Since I need more than drywall to support r 60+ cellulose in my ceilings I'm planning to add OSB.  Since I'll have roof trusses above the ceiling I trying to evaluate installation methods.  I currently favor attaching it, the OSB, to the bottoms of the trusses.  So far simple.  Once that' done it's either air seal it and hang a bunch of 2x ? to make a " service cavity"  or roll the dice with "ADA" & go on framing interior walls etc.  Though it costs in labor and material the service cavity has the potential for much better long term performance an let's  blower door testing be done when corrections are less painful.   The ceiling height can be as designed with the 'service cavity but it means building the walls, that support the trusses, a bit taller.   Plain old OSB is a fine air barrier but it's supposed to be installed with 1/8" gaps all around the perimeter.  On walls one can caulk the entire perimeter with acoustic seal during installation (I know it's hard to get good help but it works in theory).  But In the case of the ceiling to do this means adding lots of blocking between trusses, there must be a better cheaper way.   Does taping all the seams work long term?  What is the best tape?   Or should I bite the bullet & install the blocking?  Is ZIP sheathing a  better way, allows taping without blocking & is "proven"?  How about Thermostat or other foil faced sheathing & if yes what tape?  The thermostat adds a radiant barrier that's of doubtful value.

Air seal it from the attic side, its a better result than ada because instead of hoping you get it right, it's as if you can physically be in the "wall" and ensures a thorough installation assuming you have the room to manuever in the attic. 1 part, 2 part, sealant, eco seal....pick your poison.

Secondly, I think overall it would be cheaper to frame as normal, build all interior walls, set the roof then hang the osb as if it were drywall on a room by room basis, then air seal the entire ceiling plane from the attic side.

I would forget the "service cavity" and forget the blocking at all seams, seal those seams from the attic side with a good quality sealant. I wouldn't waste the money on zip sheathing but that's just me.

I happened upon yet another alternative, which I kinda like. STO 'Emerald Guard' and fabric tape on the seams only. From what I've read it should work, & last, very well with no blocking needed. I believe green tree is suggesting spray foam from above. This is difficult or near impossible as "maneuvering" in my attic will be difficult, due to headroom, within about 8 feet of the eves even though I'll have room for about 2 foot of insulation at the eves. I do believe the STO system will be more economical than spray foam as well. Delaying the interior walls till after the roof is waterproof is in the interest of suffering as little rain wetting as possible. I want to minimize the rain risk by doing no more than is absolutely necessary between floor sheathing and roofing. So my plan is sheath floor, build and erect exterior outer walls, build and erect exterior inner walls, install top caps tying inner to outer walls, set and brace trusses sheath roof & gables, lay felt, install purl ins with drainage, install flashing at gable to slope transitions, install standing seam metal. Whew, it's rain ready. If I go with the STO system, I'd make the capping of the exterior walls out of thirds of a sheet of OSB installed flush to the exterior (about 12" thick double wall assembly), seal the seams in the cap from the top & wrap over the interior edge & onto the bottom as well. Then tape the outer edges of the cap to the sheathing on the outside of the exterior wall. That way when, after the roof is on, the interior ceiling OSB was hung, taping it's perimeter seam to the capping would transfer the sealed side from top to bottom.