Our home was built in 1960 in zone 4 (western Oregon so yeah, damp).  Roof was redone a few years ago, with ridge and midline vents. The rooms under the attic space in question are a kitchen+nook and a dining room. Kitchen has soffets above cabinets such that their undersides (where cabinets attach) are plywood and their faces (and undersides in area above where there are no cabinets) are drywalled. In both rooms: cedar T&G was applied directly to the undersides of the joists (2x10s on 16” centers) and then spar varnished.  In the kitchen the T&G runs only between the soffets, not through them. In the nook and dining room the T&G runs wall-to-wall. In both rooms there are decorative beams applied below the T&G. In the attic space above, we have about 6 inches of loose fiber insulation directly atop the T&G (or in the cases of the soffets, atop their plywood bottoms) and atop that, a layer of single-faced fiberglass batts (probably product intended for use in walls – it appears to be ~R21) with the faced side up.

For us it is not a financially feasible option to remove all the beams and T&G, put up drywall against the joists, and restore the T&G and beams (which we’d feel compelled to do because the interior walls and other adjacent ceilings are of the same material and patina and we love the overall look as it is).

What I think we do want to do is remove all ancient/inadequate insulation (not to mention the sawdust and other yuck from original construction and re-roofing work, which tends to filter down through the T&G and other seams and makes the house exceptionally dusty), do air sealing (gaps everywhere!), and re-insulate to minimum R-38.

So the question is: in this scenario, what is the best solution for vapor (and dust) control atop the T&G?