1900 home, insulation and air sealing ideas (no soffits) / Climate zone 5A – GreenBuildingAdvisor

I’m grateful to be on this site, and am hopeful someone may have a recommendation for this project.

Quick background:

Climate zone 5A, Upstate NY

c. 1900 house

Balloon framed

No Soffits on house, so no opportunity for soffit vents

Attic venting done at gable ends and ridge vents

New asphalt shingle roof

Vermiculite in walls and around eaves/top plates

Fiberglass batting on attic floor

Pursuing a modest long term renovation

Existing insulation in the attic is in need of replacement. It was a planned project for this spring and the current ice damming situation has made this all the more dire.

We’ve had consults from insulation companies who recommend spray foam on the roof underside. We are steering clear of this and would like to use cellulose and keep the attic vented.

The ceilings are plaster and lathe and I want to ensure we have an appropriate air barrier between the ceiling and the attic.

Is this an appropriate idea?:

Remove existing fiberglass insulation

Add 1/2″ OSB or plywood over joists (with nothing in the void between ply and plaster ceiling).

Seal plywood seams with SIGA or Pro Clima tape

Add cellulose insulation over plywood floor

It would be particularly beneficial because the plywood or osb could function as an air barrier (?) and I would extend this whole floor to the perimeter. This would ensure no co-mingling of the vermiculite (in the top wall cavities) and the cellulose for when we remove said vermiculite from the wall cavities at a later date. It would also be helpful because we could insulate with cellulose now and if we had to pull any of the plaster ceilings down in the future, we wouldn’t be contending with that cellulose.

Would it be better to skip this and spray just the ceiling joist gaps and penetrations with closed cell foam and add cellulose on top and call it a day?

Thank you for any input you may have.