Underwriters Laboratory and Intertek list a dizzying array of tested perimeter fire containment systems, more than 300 by some estimates.
These archives represent the industry’s best thinking about how best to design perimeter wall systems for life safety. With so many proven assemblies to consider, signifying a collective investment of hundreds of millions of dollars over several decades, it’s important to identify the qualities common to all winning systems.
What is the common design criteria? What makes one system acceptable and the next one unacceptable?
“The UL and Intertek listings represent an enormous amount of successful fire containment understanding. But that impressive body of work is built on insights gleaned from hundreds of failed assemblies tested internally, as well as at UL and Intertek,” says Angie Ogino, technical services leader at Owens Corning. “Recognizing what doesn’t work is just as important as what does work.”
So, what works?
Ogino says it boils down to six design criteria. “Miss one requirement and you risk premature failure of the entire perimeter fire containment system,” Ogino says.
The six criteria:
- Reinforcement of the Curtain Wall Insulation
- Mineral Wool Insulation
- Mineral Wool Mechanically Attached
- Compression-Fit Safing
- Mullion Protection with Mineral Wool Insulation
- Smoke Barrier
Much can be said about each requirement, starting with the what, why, and how behind each of the six. Consider the first one for example: providing reinforcement of the curtain wall insulation. This is a critical design element since the safing insulation is typically installed under 25-30 percent compression between the curtain wall insulation and the floor slab to provide a tight seal to block the propagation of internal fire spread. If the curtain wall insulation is not reinforced with either an approved backer/reinforcement member or by other means of reinforcement with the use of specialty hangers, the curtain wall insulation will bow and create gaps and seams at the curtain wall/safing interface that will allow flame and hot gases to spread to the floor above. Ogino notes there are systems that eliminate the need for a backer/reinforcement element. But special conditions must exist with specific tested and listed designs observed.
“This backer reinforcement supports the vertical mineral wool insulation,” explains Ogino. “You’re looking for a 25 to 30 percent compression of the mineral wool batt, ensuring a tight fit. A gap of even 1/32 of an inch is plenty for hot gasses on the lower floor to pass through the safe-off joint and ignite combustibles on the next floor. To help ensure survivability, the support member must be made of steel and at least 20-gauge.”
Central to the perimeter wall fire containment strategy is mineral wool. “It’s the only insulation product proven to handle temperatures over 2,000 degrees Fahrenheit for up to five hours,” says Ogino. “But mineral wool must be mechanically-attached, say with positive means of mechanical attachment such as the use of screw or weld attachment. Friction fitting isn’t acceptable.”
Testing has shown that a burning building is in constant motion from the heat-induced expansion and contraction. Anything not bolted down in the containment system, notably mineral wool, will ultimately surrender to gravity.
That’s why mullion protection is a core requirement as well. If mineral wool doesn’t protect the interior face of the aluminum framing, you risk losing all connection points for the mechanical attachment of the curtain wall insulation as the aluminum melts and falls away, cautions Ogino.
She also points out a building fire’s most lethal component isn’t direct flame or heat, but suffocating smoke. Implementing the sixth design criteria attacks the smoke problem with a spray-applied barrier system. “All containment systems include a fluid elastomeric sealant that goes on wet and dries to a rubbery consistency,” Ogino says. Barrier elasticity anticipates likely joint movement, helping seal-off smoke.
In the video below, Ogino discusses these six criteria:
As you weigh your perimeter fire containment system options, keep fire containment design criteria foremost in your planning. To learn more, check here.