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Community and Q&A

One Concrete Slab, Two Vapor Retarders

nj_homeowner | Posted in General Questions on


Last year we poured a new basement slab in NJ, following the excellent advice on this forum.

Under the 4-5” slab we have a polyethylene vapor barrier, then 2” rigid insulation, then 6” stone and drains.

Now we would like to install natural stone tile on top of the slab.  We are planning to install Schluter Ditra on the slab, then the 1/4” tile.  But I’m wondering if putting Ditra, which is apparently a vapor barrier, over the slab will cause problems due to moisture being trapped between it and the vapor barrier below.

Is it ok to put Ditra over a slab that already has a vapor barrier?

Thank you in advance!


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  1. Jon_R | | #1

    The concrete won't care and the Ditra won't care. According to Joe Lstiburek, you may get mold smell (because Ditra has even more potentially high humidity air cavities than plastic sheet).

    1. nj_homeowner | | #2

      Thanks. I just read the Lstiburek article you linked, and the mold he describes occurs when you put an imperfect vapor barrier over a slab that does not have a proper vapor barrier under it.

      Our slab is detailed underneath exactly as Dr Joe prescribes, so there should be no moisture migrating into the slab from the bottom or sides.

      If we put Ditra on top, the only moisture in there will be that which comes from above and makes it through the Ditra seams (ie drips and spills) or any moisture still in the slab from the original pour that happened a year ago.

      So our circumstances are materially different than that described in the article, but I am concerned about having a similar problem.

      1. Jon_R | | #3

        Looking at Fig 7, I'd say that Joe is specifically including cases where there is a vapor barrier under the concrete. But agreed, your air pockets are likely to have little wetting. But also little drying, resulting in who-knows-what in terms of mold supporting humidity. Maybe a fungicide is an easy precaution.

  2. exeric | | #4

    I think you should be safe with your intended plans. Schluter already has implementations of showers on concrete slabs and I'm sure many of those showers on slabs already have vapor barriers beneath the slab. Ditra, when installed properly is very water and vapor proof. In showers installations Kerdi is run over the kerdi covered shower base, on the perimeter of the shower floor/wall junction. That kerdi on the junction between base and kerdi covered shower walls then water proofs the walls to the shower base.

    Since you are pretty confident in the integrity of barrier beneath the slab that means you really only need to worry about water or water vapor getting below the ditra from above. So what I would do is install Kerdi Band (a narrow roll of Kerdi) at the junctions between the Ditra covered floor and the walls. Since it isn't a shower you only have to run the Kerdi Band an inch or two up the wall. Then cover it up with baseboard of some type. It can even be tile baseboard. That should stop any infiltration of water from above the Ditra. That's what I did on my bathroom floor at the Ditra/ wall junctions. A bathroom has a lot more potential moisture problems than your non- bathroom floors will have.

  3. nj_homeowner | | #5

    Thank's for the replies. For a variety of reasons, I'm not sure I want to try to fully waterproof the Ditra across the whole floor.

    I definitely do not want any mold issues.

    I'm wondering if it would be better to omit the Ditra and just lay the tile right on the slab?

    1. exeric | | #6

      "I'm wondering if it would be better to omit the Ditra and just lay the tile right on the slab?"

      That would be a mistake. If you don't put in a flexible base, which Ditra provides, you will have extensive cracking not only on grout lines, but even on some of the tiles. Concrete moves. Natural stone tiles are even more fragile and prone to cracking than synthetic. That is the whole purpose of Ditra and water proofing is secondary, though it does that well also. Ancient civilizations learned to put sand base beneath tile and those installations still look good today. Ditra replicates it.

      Something everyone should understand (getting on soapbox here) is that individuals often don't give good advice on particular subjects. This happens even if they have good reputations otherwise. This happens because individuals are prone to overestimating their knowledge. Similarly individuals are prone to keep going to the same individuals to get their advice even if they don't know what they are talking about on some subjects.

      This is a subject I know about. I would not want to be designated an expert on this forum because I would feel bad about giving info on things I mistakenly know little about. This is not one of those subjects. It is definitely safe and good practice to install Ditra in your situation. I know what I'm saying here.

    2. exeric | | #7

      I should add one more option for you. If you are really worried about moisture being trapped, then you can skip the Kerdi on the perimeter. You can also skip the Kerdi installation that normally joins individual sections of Ditra. Do not skip the Ditra though. It is a tremendous advance for installation of tiles over surfaces that move.

  4. nj_homeowner | | #8

    Thanks for these thoughts.

    Definitely I would not seal the Ditra at the seams or edges of the slab. There are also two small rooms, maybe 150 sf out of 900 sf, that will not be tiled. So in those locations the slab will be exposed and, I suppose, would therefore provide further drying potential.

    What I am not understanding at all is how the Ditra, which is vapor impermeable, can "breathe" (per the Schluter literature), which I am reading as meaning that it can "dry." It seems like it would have to provide drying in order for the thinset under the Ditra to cure.

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