Help with engineered wood slab on grade?

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Loree

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Hello from North Central TX,

I have been searching the forums for information on installing engineered wood on slab on grade. I am hoping to get some expert advice—

I am remodeling a 1979 one story with no vapor barrier under slab (or it has failed).
Additionally, for the first 40 years of its life, the house had horrible drainage issues and no gutters. The lot has been regraded, and gutters added. The house has significant positive drainage to the front and back and is on a flattish street.

When we pulled up the ancient carpet, most of it came up ok but there was one room with rotten pad, musty odors, moisture, etc. This is the “problem room” and floor will be left open to monitor for time being. I mention this to establish that there are moisture concerns with this slab.

I am wondering if I can reasonably install glue down around the other side of the house. Carpet was a little funky in spots when it came up, but mostly ok. Slab is dry to the touch and no visible dampness. Contractor came through with Tramex meter and got readings all over the place. It was an unseasonably warm day with super high humidity and no climate control (because 72 degrees outside). I have since done CC tests for my own curiosity.
Results were 4.25, 4.5, 5.8, and 6 lbs/24 hrs. I know RH test would be best but that has not been offered as an option yet.

The flooring is Graf unfinished engineered with a limit of 3 lbs. Their documentation says that in cases with no underslab vapor barrier, a sealer should always be applied over the slab.

Am I asking for failure if we go in with a two part system (Sika MB + adhesive)? Are there particular products that would be best?

Contractor would ideally like to monitor the slab over time in the hopes that trapped moisture will slowly evaporate and levels will come down. I don’t know how realistic this is. I also don’t want to live on unfinished concrete.

Should I give up now and do porcelain tile?

If you’ve made it to the end… many thanks.
 

Mark Brown

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You are in luck. There are a myriad of options available for the installation of hardwood on slabs with known moisture issues.

I am not personally familiar with Sika MB, as I mostly use Mapei products as they are available to me, in the past I have used a few Sika products and was very impressed. After a perfunctory look at the product you have mentioned I would say you are in good shape with your plan.

On top of the Sika, if you then used a moisture control adhesive and followed the guidelines established for the use of said product, I would say go for it. If you think you manage to keep the ambient humidity in the space within parameters for the wood then you should not have a problem.

We have done many a "not so ideal" installations using the exact method, albeit different products, as you just mentioned and have had astounding results.

Sorry, I neglected to mention, there really is no need to let the concrete lose any moisture if you are within the parameters that are outlined with the products mentioned. RH testing would likely give you results in the high 90's from the numbers I am looking at from the things I have seen. That of course is not a scientific assumption as all concrete is different but the Sika allows for up to 100% RH as it is designed for green concrete.

The only thing I will say as a word of warning, is that most of these products are not intended for hydrostatic pressure situations and if you have not corrected your grading and there is a water table sitting at your slab with pressure on it, there could be adequate pressure to pop the whole works. This is much less common that people assume and most moisture issues are just the result of vapor transmission due to the imbalance of relative humidity of the two spaces above and below the slab.
 
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Loree

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Texas
Thank you for your response!
I have zero knowledge or affinity for any particular brand— Sika MB happened to be what contractor mentioned.
Is it true that it would be best to use two products from the same company (ie Sika MB and Sika T21)?
My house is full of Fortane LD at the moment (from before we knew about the issues). Fortane makes a moisture barrier product that could go underneath. I am inclined to go for whatever is best since these floors are so expensive.
Re hydrostatic vs vapor…
Is there anyway to tell? I have observed efflorescence in the garage and the previously mentioned bad area that will be monitored. There has never been a puddle of water. Engineer, architect, etc are inclined to think this is all from the previous bad drainage and will self-correct.
 

Mark Brown

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On a concrete floor with no known or a compromised moisture barrier under the slab and even a lot of times when I know they are good I will still always recommend utilizing a moisture barrier over the concrete. There is no such thing as too much insurance and like you said, flooring is not cheap.

While in our industry most people buy at the top of their budget and then try and short change on all of the things that are going to make that decision last, it sounds to me like you know what you have, know what you want and would like it to last.

If all of those statements are true, I would 100% say it is beyond a recommendation and is a requirement that you apply a topical moisture barrier as you have mentioned. Seeing as your contractor seems to like a product, let him. There is nothing wrong with taking the recommendation of the people doing the work that is for certain and it sounds to me like you have had some people on this.

One thing I can say however is that no matter how long you wait, a slab without an intact moisture barrier will always have a moisture issue when it is in regards to hardwood. There is an infinite amount of moisture in the earth and all it wants to do is travel to equilibrium. I mean, we have know this for 1000's of years

“Water is fluid, soft, and yielding. But water will wear away rock, which is rigid and cannot yield. As a rule, whatever is fluid, soft, and yielding will overcome whatever is rigid and hard. This is another paradox: what is soft is strong.”​


― Lao Tzu

So anyway, parables aside, that water will always find a way to travel to a drier source..... that is unless you kill it dead :p Your plan is exactly what I would recommend for your situation. The topical barrier is in place to disallow moisture to travel beyond it. Your hardwood will also not allow moisture to travel easily, lets say as easily as carpet, but believe you me it will and eventually will become so saturated as to fail. This is why the application of a topical barrier is so important so as to stop it from ever entering the space.

While it is not required to use products from the same manufacturer, it is recommended. They will guarantee the system for what it is designed to do where as if you mix and match you would have to get bond warranties and the like from two different sources. That is a pain in the butt and there is no reason to do it. Sika has a wonderful line of hardwood adhesives and those I am very familiar with and would recommend without hesitation.

From what you describe, I would not be overly worried about the whole hydrostatic pressure (water pressure) issue. It is rather uncommon, just something worth mentioning as I do not want to give incomplete advice.
 

Mark Brown

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Me again, I kinda glossed over the adhesive they recommended. Technically speaking if you used the T21 you wouldn't need the topical barrier. That stuff is a wonderful product when used appropriately and in most cases will preform all of the utilities you need it to. That being said, knowing you have had a damp slab and presuming you do not have an intact barrier, I would probably still make the recommendation to use both products. A little money now is worth a lot less in the long run than a big problem latter. Remember what I said about never having too much insurance :)
 

Loree

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Nov 21, 2021
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Texas
Thank you so much for the detailed reply!
I couldn’t agree more about insurance. I am a belt and suspenders kind of person in general 😂
I did read that the T21 was moisture and adhesive, but the spec sheet mentions using the primer under it on a wood subfloor so I figured they were compatible.
 

Loree

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Texas
Hey Mark Brown,
If you see this… thoughts on Mapei Planiseal PMB urethane vs Planiseal VS epoxy? The Mapei warranty looks great. Happy Thanksgiving!
 

Mark Brown

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I use the PMB all the time. It requires a nice slab which typically in smaller applications would require grinding the slab out, nothing too crazy, just enough to get contaminates off of it and rough it up a wee bit. We have use the VS a few times but typically we have shotblast the surfaces to get the csp # required to apply it.

In all honesty of all the products you have mentioned, i would go with the Sika if you could get a good price on it and have access. After that it would be PMB and then the VS.
 
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