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low and high moisture readings affected by air RH

Need some help to determine what is going on with a concrete slab and decide if it is safe to install a synthetic floor.

We have a 5 months old 3" - 4" concrete slab (with a wire mesh) with a 6 mil vapor barrier poured over an old 6" concrete slab. The new concrete slab was poured to solve some level problems and to install a vapor barrier as a precaution.

We are using a Wagner C575 analog concrete moisture meter to take measurements in points all over the floor 10 ft apart.

This is a 8000 SQFT gym in South America with no AC
The air relative humidity has been fluctuating between 60 to 85% and the temperature between 70 to 80 F.

We found that there are big differences in the readings linked to the air RH.

at 60% RH the readings are between 3.8 to 4.2 which would be acceptable to install our floor

at 77% is between 5 - 6
at 85% is over 6

Since the slab is 5 months old and it has a vapor barrier, we do not think we are having moisture problems coming from below.

Does anyone have any advice?

Can we install the floor assuming the higher readings are caused only to the air RH?

Can we use a dehumidifier to reduce the moisture content while we install the floor ( we have to start the installation in 10 days while the air RH will be around 80%) Will it be safe?

Another problem is that we are having different readings when we take readings around the same point. About 8" away we have readings with differences like 4 to 6. The wire mesh should be about 1.5" below the surface. What could be the problem?

Thank you for your help

Juan Pablo Salguero

Hi Juan,

I distribute, rep, and contract for Gerflor Taraflex (Synthetic Gym Flooring) in California. It is all we do so I know the issues of a gym & synthetic & moisture intimately. It is unique in that there is no other space other than VERY high end factory floors where you have a large enclosed space, a single large slab, that gets flooring. Most concrete industry knowledge focuses on warehouses which never get flooring.

#1 what specific product are you using? you might have an option to use a higher moisture tolerant material or a slip sheet to handle higher moisture.

#2 you should be using RH probes in the floor. An analog meter is only of use to establish where to place the probes.

#3 you have no AC, that is fine but what you need is the ambient conditions and understand that the ambient conditions are what your installed environment will be. Make sure that is within tolerance. It is the large fluctuation in conditions (hot and wet to cold and dry) that cause the moisture in the slab to come to the surface and cause issues with flooring adhesives. Understand that your slab will never get dryer than the highest your ambient humidity is.


Thanks for the questions. Hopefully I can help with clarify some of them.

1) as eaadams mentioned! analog meters, such as our C575, are not intended for making a final installation decision on ANY flooring project. At least in the US, there are only, maybe, a handful of manufacturers that even recognize this type of testing prior to installing finished floor product. The reason is pretty straight forward, there are too many variables with surface testing of concrete. At best, meters of this nature are are only looking at the top 3/4" of the slab, which really doesn't give true indications of moisture.
2) The fluctuations of ambient conditions are directly related to the fluctuations in your surface readings with the C575. Remember, concrete is hygroscopic, thus it will absorb moisture from the air when there are high ambient RH conditions.

The remainder of the questions really should be directed to the adhesive and/or finished flooring manufacturers for this product specifically. They really need to buy into and sign off on ANYTHING you do here.

Hope this helps.


Thank you Jason and Eaadams

I understand we should not take any decision based on the C575 meter.

The thing is that this concrete slab is only 4" thick, it has a vapor barrier and it shows to be dry enough or, not depending on the air RH. I know concrete absorbs moisture in the air, but I did not expect it to be that sensitive. On the other hand if it is being affected by the air RH, once we
install the flooring it shouldn't be affected by the moisture in the air.

We are installing a polyurethane floor, poured in place, over a rubber mat glued to the slab.

We plan to put some fans and do a better test before installing the floor.

Thank you for your input

Juan Pablo

Ok you are doing a pad n pray installation.

I've seen those fail due to moisture. You need to be at 80%

Juan, this should be interesting, send us pictures. Will there ever be any HVAC in this building?
Stephen Perrera dba
Top Floor Installation Co.

It isn't uncommon outside the USA. We did a project in Guam that had no HVAC. Working on one in Kona Hawaii that is essentially open breezeway. And have another going in Tahiti that is ... well I didn't win that so screw those people.

It's going to be one sweaty gym without HVAC.
Stephen Perrera dba
Top Floor Installation Co.

Amazingly Ernesto that is something a lot of gym chains look for in flooring. It is part of the reason they use the re-ground rubber products. They get a musty 'work out' smell into the flooring. Some weight lifters like it.

Gross, unless it's female sweat.
Stephen Perrera dba
Top Floor Installation Co.

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