insulation moisture retarder over panelled ceiling


I've got an extension to a 1941 house, probably added in the 50's, that has an attic space above it. I want to keep the rooms underneath warmer. The attic has a wooden floor and rafters covered with wallboard, and I've no idea what is behind any of it. Judging by the main attic, the floor probably has some loose rock wool (?) or fiberglass insulation without vapor retarder, well ventiliated by mice, and I can't even guess as to the ceiling. There's no outside ventilation to the space, it was used for storage or crude kid's playspace. My original plan was just to lay unfaced R-30 batting down and get on with life.
I noticed one posting here talking about the utility of moisture retarders on attic insulation given ceiling paint (don't need facing, paint does enough) vs. panelling (less airtight, should use facing), and got to thinking... the extension has wood panelling on the ceilings underneath. Should I lay down a sheet of plastic over the wood floor, not knowing what is underneath? I know you don't normally put vapor barriers between insulation, but could it hurt/help much here? Will it really make much difference either way?
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Vapour barrier should be on the warm side of any insulation or you risk it causing condensation when the moist air reaches the cold vapour barrier. You could cover the ceiling with 6 mil polyethylene and drywall over it or just paint the ceiling with a vapour barrier paint as a substitute barrier, not as good as heavy poly but better than nothing.

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