Update on Metal Composite Material Wall Cladding Use
Originally published by: Metal Construction News — April 1, 2018
by Andy WIlliams
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It has been almost nine months since the Grenfell Tower fire tragedy in London that claimed 71 lives. Many articles and reports have been written about this project, and the construction and design of buildings in general using metal composite material (MCM) as a component in the exterior wall cladding. Additional, large, high-rise buildings have been identified in Dubai where similar wall cladding systems containing MCM panels have been used and exterior fires have taken place. Designers worldwide are asking whether MCM material, or any wall cladding assemblies containing combustible material, should continue to be used on high-rise construction. Before any type of judgment is made, let’s review the facts behind the building fires that have been reported.
MCM panels are generally used as the exterior facing on a building as an element that resists weather and provides a substrate for an architectural finish. The MCM panel is primarily comprised of either a polyethylene (PE) core or a fire-resistant (FR) core and is generally covered on both sides with a painted aluminum facing. The core material is extruded as a solid-sheet material, so the panel provides little insulation value for the building. The painted panels are typically installed on the building using aluminum extrusions that allow for a free-air cavity behind the panels, which aids in drying any condensation that may form. This cavity is often filled with an insulation material that is either foam plastic or noncombustible mineral wool. It is this insulation that helps in meeting the requirements of the local energy code.
For high-rise construction in the United States, which is generally buildings over 40 feet in height, the entire wall assembly is required to be tested in accordance with a standard set by the National Fire Protection Association called “NFPA 285: Standard Fire Test Method for Evaluation of Fire Propagation Characteristics of Exterior Non-Load- Bearing Wall Assemblies Containing Combustible Components.” There are similar test requirements around the world including the Canadian, “ULCS134 Standard Method of Fire Test of Exterior Wall Assemblies,” and the British Standard, “BS 8414 Fire Performance of External Cladding Systems.”
Each standard test is intended to show fire performance of the entire cladding assembly, which includes the panels, support framing, insulation material and weather barriers. It is important to test the actual assembly being used because an assembly may perform differently if any of these components is exchanged for another material. There is an alternative to testing each specific assembly, that involves an equivalency analysis performed by a design professional that is well-versed in fire behavior of construction materials.
Each of the high-rise fire events identified throughout the world, including Grenfell Tower and each of the Dubai fires, has involved MCM panels with a polyethylene core. This product is generally not used in high-rise construction, and the wall assemblies have not successfully met the test requirements of any of the full-scale tests listed here. In North America, both the International Building Code (IBC) and the National Building Code of Canada (NBC) have very limited applications that would be allowed in high-rise construction. Full-wall cladding, as used in London and Dubai, would not be allowed. In most locations regulated by these codes, the maximum height above grade for a combustible wall assembly not tested to one of the full-scale tests listed above is 40 to 50 feet. There are certain limited area applications that can be installed up to 75 feet above grade, but for all practical purposes the height limitation for assemblies not tested to one of the full-scale fire tests is quite limited.
The MCM manufacturers of the Metal Construction Association (MCA) have been involved for many years in the development of these limitations and test standards, and fully support the allowable use of the correct product for the correct application: PE for low-rise construction and FR for high-rise construction.
Even though these limitations are specifically defined in the building codes, designers and architects are thinking twice about the use of any MCM materials. In fact, one insurance group has reportedly indicated that MCM cladding systems should be comprised of noncombustible or Class 1 rated materials in spite of the fact that there have been no significant fire events in North America when cladding systems have been installed as tested to NFPA 285 or ULC S134.
Some designers have attributed this performance simply to luck that no fire event has happened. However, these same standards, NFPA 285 and ULC S134, are used not only for MCM cladding assemblies, but for all wall assemblies containing combustible components used above a certain height, whether as part of the cladding material— MCM or insulated metal panels (IMP)—or as foam insulation included in the wall assembly to meet the energy code requirements.
There is no doubt that the entire industry needs to continue to be concerned with the design of combustible wall cladding assemblies so that fire events, which have happened around the world, do not continue occurring. There is also no doubt that the codes themselves are being reviewed regularly to ensure that cladding assemblies must meet the performance requirements of these full-scale tests. Reviews and code change proposals are already being considered in North America to ensure that these performance requirements are being met. We must, however, remain diligent that the right products are being used in construction of any type in the same manner as they were tested to continue with the highest quality of safe construction.
MCM manufacturers and fabricators will continue to monitor and be actively involved in the further development of the codes and test standards to help designers fully understand the correct and safe use of MCM in construction.