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Vapor barrier/insulation on concrete slab under bathroom?

8 years ago

Our new home wasn't well maintained, but we knew that going in. One thing I noticed was a toilet that appeared to have been leaking around the base for some time. I assumed I'd be removing and replacing the subfloor. Today, I finally got into that project. The bathroom will be getting a new tub, tile floor, toilet, etc.

I pulled the old toilet today, and found what I expected -- rotten subfloor. I've pulled this out, but now have a question. The bathroom is built over a slab. The floor sits on 2x8 joists which are not directly on the concrete slab, there's about 1" below them. Someone put loose fill insulation in this cavity, it was sitting on top of the concrete. The bottom half inch or so of this loose fill is damp, has compressed, and has now been removed. At this point, the subfloor is off, the old damp insulation is gone, and I'm ready to continue.

Now, the question is, should I put a vapor barrier on top of the concrete? It seems like a good idea, as moisture was clearly coming up through the concrete and getting into the insulation. So I'm prepared to put down a barrier, if that makes sense. Second, should I attempt to put insulation back in? Or better to leave out? Ultimately there will be a 3/4" plywood subfloor, a layer of concrete backer board, and tile. There's no access to this area without tearing up the floor, no vents in there, and no climate control of any kind.

Thanks for your input!

This post was edited by peter826 on Sat, Jul 26, 14 at 21:23

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