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Vapor barrier over flr joists, below sheathing-ok?


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I inspected a renovated 1950s home.

Original flooring was just one layer of tongue and groove flooring.

Remodeler replaced the floor boards with 3/4" ply but placed a layer of plastic vapor barrier on the top of the floor joists and lapped up plaster walls behind the baseboard to reduce infiltration because of the single floor layer.

Is there any problems that can come about because of this that I should note to the homeowner? [:-dunce]

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It could end up being an issue. Vapor diffusion will push moisture-laden interior air toward cooler/drier air. If things get too cool in the crawlspace vapor could condense on that cold plastic and become trapped between the underside of the floor and the plastic and lead to wood rot. He'd need to insulate the rim joist above the foundation at the perimeter of the crawl as well as the foundation wall and the underside of the floor to ensure things never cool to dewpoint. He'd need to ensure there is zero air leakage behind that plastic at the perimeter of that room, so they don't have a situation where moist air is condensing there. Lots of little details needed to get it right.



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Is the crawlspace floor covered with a vapor barrier?

Would this lead to excess moisture collecting on the joists?

It seems to me like removing the plastic, insulating the perimeter of the crawlspace, insuring adequate ventilation and covering the "floor" of the crawlspace properly is the right way to go.

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