24 CFR §3280.216

1. (a)When there is usable space both above and below the concealed space of a floor/ceiling assembly in multi-dwelling unit manufactured homes, draftstops must be installed so that the area of the concealed space does not exceed 1,000 square feet. Draftstopping must divide the concealed space into approximately equal areas. Where the assembly is enclosed by a floor membrane above and a ceiling membrane below, draftstopping shall be provided in floor-ceiling assemblies under the following circumstances:
   1. (1)Ceiling is suspended under the floor framing; or
   2. (2)Floor framing is constructed of truss type open-web or perforated members.
2. (b)Draftstopping materials must not be less than 1/2 inch gypsum board, 3/8 inch wood structural panels, or other approved materials adequately supported.
3. (c)Draftstopping must be installed parallel to the floor framing members.
4. (d)The integrity of all draftstops must be maintained.