ACM Panel Exterior Wall Assembly Specification

Photo courtesy of Henderson-Johnson Co. Inc.

This ACM installation over stone wool and a cold-formed metal secondary framing system required that designers pay special attention to moisture management and thermal bridging in the envelope.

After all of the discussion about the conflicts in the code, it may seem impossible to specify a code-compliant exterior wall assembly. However, with the right materials and the application of a few industry best practices, it is possible to satisfy the structural requirements of IBC Chapter 16, the energy code requirements of the IECC and ASHRAE 90.1, the water management requirements of IBC Chapter 14, and the requirements of NFPA 285. Unfortunately, the solutions tend to be based on proprietary assemblies and can never be specified in a manner where they could be competitively bid.

One such assembly is based on a generic wall that was tested to be NFPA 285 compliant by an aluminum composite material (ACM) manufacturer. The manufacturer tested the ACM panel in an installation system over a very generic, cold-formed steel-framed base wall with 5⁄8-inch glass-mat-faced exterior gypsum wall board on the exterior and 5⁄8-inch type X interior gypsum wall board on the interior. The glass-mat-faced gypsum wall board was coated with a fluid-applied, vapor-permeable air and water barrier. The cavity did not contain anything other than typical wall system installation components. The intent of the testing such a generic, typical assembly was to allow a designer to easily use the NFPA 285 compliance test as a basis for a code submittal package, which would then include the engineering judgement letters from the manufacturers of any component they intended to add to the wall assembly, including, but not limited to: alternative air and water barriers, insulation, and secondary framing elements (if combustible).

The designer added a proprietary glass-fiber reinforced secondary framing system that was designed to incorporate ISO rigid board insulation into the framing to create a managed drainage plane on the exterior side of the rigid insulation. The seams of the insulation were taped with a proprietary, pressure-sensitive acrylic sealant tape so that the moisture was exclusively managed at the face of the insulation. The building was under 40 feet tall so the glass-fiber reinforced secondary framing system met the exemption for NFPA 285 compliance requirements in IBC. Documentation of compliance with the exemption was supplied by the manufacturer of the proprietary framing system. It is important to note that it is atypical for a wall assembly component to be allowed exemption if another element in the wall assembly had triggered the need for NFPA 285 compliance, but in this case it was allowed.

Though the secondary framing system received an exemption from the need to meet NFPA 285, ISO insulation is required to be NFPA 285 compliant regardless of building height. The designer chose a type of ISO insulation that could maintain NFPA 285 compliance when placed behind an NFPA 285-compliant ACM wall system and was rated to function as the air and water barrier when properly taped and sealed. The manufacturer of the ISO supplied the engineering judgement letter to complete the demonstration of compliance.

The designer wanted to introduce a drainage cavity into the wall that had a greater depth than could be achieved with the standard ACM installation. To achieve a greater cavity depth without introducing additional code concerns, the designer incorporated 7⁄8-inch deep, vertically oriented, cold-formed metal hat furring channels that were fastened to the faces of the horizontally oriented glass-fiber reinforced framing system. Because the furring channel was steel, it was considered noncombustible and did not impact the NFPA 285 compliance of the wall assembly. Some fire experts suggest that the introduction of additional airflow associated with this larger drainage plane could impact NFPA 285 performance, but the overall cavity depth was the same as another NFPA 285-compliant system from the same manufacturer, so it was deemed acceptable.

It is important to consider that, while in this case the local code official and authority having jurisdiction were comfortable accepting the NFPA 285 compliance material, not all will be. It is important to interact with the manufacturers of the alternate materials being proposed in wall assemblies as well as the manufacturer that tested the base wall for NFPA 285 compliance. Local code officials will often defer to the engineers from the design team, if they are able to assemble a reasonable compliance package for NFPA 285 wall assemblies that include engineering judgement letters. Also, though this assembly incorporated elements to comply with all four of the code conflicts, it is very proprietary and could not be competitively bid.