April15 1 Lowrez

This weather-resistant wonder looks great and reduces structural requirements

Bob Dylan's 1962 song "A Hard Rain's a-Gonna Fall" has more relevance to the metal construction industry than just the Cuban missile crisis for which the tune was written. For weather events like hard rain, hurricanes, snowstorms and heat waves, metal composite material (MCM) panels are increasingly being specified by building owners and architects for rainscreens.

Because they can be curved, bent and joined together in diverse configurations, MCMs make an attractive visual element offering low-maintenance longevity. Especially effective for rainscreens, they produce a reliable building envelope that resists the elements and protects against air and water infiltration.

Two sheets of metal sandwiching an extruded plastic core material comprise a MCM panel. A flat and rigid plank of material is the result. "The outer metal sheets of the MCM provides the strength required to withstand wind pressures applied to the MCM as well as to provide the rigid component that allows the MCM to be formed into complex shapes," says Tyler Bruton, PE, product development and technical specialist at The Miller-Clapperton Partnership Inc., Austell, Ga. "The plastic or fire-retardant core (FR core) material gives the product extremely elastic properties allowing the MCM to deform while under load and return to its original shape without permanent set deflections. The combination of the metal skins and plastic core enables MCM panels to be extremely durable and sturdy when used in rainscreen systems."

 

Resist the Rain

MCM panels are non-porous and impermeable, making them ideal for systems designed to divert and keep air and water from penetrating building envelopes. As part of a pressure-equalized rainscreen they can quickly shed water and allow the wall assembly to dry out more effectively.

"The panels can be fabricated to form compartments to manage and balance the air pressures between the interior and exterior of the system," says Melissa Grimes, marketing manager at Laminators Inc. in Hatfield, Pa. "When water and moisture does enter the wall cavity, the drying process can begin immediately without having to wait for the cladding to dry. Unlike other types of cladding that absorb moisture and can swell, the MCM panels remain flat and securely in place."

On most MCM rainscreen systems there is a MCM joint filler piece called a spline. "With correct fitment of the MCM spline, along with a properly installed MCM system, you can achieve a rainscreen system that allows less water infiltration than other types of rainscreen systems," says Greg Samples, operations manager at The Miller-Clapperton Partnership. "Many other rainscreen systems use an open joint that allows water to more easily infiltrate the rainscreen system."

Bruton acknowledges that there are many different classifications of MCM rainscreen systems; some minimize the amount of water and air penetrating the MCM system while others allow for substantially more air and water to penetrate the system. "MCMs are ideal regardless of what classification is trying to be obtained due to the fact that they can repel as much air and water from entering the rainscreen system as the designer has allotted for," he says.

And the fact that some MCM rainscreens do allow penetration must be accounted for. Dan Nicely, managing director, VMZINC USA + Mexico, Umicore Building Products USA Inc., Raleigh, N.C., contends the very nature of a MCM rainscreen allows for the infiltration of air and water. "This acknowledgement up front in the design process allows the rainscreen to manage those variables, instead of taking an approach of trying to keep them out of the assembly, usually through temporary and easily foiled means of 15- to 25-year caulking or sealants," he says. The assembly actually needs air flow to really perform as it should. Water drainage at the cavity or at each individual panel not only deals with the moisture from without, but also with the moisture from within the building. The airflow can be instrumental in making this happen."

 

Appearance, Aesthetics

One of MCM rainscreen's biggest drivers is the growing list of available colors (both standard and custom), finishes, materials and designs available for them. "Multiple applications and design opportunities, accessories, trims and integrated profiles are available to enhance the architectural make-up of the project," says Brad Kirkland, regional manager at Kingspan Insulated Panels Inc., Deland, Fla. "Various profiles, radius panels, irregular-shaped panels, trimless ends, finishes and colors contribute to the architectural appeal."

Pre-patina copper is a recent introduction for MCMs. "Pre-patina copper is a chemical process that accelerates the natural patina process of copper materials," says Doug McIntyre, director of research and business development, Altech Panel Systems Inc., Cartersville, Ga. "Instead of waiting 20 years for your copper to patina naturally you can now have the exact same finish in about four to six weeks. Pre-patina copper will also have an organic clear coat applied to the surface that will help protect the finish. Over time this organic clear coat will breakdown exposing the natural copper to the elements. This will allow the copper to begin its natural patina process."

Nicely says zinc composite material usage is growing for MCM rainscreens for what has traditionally been a painted or aluminum market. "In the case of natural metals like zinc, one is looking at a material that will self-heal minor scratches," he says. "In addition, the panel is bonded to a fire-rated core and then bonded to another skin of zinc. This sandwich of zinc-FR core-zinc provides the panel with the correct expansion and contraction coefficients on each side of the panel, a self-healing skin and a natural longevity due to the patina process. This equates to a win, win, win scenario for a rainscreen assembly."

In addition to these natural metals, new MCM rainscreen finishes include wood grain finishes and stone finishes. McIntyre says he has even begun to see the use of embossed face sheets on MCMs. One MCM rainscreen design component that aids its aesthetics is that it has no exposed sealant at the panel-to-panel joinery according to Bob Griffiths, architectural technical sales specialist at Firestone Building Products, Anoka, Minn. "No exposed joint sealant means the system has a clean look with little to no maintenance," he says. "In addition, many designers will use the sealant-free panel joinery as a design opportunity to incorporate panel reveals, shadows and colored joint features. [Since] there is no exposed joint sealant to add after the panels are installed, this is where installation time can be saved."

 

Installation

Another bonus of MCM rainscreens is because of their significantly lighter weight, they requires less people and equipment to install them. With this decreased panel weight, larger panel sizes can be fabricated and handled with greater ease. Also, "Their attachment systems are designed for easier installation," says Jackie Joyner, engineering manager at The Miller-Clapperton Partnership. "Most MCM panels arrive on-site ready to be installed while other alternative rainscreen systems might require pre-installation layout, sub-framing members, clips, angles, etc."

Typically, all rainscreens will have some sort of attachment system that will hold the rainscreen system off the face of the substrate. This system is typically an extruded aluminum system or galvanized steel. These components have been designed to withstand the loads that are exerted upon them during a weather event. "However, with modern energy codes progressing to higher energy efficiencies, the need for exterior insulation arises," says Brian Nelson, general manager, Knight Wall Systems, Deer Park, Wash. "This is one of the primary purposes and reasons a rainscreen attachment system is used. It provides a solid structural substrate for the panels to attach over the exterior insulation. Additionally, a well-designed rainscreen attachment system will have minimal effect on the exterior insulations R-value by keeping thermal bridging minimal without impacting the versatility of the panel layout."