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T&G on interior ceilings8/6/19
I have a large cathedral ceiling that I intend to install pine tounge and groove on.
If its 4/4 material I would opt to sheath the space first with ply or at the least OSB. Installing T&G over rafters/vapor barrier has been a nightmare any time we were forced to do it. You wind up poking all sorts of holes in the vapor barrier and you have no opportunity to pop a nail in a spot where you need one. The other bonus to sheathing it is you wont have to land your butts on a rafter/member so you can let them fall anywhere just like hardwood flooring. Your savings in time and waste will more than cover the cost of the sheathing. Id never bother with the drywall.
I lived in a house with knotty pine tongue and groove ceilings, nailed to rafters. There was 4" on insulation over the pine.
Air leaked into the cold space anywhere and everywhere. This created suction at all the windows and holes in the lower part of the house. In the attic, the warm air would cool and condense, and water would drip out of the ceiling where it wasn't water tight. Being wet, the attic was a good place to grow mold, even though I hadn't planned on having a mold farm in my attic.
The ceiling needs to be air tight. There is probably a way to do that and use decorative lumber, but I don't know how to do it.
Your experience Harold is exactly what I was referring to when I mentioned poking all sorts of holes in the vapor barrier. If the ceiling is loose the stack effect in a building is enormous. In the instances we were forced by the homeowner to install directly over framing we stayed with our default continuous poly vapor barrier. No seams. And wrapped the interior of the ceiling before the T&G went on but its still a poor solution which the homeowners found out when we used far more material having to cut to land on framing as opposed to being able to just run random joint stagger.
Running T&G with loose fill above and no vapor barrier or even paper faced insulation sloppily installed would be a disaster which you had to endure.
Your advice of a tight envelope is spot on.