I am in the process of restoring a 100+ year old farmhouse, which mostly entails finishing the second story which was not winterized when we bought the house (we live in the upper midwest, zone 6/7). The floors of this second story were insulated in the 1950’s with vermiculite and fiberglass batts; I have since removed that insulation and am now focusing on insulating the knee-walls and ceiling of the second story. 

The original construction is 2X4 with shiplap exterior siding and then bevel pine siding. no insulation or interior siding currently.

So far on the knee-walls I’ve added 2×2 stock to the 2×4’s to make a deeper cavity for insulation; I’ve added R13 fiberglass within the cavity and have installed 2″ rigid foam between the studs.
For the ceiling I’ve added the 2″ foamboard between the rafters and overlayed it with two layers of R19 fiberglass in a crisscross fashion in the attic space above.
The roof is vented with soffit vents and a ridge vent.

My question is whether to apply the 6 mil poly across the face of the ceiling/walls now before I install interior siding, which will be wood siding (cedar, pine, mixed hardwoods)

I don’t believe I have a completely air-tight construction with what i’ve done so far, so adding the poly would help maintain air tightness. But I also don’t want to hinder drying to the interior if I do get moisture in the wall cavities and attic space.

Your feedback is much appreciated

thanks
Levi