Vapor barrier/insulation on concrete slab under bathroom?

10 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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Comments (9)

User
10 years ago
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.
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snoonyb
10 years ago
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?
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worthy -- thanks for replying... unfortunately, I can not slide in the XPS behind the studs. By local code, we had to install fire blocking on the end of the wall. So now it is closed off. And, I can't install XPS in between the studs becuase I already have electrical and piping running through the studs. The gap between the wall and the stud is only a little more than 1/2". So even if I could slide in the XPS behind the studs, I would only get about r 3 value.... Thats why I figured it is almost better to put no insulation. The pipes arent gonna freeze; Plus, if I do get moisture back there, then there will be no fiberglass insulation to get wet. With no insulation, the wall will just dry out. (with the help of my dehumidifers and the bathroom vent fan, of course) If I get a chance, I will post a picture tonight of the studded wall with the roughed-in plumbing and electrical. thanks again. mm I'll try to post a picture later....
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The most expensive but best insulating process is closed cell spray foam. For a large space, with ten foot ceilings it might be no more costly than board insulation. XPS is more labour intensive. For NYC, I'd use t&g 4'x8' x1" boards; if you can't find those, then use 2'x8'x1" t&g. Seal all the joints with building tape--Tuck and Tyvek and two popular brands. I've not used sealers, but a building writer whose information I've always found to be sound, swears by Drylok. (I assume this is not a rubble or brick foundation.) The advantage of steel framing is it's neater to put up (no sawdust) and there are no subsequent nail or screw pops. However, as it's sensitive to moisture, I'd have the baseplate resting on strips of XPS not only to create a thermal break but to keep it away from any minor floods that may occur. Since one inch of XPS is R 5, I always add high density fg batts, adding another R 15. No vapour barrier is necessary, though if this is a permit job, the inspector may insist on one. It wont' hurt anything. (See Building Science links in my first post.) After two years, a number of tiles cracked at pressure points on a brand new home I built. And that's with the tile on top of at least 10" of reinforced concrete and power tamped gravel. Since then, I figure it's a lot cheaper in the long run to use an uncoupling substrate such as Ditra. (If you dont', be sure to keep a lot of extra tiles just in case.) Ditra and cement add maybe 3/8" extra to the flooring height. NYC labour. My condolences!
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I just read this thread and want to add my question to the original. I am in East Texas. I plan on converting an old woodworking room into a studio. It is 30 years old and was done the same way; i.e., rough plywood sheathing directly over the studs. It is paneled in the cheapest rough particleboard the owner could find (looks like a pile or large wood ships and shreds were simply saturated with resin, then pressed into shape), but the paneling is in good shape and I don't really need to remove it. We though about just adding a vapor barrier, then covering the entire thing with a new layer of paneling or drywall. The questions are: If we add a vapor barrier, where does it go - on the outside of the building with a new layer of siding over that or on the inside with a new layer of paneling over that? And where would the insulation go? Dian
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there Is a diference between vapor barrier and water proof redgaurd waterproof well maybe not if you only apply 1 coat? Wrong. Taken from CBP redgaurd tech sheet.: Redgard can be used as a slab-on-grade moisture vapor barrier under all types of floor coverings. Dennis: always go direct to the product tech spec( as you did) never fight with CA inspectors :). Ditra has been tested for California.exceeds both vapor perm and is VOC compliant theres many better products regardless and sounds like youve found one that will be fine.. DITRA and DITRA-XL were evaluated according to the "Standard Method for the Testing and Evaluation of Volatile Organic Chemical Emissions from Indoor Sources Using Environmental Chambers, Version 1.1" for California Specification 01350 and found to comply with the VOC requirements. California Specification 01350 is referenced by various green building standards and rating systems.
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peter826
Original Author
10 years ago
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.
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snoonyb
10 years ago
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.
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peter826
Original Author
10 years ago
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"..
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worthy
10 years ago
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
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User
10 years ago
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.
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peter826
Original Author
10 years ago
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....
0
User
10 years ago
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.
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Vapor barrier/insulation on concrete slab under bathroom?

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Vapor barrier/insulation on concrete slab under bathroom?

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