Vapor barrier in unvented attic living space
We have an old two story log farmhouse in Northern Michigan that has been in the family for 100 years. It is a simple saltbox design with just an up and over roof. It has a full second floor (with full stairway and door, etc). The roof assembly is unvented and the underside of the decking has been sprayed with closed cell foam. All sides of the space up to about 4 feet high have knee walls built that were insulated with r-23 rockwool and a vapor barrier under the drywall. The areas behind the knee walls have not been sprayed – you are looking at the inside of the top 4 feet of the log construction. Roof assembly is blocked off from the knee walls.
The existing attic living space has a flat ceiling which causes a 6 foot high (at peak) triangular void between the attic living space ceiling and the ventless, closed cell insulated roof assembly.
The floor is uninsulated and there is an old time vent installed in the floor that acts as a secondary heating source from the rising heat on the first floor (there is a primary heater in the second floor space as well. The living space has windows on either end of the house that are open from late spring through early fall and there is no air conditioning in the home.
We are now renovating a few things.
My question is this: Should we install a vapor barrier under the new drywall ceiling in the upstairs living space? Or will that cause issues with the closed cell foam above that that acts as the conditioned space’s own vapor barrier?