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Unread 10-12-2022, 06:07 PM   #1
Just In Tile LLC
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Slab with moisture, plastic over it.

Not a tile question, but had a great discussion about laying poly down underneath a laminate or vinyl plank floor. IF the slab has moisture (most homes around here aren't really built with a moisture barrier underneath the slab). When installing most laminate or vinyl plank floors the manufacturer requires a vapor barrier underneath the flooring product.

The discussion today was about are you creating a moisture/mold issue underneath the poly? I would think yes, since the crude method of aluminum foil taped down for a moisture test creates a darker slab when moisture is present.

What is the science or thoughts on this practice to protect the laminate or stop moisture from accumulating underneath the vinyl plank without creating a moisture problem underneath the plastic or vapor barrier?
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Unread 10-12-2022, 06:43 PM   #2
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I've never done any of the vinyl plank, Justin, but I know all the engineered wood laminate floors I installed or had installed required the underlayment you're talking about. Most of them were actually even a cushion of sorts, being nearly an eighth-inch thick made of some sort of plastic with tiny foam balls, etc.

All of my SOG foundations had a vapor barrier material under the concrete, but the laminate flooring made no distinction at all; put their flooring over concrete, put their vapor retarder under it. And I found it interesting just how little moisture it required to keep those things from snapping together and I expect no much more to cause it to buckle in place, but I never experienced any of that.

As for having moisture build up under the vapor barrier material, there would be nothing between the plastic and the concrete to feed any sort of growth, so I wouldn't expect any real harm. Not like the "moisture sandwich" we often caution about here where there might be organic material within the package and you could very well have some problems.

I would never want to intentionally create such a situation, but in the case of the flooring, where there is nothing but impervious plastic, water, and Portland cement concrete, I don't know what hazard there might be. Once there was liquid water on the top of the slab, the moisture vapor pressure differential would be substantially diminished, despite whatever temperature difference might exist, I would think. Would the liquid moisture get nasty in there? I don't think it actually would, but..........

Perhaps some building science expert will stop by and correct me.
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Unread 10-13-2022, 10:41 AM   #3
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Agree-not allot different then bonding a bondable membrane down under tile..
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Unread 10-13-2022, 11:10 AM   #4
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Don't y'all have some maximum limit to MVER for the use of your membranes, Eric?
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Unread 10-13-2022, 01:14 PM   #5
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Bonded with EXT 85% and under per ASTM F2170 or 4 lbs/1000sf/24hrs. ASTM 1869

Bonding with Thin-Set(whos ,which one) doesn't really matter as long as there isn't standing water and drying. Modified, Rapid Set ?
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Unread 10-13-2022, 05:00 PM   #6
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Quote:
Once there was liquid water on the top of the slab, the moisture vapor pressure differential would be substantially diminished, despite whatever temperature difference might exist, I would think.
Can you simplify what that means for me CX
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Unread 10-14-2022, 09:12 AM   #7
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I think he means this, Justin:
The moisture drive of the damp slab is upwards to the relatively drier surface of the slab. But if a layer of plastic at the surface causes the vapor to condense to liquid water, you’ve now got 100% moisture content. This layer of water will sufficiently push back/equalize with the vapor coming up from the slab, bringing it almost to a halt. Not quite to a complete halt. Some moisture will harmlessly evaporate from the liquid layer at the seams or perimeter of the plastic…and be replaced with a little more vapor condensing to liquid. But the liquid layer will be harmlessly separated from the vinyl plank/laminate material during its life.

Switching from vinyl plank/laminate flooring to hard tile:
The creation of the 100% moisture “microclimate” is the very reason that the only floor membrane that performs well on an excessively damp slab is Ditra. On a slab with lots of moisture coming up through it, the air pockets between Ditra’s bonding fleece and the orange plastic membrane creates a “microclimate” of 100% moisture that very effectively pushes back on the slab’s moisture. It’s not really the orange plastic that is pushing back, but rather the 100% moisture content that pushes back.
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Last edited by Tool Guy - Kg; 10-14-2022 at 09:23 AM.
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Unread 10-14-2022, 09:22 AM   #8
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Yeah, mostly what Tonto said, Justin. There is also a temperature differential to consider when evaluating moisture vapor drive, but that could actually go either direction in a concrete SOG foundation situation.

And I must take issue with this part:
Quote:
Originally Posted by Goldstein
It’s the reason that the only floor membrane that performs well on an excessively damp slab is Ditra.
While Ditra may better accommodate the situation in some instances, it is not the only membrane that will do so.

My opinion; worth price charged.
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Unread 10-14-2022, 09:31 AM   #9
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Sorry, I edited my post while CX was typing so his quote of my words is not exact.

Yeah, another copy cat membrane might do the same thing. But copy cats aren’t getting credit and I’ll stand by what I said. I’m not a mindless troll for any company, but I’ll give them kudos for their invention that others copy. In this case, the credit goes to Schluter.
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Unread 10-14-2022, 02:26 PM   #10
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Strata_Mat may look similar to Ditra, but it purposely has holes in it, so I'm not too sure it would perform the same way as Ditra to stabilize the vapor content. There are other copy-cats out there, though.
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Unread 10-14-2022, 09:15 PM   #11
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Quote:
Originally Posted by e3, Post #5
Bonding with Thin-Set(whos ,which one) doesn't really matter as long as there isn't standing water and drying. Modified, Rapid Set ?
That's what I thought. I can recall years ago in a MMSA meeting (back when I attended such) where some round-robbin testing was going on in still another effort to find a way to differentiate uncoupling membranes from crack isolation membranes in hopes of someday having a standard for the uncoupling membranes (still hoping? ), and the results of the test for bonding to "green" concrete were reviewed.

Seems at least one of the flat membranes faired better than any of the profiled membranes in that test. That would appear to me to also indicate that such flat membranes would work just as well in stopping moisture vapor emissions as are the profiled membranes. That is, unless the bonding mortar was somehow degraded to the point of failure by the moisture. But that would apply equally to the profiled membranes if that were in fact the case.

I find it easier to explain, at least to myself, that the accumulation of liquid moisture tends to stop the vapor drive, but it appears that the saturated mortar and the impervious flat membrane might be doing the same thing.

SOMEONE PLEASE CORRECT ME BEFORE I GO ON BELIEVING THAT!!!
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Unread 10-14-2022, 10:09 PM   #12
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I appreciate you taking the time to further elaborate on CX’s answer Tonto, that makes sense. And I’m sure the little moisture that make its way to the edges/wall area is not enough to cause damage?

I’ve always read or known ditra handles moisture underneath itself but never heard it put quite like that which helps me understand the principle behind it.
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Unread 10-14-2022, 11:07 PM   #13
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Quote:
Originally Posted by Just In Tile LLC
…And I’m sure the little moisture that make its way to the edges/wall area is not enough to cause damage?
Around the perimeter of the Ditra, the sliver of standing water is exposed to the air (even if it’s hidden under base or shoe molding) where it evaporates harmlessly. The slight drop in moisture content allows a tiny bit of moisture from the slab to replace it. All other things being equal, there’s a very, very slow transmission of moisture from the slab into the room through the perimeter.
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Unread 10-15-2022, 07:46 AM   #14
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CX I guess I missed your last reply somehow. My coworker also suggests water gets into the slab from the sides, since alot of people have dirt against the exposed sides of the slab. Or for reasons unknown the property is graded where water runs into or past the house. So even if the slab has poly underneath aren’t the sides still allowing some moisture to wick from all sides?
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Unread 10-15-2022, 09:18 AM   #15
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To be clear, Justin, we're not talking about liquid moisture, aka water, coming up through the concrete, we're talking about moisture vapor coming up through the concrete. If you have liquid water entering the concrete, you likely have a hydrostatic pressure problem and an entirely different issue.

We put a vapor barrier material under all our concrete SOG foundations, and you are bound by best practice and code (in compliance jurisdictions) in your area to do the same. There is some disagreement amongst compliance inspectors, in my experience, whether one should/should not continue that barrier material up the outside of the perimeter grade beams. I always wrapped mine up the sides.

Properly contouring the grade adjacent to the foundation is always important. And in areas with a relatively high water table (all of east Texas?), it's also critically important to provide proper drainage around the foundation below grade.

All that done correctly for the particular foundation and site location will help to limit the presence of moisture under the foundation and, therefore, the moisture vapor emissions likely to be experienced inside the structure once the interior climate control is functioning. It's important, but frequently overlooked, or disregarded, especially by tract builders.
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