It will help reduce air movement through the batts within the joist. Moisture drive may be affected as plywood can be considered a vapor retarder. To what level of effect it will have depends upon the installation, thickness, and quality of the sheathing.Going to help a buddy insulate the ceiling of his home addition, and the initial plan is to roll fiberglass on top of and perpendicular to to the joists, working through the joists, then packing between the joists from below. At some point I happened to think it would make laying the top layer of insulation easier, and provide better mechanical access, if we sheathed the tops of the joitsts with say 3/8" plywood or something along that line. Anyway, nothing new or radical, as floored attics are insulated routinely, but my question is, given the option, would the inclusion of this sheathing help, hinder, or have no affect one way or the other in regard to insulation, moisture, etc. Thank you.