This post is a follow up to a Q&A on how to prep for a blower door test. 

My new construction house has zip/taped exterior walls and I spent a good bit of time attending to sealing every penetration through the framing. I also used airtight electrical boxes on exterior walls and for smoke detectors, avoided ceiling lights under attic areas, and caulked the bottom plate to the subfloor.

At the end of last week I had a contractor hang the ceiling drywall. Ceiling only, no tape/mud, no insulation yet, no wall drywall yet. Correct me if I'm wrong, but is seems to me that the plane of the ceiling drywall, if I get it (drywall) properly sealed, would form the top of the building envelope (the attic is vented). Now it's my turn to finish up the air sealing so we can pressure test, fix any misses, and continue on down the long road to CO.

So... how does one adequately seal the ceiling DRYWALL to the framing to ensure that over the life of the building it remains a good air barrier? From reading about ADA, it seems that the tape and mud applied to the joints between the drywall are considered a sufficient air barrier. OK, I'll buy that in the 'field' of the drywall ceiling, away from the wall partitions. But what about the long (albeit generally narrow) gaps between the ceiling drywall and the wall framing (top plate)? If the gap is less then 1/2", the wall drywall will cover it, then it will be taped and mudded - OK. But in some places in my house, the contractor left a gap that is wider than 1/2". And unfortunately, one of these locations is in the master bath, above the shower stall of all places. Jeepers. Do you spray foam these gaps with 1-part can style stuff from the attic side? If yes, does it make sense to use the window/door spray foam since it remains a bit flexible and could maybe take the natural movement of the building lumber a little better without letting go of the seal between the lumber & the edge of the drywall? Or should some other means entirely be used? 

When I go into the attic, I can see daylight coming from all the gaps between the drywall and interior wall partitions. Should ALL of these be sealed with foam or something? Or does the 'wall drywall + tape + mud' and cellulose on top of it all (in the attic) create a sufficient air seal if the gap is 1/2 or less?

I'm all ears, Thanks!

We see top plate leakage through existing insulation all the time. The air goes behind the corner tape, behind the wall drywall at the top plates and into the wall cavity. The corner mud will not effectively stop it. Tight fitting ceiling panels will not necessarily stop air infiltration either, but every time we have seen significant air leakage at the plates with an IR camera and then go into the attic the gaps of the ceiling drywall have been substantial.

We seal with 1 part foam, some will poo poo 1 part in favor of 2 part but economically it's not really feasible if you're into making a profit and keeping prices reasonable (speaking as a business owner).

So yeah, seal it all, and dont seal w/ window and door, you want expansion and you want it to adhere so be generous and force the foam into the gaps, I want to see 1" diameter beads or better when I check our work. Touch n seal all season formula is good.

I think the best approach woud be to drywall in ceiling before you installed interior walls.