NFPA 13 (2019) section 220.127.116.11 addresses a "concealed space" with studs or joists with less than 6" between the inside or near edge (of the stud or joist) and then references figure 10.2.6.1.5.1. Figure 10.2.6.1.5.1 clearly delineates no floor or ceiling on the lower set of joists. My contention is that the illustration does not depict a concealed space because the area below the lower joists (or on top of the lower joists) is not covered, but instead is related to section 18.104.22.168.5 "Double Joist Obstruction" section . I have a designer that has locked in on the "less" than 6" language above and does not think he needs to protect a true concealed combustible space because the nominal lumber 2 x10 floor joists above are only 4 1/2" above the 2 x 6 ceiling members below with drywall fastened to the bottoms. I would agree that sprinklers would not be a good choice in this instance however, the space still needs to be protected by ASTM E136 listed insulation filling the space. Any comments would be appreciated.