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**EXPERT Advice Needed** - VERY Moist Slab With LOTS of Vapor! What to do?

Good evening,

My name is Luis, and it is a pleasure to make the acquaintance of anyone kind enough to assist me in this predicament. I am a lowly homeowner and I request the advice of the fine minds that roam through this forum.

I am dealing with a slab that is internally moist that is releasing the moisture in the form of vapor. To make a very long story short, we had a leak beneath the slab which caused the slab to become quite wet. The wet slab over a period of time created a substantial amount of water vapor, ultimately leading to my having to remove my beautiful large porcelain tile (with very thin grout joints).

Now, the slab has been open for approximately 3 months (with nothing on it but some thinset residue) but it remains quite moist. Although the slab appears dry to the naked eye, there is a substantial amount of vapor being released. The results of the multiple calcium chloride tests have been in the HIGH teens.

I have recently employed the use of two industrial size dehumidifiers and five drying fans which have been on for about two weeks. The dehumidifiers bring down the ambient humidity within the room to about 35% when they are on (otherwise it is usually 45% in the house with the a/c set at 72 degrees).

What I am seeing is that when using a handheld moisture meter (the type with the two prongs that you use on top of the slab), the moisture content has gone down significantly on the slab (the digital display on the meter has gone from the high 80's to the 30's) as measured by that device.

However, I was smart enough to have installed 5 separate Wagner Rapid RH 4.0 relative humidity sensors at different locations in the slab (within this area where the tile was removed). Each of these sensors are showing a relative humidity of 99%! This relative humidity has remained at 99% for the past three weeks, and unfortunately the dehumidifiers have not changed the RH at all. (The slab is 5" thick, so they were installed 2" down, as that is 40% of the thickness of the slab, per Wagner specifications). In addition, in areas where I have tried the old plastic taped to the slab test, I see discoloration (darkening) in the slab within 24 hours and moisture beads on the inside side of plastic within 48 hours.

BOTTOM LINE: My slab is still moist. The top layer of the slab has clearly dried a bit with the dehumidifiers and fans, but nothing is drying the middle/bottom of the slab AT ALL!

So, what should I do?

I would like to put some sort of coating or product on the slab now before I put a new floor down (the slab is obviously too moist to put any floor down). I am particularly interested in Ditra by Schluter systems. The reason for this is because I know that I am experiencing significant vapor issues that will likely continue, and Ditra will allow that vapor to channel away from the slab. Also, I love the idea that it is an uncoupling membrane.

In addition, I am also considering: (a) bead blasting the slab or (b) scarifying the slab, to release the locked-in moisture within the slab.


(1) Do you recommend Ditra? For my problem, would you recommend Ditra, or instead a sealer like one by Ardex or Laticrete? My concern with those products is that they may "pop up" after a number of years during the time that the heavy vapor is being released.

(2) Is there an issue with using Ditra considering that there is so much vapor coming up? Particularly, will my walls/baseboards (where the vapor would be funneling to with the Ditra) become saturated with vapor and ultimately become wet and moldy? Is there so much vapor coming up that it will condense into water (like it did previously beneath my old tile) and channel under the tile within the Ditra to the walls?

(3) Do you recommend doing something (scarify, bead blast) to the slab now, before I put down the Ditra? The theory is: if I scarify or bead blast the slab, it will allow all of the excess moisture to be released from the slab before I put down the Ditra, especially if I keep the dehumidifiers and fans on after the scarification/bead blasting. Please note that right now there is a thin layer of residue thinset on the slab likely inhibiting the drying. On the other hand, will scarifying or bead blasting the slab cause issues in the future? To be more clear as to my concern: since I know there are vapor issues, will doing this to the slab allow more areas for the vapor to easily rise up through the slab and cause it to condense into water that much quicker?

(4) Is there any benefit to having a french drain created around the perimeter of the slab? I have heard of others doing this for vapor issues within the slab (usually if they have a grading issue), but I was not sure if there is any benefit for my purposes (i.e., does it aid the drying process of the slab?). I can't see how a french drain could help me, but I thought I would ask the experts!

Thank you for any help you can provide! I appreciate it!

Messages In This Thread
**EXPERT Advice Needed** - VERY Moist Slab With LOTS of Vapor! What to do? - tileguy555 - 06-23-2011, 05:29 PM

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