Vapor barrier/insulation on concrete slab under bathroom?

peter826July 26, 2014

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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The water could also be from condensation on the cool slab. This is not a simple problem.

Is there no possibility for venting?

You might be forced to spray closed cell foam on the slab.

    Bookmark   July 27, 2014 at 7:29AM
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Since there can be a reasonable assumption that the bathroom may have been built over a patio slab.

What is the floor structure of the adjoining dwelling built over?

    Bookmark   July 27, 2014 at 11:34AM
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The only option I can see for venting would be to cut holes in the concrete block foundation...

House is mainly on a basement, some crawl, and some slab. This room is behind the garage and likely shares the same slab.

    Bookmark   July 27, 2014 at 12:39PM
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You would know better than I, but it is uncommon to set a wall on concrete block.
It is more common to frame the floor system, setting the sill, rim and floor joists on top of the foundation, which would increase the opportunities for cross ventilation.

    Bookmark   July 27, 2014 at 1:28PM
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You're right -- I should have said, cut holes in the concrete block wall. That would have to be done in the lower 12" or so of the wall, as that is the part which comprises the "crawlspace"..

    Bookmark   July 27, 2014 at 2:59PM
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Seal the floor, then spray foam the cavity. Or combine well-sealed layers of XPS or EPS. Do not vent.

Here is a link that might be useful: Building Science Corp.: Crawlspace Insulation

    Bookmark   July 27, 2014 at 4:58PM
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The miniature crawlspace must be passively or actively ventilated. The former can be by vents to the outside and the latter can be done mechanically from the interior of the house.

    Bookmark   July 27, 2014 at 6:15PM
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Responses here seem to follow other information I've found online, a real difference of opinion whether this miniature crawl should be vented or sealed off....

    Bookmark   July 29, 2014 at 8:40AM
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Be sure you understand the options.

There are 2 kinds of crawlspaces allowed by the IRC:
Vented and Unvented (aka "sealed"). The terms "unvented" and "sealed" are misleading because this type of crawlspace is not completely sealed or unventilated.

The IRC code requires that both of these types of crawlspaces be "ventilated" so that moisture does not collect and encourage the growth of fungus (whether from the ground, condensation on the slab, etc.) by one of the following methods:

1) "Vented" Crawlspace - install vent openings in the exterior wall of the crawlspace

2) "Unvented" Crawlspace - inside the house supply to or exhaust from the crawlspace a small amount of air with a return air path to or from the house and insulate the exterior walls of the crawlspace.

Both crawlspaces must have a vapor retarder at ground level and an access opening.

For more information see IRC section R408.1, 2, & 3.

    Bookmark   July 29, 2014 at 3:20PM
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