Tiling over slab with water vapor transmission

I haven't been able to find an answer to my issue, although I suspect its not that uncommon.

I have a slab through which it seems water vapor passes. While lifting up old tile, I found some slight mold behind some baseboard molding and some screw heads on the screws on the bottom of drywall which were behind baseboard are rusted. The old tiles were set on top of tar paper (house and original tiles are about 7 years old). After lifting old tile, a mildew smell was present, which was not noticed before tile was lifted. Now that tile has been removed and tar scraped off concrete foundation, smell has gone away. This is inside of the house and I dont believe its any condensation, or water intrusion, but vapor. Also, other neighboring homes are having similar issue. I could do a calcium chloride test to confirm the vapor, but it seems like a sure thing to me.

So, question is, do I try to seal it and keep water vapor from coming up or apply some kind of breathable membrane (I want crack isolation also) so that vapor can come through and dry out (I was thinking of Ditra, as it seems to be highly regarded on this forum and others).

I was thinking that if it was sealed, mold would just continue to grow down there, which doesnt sound good.

If vapor was allowed to pass, then it could dry out, but would the vapor lead to failure of adheasion of the thinset?
If allow to pass, do I need to choose from a certain kind of tile which will allow vapor to pass, or will all ceramic tile and/or marble have this property?

This seems like such a basic issue, but I havent seen it answered directly in Byrne's book (sorry John Bridge, but I bought his before I saw yours for sale) or others that I have read or on the web anywere.

[John Bridge]

Hi David, Welcome.

You don't have to be sorry for not buying my book. It doesn't cover your concern, anyway.

Except for the crack suppression membrane you want, there is no problem. Many slabs transfer moisture up from the ground. It only causes problems under floors that have been sealed off, such as linoleum (vinyl) floors (or floors installed over a membrane, even tar paper).

The truth is I don't know of a membrane that won't trap moisture underneath, and this includes Ditra, which is plastic and waterproof.

If you install directly to the slab, it's no issue.

Maybe somebody more informed than I can come up with something.

John

Thanks for prompt response John.

One thing that caught my eye about the Ditra is that they claim its vapor permeable and water proof, which seems kind of contradictory. (I have to admit, I don't understand the physics here).

From their web site, "Schlüter®-DITRA serves as an uncoupling layer for problematic substrates, a waterproofing membrane, and a vapor pressure equalization layer to accommodate moisture occurring at the underside of the substrate." So, it seems like they are claiming that moisture from vapor would not be trapped by the membrane, but pass up through the thinset and tile. However, water would not be allowed to pass.

One of my neighbors claims that the vapor caused his tiles to buckle. Is that possible? What would cause this? Adheasion failure? I had assumed that he had a ceramic tile. Maybe he had a linolium? Have anyone ever heard of this happening with a ceramic or marble tile floor due to moisture?

[Bri]

Hi
With Ditra, there is a flow of air beneath it,because the grids on the bottom are left open..I would guess water would evaporate out the ends of the Ditra..close to the walls?

[John Bridge]

I hope someone else joins us here.

The Ditra will allow moisture to migrate laterally to its edge, but believe me, nothing is going to go straight through it. It's solid plastic. There are air channels underneath.

Serious moisture in a concrete slab can cause excessive expansion. Then when it dries, it can buckle the tile. Usually, though, when tile buckles up it can be attributed to other causes, including improper installation. The phenomenon is called "tenting," or simply delamination.

You don't have siginificant moisture, and I wouldn't worry about it. As I said, I could install tile over your slab right now and not worry about moisture. The potential for cracking is always there, though.

P.S. Looks like Bri beat me to the draw. I'm too long-winded.

[Jason_Butler]

John's right. The only way the water would affect the adhesion of the tile is if the installer used MMMMMastic.

Sorry John...had to bring it up.

Jason

Hi David

The way Bri explained it is the way it has been explained to me by my Schluter rep.

I use Ditra on basement slabs for just that reason. Even dry slabs now may get some moisture under them later. It is good protection.