April 18, 2016
Impact of Insulation on the Allowed Volume in Joist Channels
We have a project where the contractor has fire blocked the areas between grid
lines at 13 ft on center, the length is 30 ft, and the joist is 12 in. where
that equals 390 cu. ft. The area is filled with insulation within 4 in. from
the plywood roof that creates an area of 13 ft x 30 ft x 4 in. that equals 117
cu. ft. Do we measure from the entire TJI depth or from the insulation?"
We have reviewed NFPA 13, 2016 edition as the applicable standard. Our
informal interpretation is that the volume is based upon the open air within
the channel and does not include the portion filled with insulation.
This is an interesting question since it is another example of many items that
was assumed to be understood. In reality the standard simply says the joist
channel which literal means the whole channel. That is not the intent and
fortunately there are enough related requirements to shed some light on it.
The most obvious is the allowance to omit sprinklers when the space is filled
with insulation. If the size of the space is irrelevant when it is filled,
this tells us that it is the open air (where the fire will actually develop)
that is the concern. The fact that the insulation is considered a thermal
boundary the same as a fixed the ceiling (as discussed in Section 8.5.4.1.3)
for the allowed distance below the ceiling, tells us it is robust enough to be
the bounding edge in assigning the boundary for the allowed void within a joist
channel. The last related section is Section 8.15.1.2.8 that allows for the
omission of protection when the space between a drop ceiling and the bottom of
the joists is filled with insulation. It too imposes the common 160 cu. ft.
restriction on the joist channels. If the insulation itself were not the
defining boundary in determining the volume, then the area beneath the joist
would also need to be included in the determination of the volume. This
terminology originated with the ceiling being tight to the bottom of the joist
and there was no expectation that insulation would also be installed. When the
section was expanded to account for the sound channel at the bottom of the
joist (and the related 3-1/2 inches of insulation), it would have been
appropriate for the committee to address this nuance. At the time, though,
they were focused on addressing the potential for fire to pass between joists
due to the channel.
Technical Update is prepared by the Technical Services Dept. of the AFSA: Vice
President of Engineering and Technical Services Roland Huggins, a PE registered
in fire protection engineering; Phill Brown, a NICET IV certified automatic
sprinkler technician and NFPA Certified Fire Protection Specialist (CFPS) and
Tom Wellen, a PE registered in fire protection engineering. This is provided
with the understanding that the AFSA assumes no liability for this opinion or
actions taken on it and they are not to be considered the official position of
the NFPA or its technical committees.
Copyright 2016, American Fire Sprinkler Association. All Rights Reserved
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