Return to the Remodeling Forum | Post a Follow-Up

 o
Vapor barrier/insulation on concrete slab under bathroom?

Posted by peter826 (My Page) on
Sat, Jul 26, 14 at 21:17

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


Follow-Up Postings:

 o
RE: Vapor barrier/insulation on concrete slab under bathroom?

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.


 o
RE: Vapor barrier/insulation on concrete slab under bathroom?

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?


 o
RE: Vapor barrier/insulation on concrete slab under bathroom?

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.


 o
RE: Vapor barrier/insulation on concrete slab under bathroom?

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.


 o
RE: Vapor barrier/insulation on concrete slab under bathroom?

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"..


 o
RE: Vapor barrier/insulation on concrete slab under bathroom?

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


 o
RE: Vapor barrier/insulation on concrete slab under bathroom?

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.


 o
RE: Vapor barrier/insulation on concrete slab under bathroom?

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....


 o
RE: Vapor barrier/insulation on concrete slab under bathroom?

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.


 o Post a Follow-Up

Please Note: Only registered members are able to post messages to this forum.

    If you are a member, please log in.

    If you aren't yet a member, join now!


Return to the Remodeling Forum

Information about Posting

  • You must be logged in to post a message. Once you are logged in, a posting window will appear at the bottom of the messages. If you are not a member, please register for an account.
  • Posting is a two-step process. Once you have composed your message, you will be taken to the preview page. You will then have a chance to review your post, make changes and upload photos.
  • After posting your message, you may need to refresh the forum page in order to see it.
  • Before posting copyrighted material, please read about Copyright and Fair Use.
  • We have a strict no-advertising policy!
  • If you would like to practice posting or uploading photos, please visit our Test forum.
  • If you need assistance, please Contact Us and we will be happy to help.


Learn more about in-text links on this page here