Successful Perimeter Fire Containment

Shedding light on common misconceptions about PFC systems
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Sponsored by Owens Corning
Presented by Juliet Grable

Basic Criteria for the Design and Installation of PFC systems

Most curtain wall assemblies are designed around a tested and listed third-party PFC system. Though these systems vary when it comes to exterior spandrel panels, heights, and locations relative to the floor, most share six basic design components, which are critical to ensuring the system functions and contains fire to the room of origin, allowing occupants time to escape a burning structure.

Firestop system designs are tested and listed by independent testing agencies, such as UL and Intertek. It is important to note that, although third-party fire-resistance directories include hundreds of tested PFC systems, many architectural designs do not match these systems exactly. The design professional will almost always need to seek an engineering analysis or judgment to address any deviations in the designed system from a tested system.

An engineering judgment, or EJ, is an evaluation of the anticipated performance of a proposed firestop assembly that has not itself been fire tested. The evaluation is conducted by comparing the proposed system to listed and tested systems that is similar in nature. In any case, systems requiring EJs must still address the six critical criteria outlined below.

  1. Use Underwriters or Intertek Laboratories approved mineral wool insulation tested to ASTM E2307. Mineral wool insulation, at the required densities and thickness, is the only tested and proven material that will provide protection to both the curtain wall spandrel and the interior joint. Of the many insulation options available, mineral wool is best suited to the challenges of PFC, primarily because mineral wool has extremely high melting temperatures (upward of 2,000 degrees Fahrenheit). Mineral wool is the only insulation material that has been tested and proven to protect spandrel wall components. However, not just any mineral wool insulation will do, and it must be tested and approved in UL or Intertek designs as per ASTM E2307.
  2. Follow the required mechanical attachment method per the tested and listed ASTM E2307 compliant system for attaching the approved mineral wool insulation. During a fire, building components are subjected to turbulence, movement, and gravitational pull. Without mechanical fasteners, the insulation can become dislodged, allowing fire to propagate to the next floor. A range of fasteners may be used to attach mineral wool insulation to the curtain wall. However, the fasteners must be installed per the UL/Intertek listing’s installation requirements to make sure that the system functions as it was designed in the event of a fire.
  3. Provide backer reinforcement at the safe-off line per the UL/Intertek listed assembly. All systems require some type of reinforcement of the mineral wool insulation at the safe-off line. This prevents the spandrel insulation from bowing due to the compression force at the safing joint. Most listed systems reference either a 20-gauge steel T-bar, L-angle, or hat channel, but other systems may use different components to reinforce the curtain wall insulation.

    The reinforcement also ensures a tight seal at the interior joint. If the joint is not sealed properly, the spandrel insulation will flex, creating gaps or seams where flames and gases may penetrate and potentially ignite combustibles on the floor above.

    A common misconception is that metal panels such as aluminum or steel back pans will provide the necessary reinforcement. However, testing has proven these panels to be a failure point at the safing line if not properly reinforced, no matter what the material. We will discuss steel back pans in more depth later in this course.

    Note that some listings do not require backer or reinforcement members; in these cases, the design has specialized components, such as the location of the window sill transom in combination with mechanical fasteners or additional mineral wool insulation to provide the support that is required to maintain compression at the safe-off void.

  4. Compression-fit UL/Intertek approved mineral wool safing insulation must be installed within the void between the floor assembly and the exterior curtain wall insulation per the tested and listed system. The mineral wool insulation must be of the correct density and compression to create a tight and proper seal at the interior joint, so that gases cannot pass through the joint. Safing can be installed with the fibers running either vertically or horizontally. However, designs are very specific about approved materials as well as fiber orientation, depth, density, and compression of the installed safing.
  5. Exposed vertical aluminum framing must be protected with UL/Intertek approved mineral wool insulation mullion covers. Because this detail is seen as contributing little to the performance of the assembly, mullion covers are often removed from the system, especially if they obstruct aesthetic elements, such as interior finishes or window shade pockets. However, these covers play a critical role. They protect both the aluminum framing and the mechanical fasteners that keep the spandrel insulation in place. The framing also helps keep the exterior wall in position so that the safing joint materials continue to block fire and smoke. Aluminum will melt at 1,220 degrees Fahrenheit, or as early as 9 minutes into a fire. If mullion covers are eliminated, the exterior wall may fail sooner, causing loss of compression of the joint safing material. The end result is a system that provides a much shorter window of protection than required.
  6. Smoke must be prevented from passing through the safe-off area per the approved tested and listed UL/Intertek assembly. Smoke inhalation is responsible for the majority of fire-related deaths. To prevent smoke from entering the safe-off area, smoke sealant must be applied on top of the safing insulation on the nonexposed side of the fire-containment system. This essentially creates a smoke barrier that compartmentalizes the smoke and keeps it from passing to another compartment. Specific UL or Intertek designs include approved smoke sealants. The smoke seal is commonly spray-applied to the top, or non-fire exposure side of the safing, forming a smoke barrier which contributes to the assembly’s L rating or leakage rating.

 

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Originally published in October 2020

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