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Adhesives and Moisture

Hi everyone... I'm new here and love this thread. There is no question that there is a lot of confusion over the evaluation of moisture in concrete slabs. I especially love the people that put their electronic meter on a slab - observe the red light flashing and the beeping sound coming from this device - and then pronounce the slab to be "too wet". Sheesh!

Meters are Guides... NOT Gods!

So - I have a question for CC... Based on your message dated 03/18, you have a somewhat similar ERH reading in the slab - yet the MVER are on two ends of the 'acceptability spectrum'. When using the Rapid RH device, what is considered to be 'acceptable'?

Thanks guys!

Ken Larsen

Hi Ken, glad you came on board! There are a lot of great questions here, and we all learn something with each post. I'll second your comments that meters are not gods, and add that interpreting the meter's information is 90% of the battle.

As for acceptable RH levels, I guess I could go two ways with this. The safest is to say: Follow the manufacturer's recommendations. If the manufacturer does not specify an acceptable RH level, call their technical department and ask! I have done this several times (back before RH became more a common parameter in the US) and was always pleased to be able to have a good discussion with the manufacturer's technical team and come away with a warranty based on RH levels.

The second way you can look at RH is to go with your own knowledge and experience. I would have no problem putting any floor covering over a slab that is at 75% RH, and almost all manufacturers will agree with that. Yet I have been approving test installations on 80% RH slabs for many years and have not had a failure on those. Yet. I should say 'yet' because sometimes failures take time to manifest. It is some consolation to me that there are several flooring manufacturers that specify 75% RH and will write a warranty extending to 80% RH on a case by case basis. I believe this is because we have found that if the concrete is well prepared and the installation of the floor is done well, 80% RH will be fine. (Disclaimer: Always check with the manufacturer before exceeding the written RH or MVER rates to receive their approval, or the manufacturer's warranty could be voided.)

I have had the good fortune of working with very large companies that are willing to take the risk of stretching the boundaries of installation requirements. In that context I have had moisture sensitive sheet goods installed on slabs with up to 96% RH without failure. But that installation was done by gluing a homogenous sheet vinyl directly to concrete using an epoxy adhesive. No patching compounds were allowed. So far it's been 6 years and is still failure free!

So to get back to the question, 75% RH is almost universally accepted for moisture sensitive flooring, and 80% RH will be acceptable most of the time, even if it takes some leg work to get approval. Above 83% - 85% the concrete is still actively hydrating and you are walking on thin ice if you don't install a breathable floor covering.
JD Grafton
Concrete Answers for Flooring Problems

Outstanding. Thanks CC...

Have you given any thought to the other risk associated with 'damp' slabs in contact with organic materials: water activity (aW)?

If some species of mold can be supported when the aW is greater than 0.68, then one could argue that the slab should never exceed 68% Rh - (or for the sake of easy numbers, 70% Rh).

I come from the water damage restoration industry, and in the past I have been asked to assist in determining 'how dry - is dry enough?' when evaluating a concrete slab. This has been difficult for me to answer, but my approach began with identifying the risks that were present. The warranty of the new flooring was secondary to my concern over the potential of the wet slab supporting microbial growth - especially in areas where organic materials are in direct contact with the concrete. If there was wood framing in contact with the slab, I held for the 70% ERH threshold.

Any thoughts on that logic? Cool

Thanks again.

Ken Larsen

Thanks Ken,
This is far from my area of study, so I won't try to embarrass myself too much.. Smile

I have found mold on very wet slabs and in drywall on the edges of slabs. But typically the moisture sensitive floors are mold resistant, the adhesives have antimicrobial agents added, and the floor is well sealed from the surrounding air. Where we could run into trouble is in flooring cuts and gaps, if they get organic material in them, mold could be supported.

I am not scared of mold; after watching dozens of tests performed in hospital rooms with mold that looked like it could be mowed with a lawn mower, and seeing the spore activity in the clean fresh outdoor air far exceeded the interior spore count, I just don't think the terror is warranted.

That said I realize some folks are susceptible to mold induced illness, and no building owner wants to have a bad strain of mold growing that may harm someone! But for the most part, concrete will not support mold, the adhesive won't support mold, and the flooring won't support mold. After that I have to leave things to much smarter people like yourself to help figure a way to minimize the risk associated with mold and slabs.

I would be willing to bet that well over half the concrete out there has greater than 70% Rh though!
JD Grafton
Concrete Answers for Flooring Problems


Your last sentence really has my attention.

Thanks CC. I will have to give this some more thought before I make any more comments.

Ken L

Here's why I say that:
Years ago we had more asbestos in flooring materials, and our adhesives were much more forgiving. Under-slab vapor retarders were not as common and concrete moisture was seldom an issue. If there was a vapor retarder installed, it was probably 6 mil poly, and those poly retarders degrade over time. There was also a time when sand blotter layers were the norm, and we now know that blotter layer is a great conduit for water movement under the slab.

I have been finding the majority of slabs I test that are over 10 years old have very high moisture levels.
JD Grafton
Concrete Answers for Flooring Problems

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