Why 3-1/2" of Insulation for Wood Joist Exception?

5/26/2020

For the 2016 NFPA 13 code book it appears that 8.15.1.2.6 and 8.15.1.2.8 contradict each other.

8.15.1.2.6* Concealed spaces formed by ceilings attached to composite wood joist construction either directly or onto
metal channels not exceeding 1 in. in depth, provided the joist channels are firestopped into volumes each not
exceeding 160 ft3 using materials equivalent to the web construction and at least 3-1⁄2 in. of batt insulation is installed at the bottom of the joist channels when the ceiling is attached utilizing metal channels, shall not require sprinkler protection.

8.15.1.2.8 Concealed spaces within wood joist construction and composite wood joist construction having noncombustible insulation filling the space from the ceiling up to the bottom edge of the joist of the roof or floor deck, provided that in composite wood joist construction the joist channels separated into volumes each not exceeding 160 ft3 to the full depth of the composite wood joist with material equivalent to the web construction, shall not require sprinkler protection.

If you had a insulation up to the bottom edge of your joist wouldn't you be able to exempt protection per 8.15.1.2.8? Couldn't you call that out and not use the extra 3.5" required per 8.15.1.2.6?

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5 Comments

Anon

5/26/2020 10:26:11 am

2.6 is describing a concealed space within the composite wood joists ("TJI's") where sprinklers may be exempted from between the joists. However, if they are attaching the ceiling with hat-channels, creating a small pathway between the bottom of the joists and GWB, then they need to lay some insulation on top of the GWB to seal-up this gap.
2.8 allows one to layer insulation from below, up to the bottoms of the joists.
Forgive my if I misunderstood the question, still working on waking up.

TJ Jones

5/27/2020 10:29:06 am

If the hat-channels add 1/2" of depth than wouldn't adding 1/2" of insulation in that space satisfy 8.15.1.2.8? But, at the same time doing that wouldn't satisfy 2.6.

Mike

5/26/2020 10:40:14 am

I am not sure if I understand your question. It's either one or the other. I don't think it means you need both 3.5“ and completely filled, though.

For background, TJI's burn like match sticks in fire tests. They are only made of small chips of wood and glue so the fire spreads faster through them hence the requirements for insulation and fire blocking thereby reducing thermal transfer. They are very lightweight and therefore have a shorter time for combustion to occur. They also become very weak and are prone to 100% failure during a fire unlike a wood joist.

This is also why they require a 3,000 Sq ft area, if I'm not mistaken.

5/27/2020 10:31:47 am

Let's say 1/2" of hat-channel exists. So if I add 1/2" of insulation as to "fill space between ceiling to bottom edge of TJI" I would satisfy 2.8. But, wouldn't satisfy 2.6. So why does the code require the additional 3.5" of insulation when hat channel is involved?

Jonathan Joseph

5/26/2020 12:23:31 pm

8.15.1.2.8 is referring when there is a concealed space between the drop ceiling below the composite joists. This is used when contractors do not want to use prelim drywall to separate the floors assemblies to their concealed space.

If the ceiling is directly attached to the composite joists and you fully insulate, you do not need to separate the spaces into volumes of 160 cubic feet and need sprinkler protection. 8.15.1.2.7

Why 3-1/2" of Insulation for Wood Joist Exception?

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