The Importance of Mullion Covers in Perimeter Fire Containment Systems
Are mullion covers being overlooked in your firestop inspections? Perimeter Fire Containment (PFC) systems are essential for preventing vertical fire spread in high-rise buildings—but there’s a critical component that’s often missing or ignored: mullion covers. Approximately, 95% of listed PFC systems require mullion covers to achieve their tested fire-resistance rating. Aluminum mullions can fail within minutes in a fire. Without proper protection, the entire curtain wall assembly—and everyone relying on it—is at risk.
This new IFC Technical Bulletin breaks down:
• Why mullion covers matter
• What the codes require
• How they impact system integrity and thermal performance
• When they’re required—and when they’re not
You can view the bulleting below. Or download a printable pdf here: The Importance of Mullion Covers in Perimeter Fire Containment Systems .
Want to learn more? Request our free AHJ training that covers Perimeter Fire Containment, Mullion Covers, and Engineering Judgments in-depth.
The Importance of Mullion Covers in Perimeter Fire Containment Systems
Perimeter fire containment (PFC) systems, commonly referred to as “curtain wall firestopping” are required by the International Building Code, Chapter 7, Section 715.4 in order to prevent interior spread of fire from one story to the next via the edge-of-slab gap.
IBC 2024, Chapter 7, Section 715.4 Exterior curtain wall/fire-resistance-rated floor intersections
Voids created at the intersection of exterior curtain wall assemblies and fire-resistance-rated floor or floor/ceiling assemblies shall be protected with an approved perimeter fire containment system to prevent the interior spread of fire. Such systems shall provide an F rating for a time period not less than the fire-resistance rating of the floor or floor/ceiling assembly.
715.4.1 Fire test criteria. Perimeter fire containment systems shall be tested in accordance with the requirements of ASTM E2307.
Whereas most inspectors seem to recognize the importance of the safing gap insulation and sealant, and will verify that they are installed per the referenced tested system, the importance and inspection of mullion covers seem to be often neglected. Approximately 95% of all tested and listed PFC systems require mullion covers in order to achieve the required fire rating for the PFC system.
The primary function of mullion covers is to protect the vertical aluminum mullions within the spandrel area from fire exposure, ensuring the integrity of the curtain wall assembly. Given that aluminum melts at approximately 660°C (1,220°F) and can begin failing as early as nine minutes into a fire, proper protection is essential to prevent premature system failure.
Fire Performance and System Integrity
A key function of mullion covers is maintaining the stability of the perimeter fire containment system. In a fire event, unprotected aluminum mullions can rapidly lose structural integrity, leading to the failure of mechanical fasteners that hold spandrel insulation in place. As these fasteners fail, the spandrel insulation can detach, causing the safing insulation to dislodge and fall out due to gravity. This compromises the fire containment, allowing flames and hot gases to spread vertically to the floor above via the building’s interior.
By installing mineral wool insulation mullion covers over the exposed faces of the mullions, the system slows down structural deterioration, ensuring that the curtain wall assembly remains intact for the required fire resistance rating of the floor assembly. The covers also help maintain the effectiveness of the perimeter fire containment system by securing insulation layers in place during fire exposure.
T-Rating Considerations and Thermal Bridging
Although T-Ratings are not a pass/fail criteria of ASTM E2307, T-Ratings are still measured and reported during E2307 testing. T-Ratings measure the temperature rise on the non-fire side of the assembly. This data can be used to analyze the contribution of mullion covers to the overall fire resistance of the assembly. Since aluminum is highly conductive, mullions without protection can transfer heat rapidly, potentially igniting adjacent combustible materials. Adding mullion covers can improve the T-Rating of a PFC assembly and mitigate this risk by acting as an insulative barrier, slowing heat transfer and reducing the likelihood of secondary ignition.
Additionally, mullion covers can be required in some PFC system designs to address thermal bridging concerns. In non-backpan conditions, they help knit the mineral wool batt insulation into a continuous fire barrier, further enhancing the system’s performance.
Methods of Mechanical Attachment
Where the design listing specifies mullion cover protection, mechanical attachment is required to secure the mullion covers to the curtain wall framing. Alternatively, specialty spiral anchors can be used to attach the mullion cover insulation to the spandrel insulation. Each PFC system listing outlines the specific type and spacing of required mechanical fasteners.
When Mullion Covers Are Not Required
Even though about 95% of all tested and listed PFC systems require mullion covers, a small fraction of available system designs may achieve the desired fire resistance rating without them. Those tested designs demonstrate that alternative methods can provide sufficient protection. It stands to reason that about 95% of engineering judgments (EJs) for perimeter fire containment should probably specify the need for mullion covers, unless the project condition matches a referenced system which does not require the need for mullion protection. Yet an abnormally and surprisingly large proportion of PFC EJs do not specify mullion covers. Those charged with review and approval of PFC EJs should stay aware that proposed EJ’s without mullion covers should require careful evaluation and additional scrutiny, considering factors such as spandrel height, additional protective elements, and anchor locations where flame penetration risks may be higher. Don’t be afraid to ask for the referenced tests or listings that would support the lack of mullion covers in a PFC EJ.
Conclusion
Mullion covers serve a critical role in ensuring the fire resistance and thermal performance of perimeter fire containment systems. By protecting aluminum framing, securing insulation layers, and mitigating heat transfer, they help maintain the system’s integrity under fire exposure. Fire testing done to obtain a PFC listing meeting ASTM E2307 will clearly indicate where and how mullion protection needs to be provided. While certain conditions may allow for their omission, careful assessment is necessary to ensure fire performance is not compromised when it is proposed to be omitted. As a best practice, incorporating mullion covers remains a highly effective strategy in perimeter fire containment design.
Download the PDF here: The Importance of Mullion Covers in Perimeter Fire Containment Systems
Want to learn more? Request our free AHJ training that covers Perimeter Fire Containment, Mullion Covers, and Engineering Judgments in-depth.
