Air Barriers and Metal Panels

Change is on the horizon for green building—and that's the case for metal cladding systems, too. Among those major changes, says MBCI's Buchinger, is the increasing reliance on air barriers to ensure that façades do not leak or let in air—that is, by unintended exfiltration or infiltration.

Some façade products provide for a “backup IMP” that serves as an air, water, thermal and vapor barrier panel that can be used behind any type of façade. These backup panels are installed in a horizontal orientation exterior of the perimeter structural supports. A separation from the interior of the building is required by IBC, to provide a cavity to allow for moisture drainage. A minimum 1/2-inch gypsum board is also required on the interior to provide a 15-minute thermal barrier. The cladding over the backup IMP can be a single-skin metal panel, brick veneer, terra cotta, high-pressure laminate or one of many other options. “There are no thermal bridges to reduce the energy efficiency of the wall,” Buchinger explains, “and it's an economical solution to conventional air, water, thermal and vapor control without sacrificing thermal efficiency.”

In this way, IMPs provide a number of cladding needs—insulation, durability, structural capacity, and a continuous air barrier in single, relatively easy-to-install panel. Unlike fiberglass insulation, which can become compressed in the building cavity, compromising its effectiveness, the IMP maintains its form for the life of the building. Even more critical, the air barrier is continuous, self-supporting and integral to the enclosure system—without requiring significant coordination among various construction trades.

In his WBDG article, “Air Barrier Systems in Buildings,” one of the foremost U.S. experts on the subject, Wagdy Anis, FAIA, of Wiss, Janney, Elstner Associates, Inc., describes three kinds of air leaks through an architectural enclosure:

• Orifice flow. An example is “a crack between a window rough opening and its frame,” says Anis.

• Diffuse flow. This describes materials that are ineffective at controlling air infiltration and exfiltration, such as uncoated concrete block.

• Channel flow. The most prevalent problem in envelope design, this describes air leaks where “the air entry point and exit point are distant from each other, giving the air enough time to cool below its dew point and deposit moisture in the building enclosure.”

As Anis and other experts such as Joseph Lstiburek, Ph.D., P.Eng., principal of Building Science Corp., Westford, Mass., have emphasized, the problem of uncontrolled air movement is a moisture management problem, too.

To deal with these air leaks and potential moisture damage, panelized metal wall systems and IMPs have been used to create an effective barrier, according to Wiss, Janney, Elstner's James N. Chiropolos, RA. A secondary air/moisture barrier is not needed for face-sealed systems, while rainscreens and drainage-type systems do require the dedicated air barrier system behind. In these cases, choice of the barrier depends on various factors, including the metal panels used, the sheathing type and even anticipated HVAC needs.

In all events, the key is to specify and detail the panelized metal façades properly, according to Chiropolos. Metal will keep water out as long as panel joint design is effective, and if the panels constitute an air barrier, the system's face-sealed joints must be carefully designed and coordinated—and, later, inspected during construction. Among the key design considerations are the tolerances for joints considering the relatively high coefficient of thermal expansion that architects should expect with metal panel systems. A 20-foot aluminum extrusion experiencing a 100 degree F temperature increase will expand about 0.30 inches, Chiropolos points out.

In addition, sealed joints typically require returns on the edges of the metal panels, Chiropolos notes. Regardless of the sealant used, the thickness of the metal panel is critical to proper adhesion—as is the use of two-sided adhesion details to allow for joint expansion and contraction.

Architects can work with experienced installers as well as manufacturers on all the variables that affect cladding thermal movement—and therefore air barrier effectiveness. The proper design process includes reviewing the size and orientation of panels, their location on every façade exposure, and the specific tolerances of the IMP under consideration. When these aspects are considered, the resulting buildings not only work well, but they'll also look great for a long, long time.

C.C. Sullivan is a marketing and content consultant specializing in the AEC industry.

 

 

MBCI is the leading manufacturer of metal roof and wall systems. Product solutions include both single-skin and insulated metal roof and wall panels and retrofit solutions. www.mbci.com