For the 2016 NFPA 13 code book it appears that 220.127.116.11.6 and 18.104.22.168.8 contradict each other.
22.214.171.124.6* 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.
126.96.36.199.8 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 188.8.131.52.8? Couldn't you call that out and not use the extra 3.5" required per 184.108.40.206.6?
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