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Old 01-15-2018, 07:44 PM   #1
kba1980
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Acceptable RH % for concrete slab

Hello I was wondering if any of you folks test the RH level before installing your floor coverings. I need to do a test but none of my box stores around me have a MVER test available which measures in pounds. Is there anyway to figure this out using a RH test vs a MVER test.

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Old 01-20-2018, 08:46 AM   #2
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RH test for floor coverings? Yes. For tile? No

Order a Calcium Chloride (CaCi) kit to test MVER. Old test method but cheaper than buying RH meter for one time use. With math calculation you will get lbs/1000sf.

What type of floor covering are you installing?
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Old 01-20-2018, 09:42 AM   #3
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I have this stat on my other computer, but memory serves its very high for tile
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Old 01-20-2018, 07:27 PM   #4
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High moisture content in slab with Ditra

I am remodeling a house built in 1954 on a slab. I am planning on using Ditra but I tested the slab for humidity and it reads 100% to 93% there is no water standing and the slab soaks up water. I have a MVER test coming Monday to to check pounds per 1000 sq.ft.

Will there be a problem with a lot of moisture in the slab with Ditra even though there is no visible wet spots on the surface. How about the bonding strength of the thinset will it be effected.

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Old 01-20-2018, 07:43 PM   #5
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The structure of Ditra does help to manage vapor from a cement slab, but there are limits. Have you read their installation manual? FWIW, cement cures to a stronger bond when done slowly, which moisture obviously creates (unless there is freezing involved). A modified thinset is more affected by being wet (at least some of them) in that if it is a latex based modifier, needs to dry to attain its design strength. FWIW, Ditra is the only substrate I'm aware of that is suitable to be installed over a green cement slab, which obviously is releasing a lot of moisture (often, the next day after it is poured).
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Old 01-20-2018, 07:55 PM   #6
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Hi Jim thanks for the reply I was going to use the Schluter All-Set under the Ditra which Schluter says can be used. I also have Bostick Ditra Set, which thinset would you recommend. I am definitely planning on using Ditra Set to install the Kerdi and the tile but I thought I would try the Schluter All-Set under the Ditra.
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Old 01-20-2018, 08:29 PM   #7
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I posted another thread I have decided to use Ditra and I do have a mver test on the way. The moisture meter I bought was chap like $40 from Lowe’s and it measured it from the surface no pins or anything. I was getting 93% to 100% on a slab poured in 1954 probably no vapor barrier but there is no visible water on slab and no miss colored dark spots that could be moisture related.
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Old 01-20-2018, 09:40 PM   #8
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The Ditra thinsets do not use latex, so excessive moisture probably won't be an issue, but it's best to follow the instruction manual. I think that over a slab, they recommend the unmodified. Haven't read their latest copy since they introduced their new thinsets, though. FWIW, the fleece's bond strength is only in the order of 75psi or so (minimum of 50psi), so extra strength of flexibility of a modified in this case probably only cost more, doesn't add much, if anything. Now, having flexibility over a wooden subfloor is much more critical, but a slab doesn't move much, and the delta T between the tile and slab won't typically be super high.
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Old 01-21-2018, 03:31 PM   #9
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With high moisture transmission over a clean slab that absorbs water, my go to is Ditra with DitraSet. And like Jim said, a more expensive mortar isn't going to add a benefit to you...just cost you more money.

Are there any things goin on outside the house that are contributing to the moisture in the slab? Like rain gutters that aren't redirecting the water away from the foundation? Or a reverse slope on the grade within the first several feet around the house? Or something going on with a driveway that allows water to puddle around the house?

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Old 01-21-2018, 06:50 PM   #10
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I havenít seen anything out of the ordinary as far as slope towards the house or any puddles I can about guarantee there isnít a vapor barrier under the concrete being the hose was built in 1954. I should also mention that when I was removing the 2 layers of linoleum and the layer of vct tiles I used a wallpaper steamer so the slab got wet but that was over a month ago. I donít know how long it would take to dry but the slab soaked the water up and the surface looks dry. I have a mver test coming tomorrow.
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Old 01-22-2018, 12:36 AM   #11
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When you put something like Ditra over a slab like that, the vapor pressure tends to equalize and stop the water vapor. If there was liquid water being forced up, that's a different story.
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