Water coming up underneath gym flooring

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Well-Known Member
Apr 2, 2016
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Will 1/2” foam padding catch water vapor coming up from a concrete floor and cause pooling? Our basement has stick down waterproof flooring and we put mat down for a gym and keep finding water pooling underneath. It’s in the middle of the room and doesn’t appear to be coming from a foundation wall. It’s difficult to see because it’s been wiped up in the pic but it gets pretty wet.
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Hello Dborns,
Even if the foam padding stops the water from reaching the mats, mold & bacteria will grow. Virus' also thrive in wet environments. All three like dark, damp environments. Mold also enjoys the alkalinity of concrete.

And, foam padding is usually open cell. It'll absorb water as sponges do. If you do use foam, get closed cell.

Perhaps a better plan, if you can't stop the water, is to use something like Dricore Subfloor. It will allow air to circulate between the Dircore and the concrete. https://dricore.com/products/dricore-subfloor/

A sheet drain like Dimple board will also let air circulate, but I don't know if it'll wiggle around under heavy exercise.

I have to think, however, if lots of water or heavy moisture is under either product mold & bacterium will still grow.

Not to be Mister Lecture, but stopping the water is quite important for your & the building's health.
If you can't stop it, be sure to ventilate the room very, very well and use a hospital grade UV air circulator with 254 NM wavelength. It's not ideal, but can't hurt. A HEPA filter will also help. (Note than many household UV don't do anything. You need the tested & certified hospital grade models. NSF may be helpful in finding one.

Whatever you do, don't try to clean mold with chlorine products, such as laundry bleach!
Mold has a defense trigger that senses chlorine and will throw spores quite far when it detects chlorine. I learned that in EPA classes I had to take at work. One of the videos showed spores flying more than 7 meters.

Quaternary Ammonium products will kill the mold spores & bacteria without triggering a defense. You still have to remove the living mold manually. It has a residual action, so it starts working again when next damp. This lasts until it's washed away. (Residual is for mold & bacteria. I don't know if virus' are included.)

Kind of an interesting article:

Thank you for the info! We already have 1/2” foam flooring down for the gym floor which the foam sits on top of a type of glued down water-resistant floor. I was under the impression that it’s natural there is always moisture coming up through concrete, and that was what’s coming up through the seams of the flooring and causing the water underneath the foam? It’s all out in the middle of the room, and the outer foam tiles next to all the walls are dry. And it’s not sitting water, it’s just enough to cause the tiles to be discolored and damp.
Concrete, even below grade, should never have moisture coming up.
Often these days, 6 mil or heavier plastic sheeting is laid before the concrete is poured to block the moisture.

In the olden days, like at my house, there would be a network of clay pipes under the slab. They tied to the sanitary piping system. Any water that rose would hit the joints between pipes & be led to the sanitary piping. It never gets a chance to hit the bottom of the concrete. Where we live the water table is very high & my basement is so dry the floor gets dusty. The newer houses in the neighborhood don't have it and they often have 3 or more sump pumps that run constantly.

Some Hope:
You mentioned that there is no moisture at the outer tiles. Maybe (hopefully) this is good news and the moisture isn't from the floor at all. It may be humidity in the gym that condenses in the coldest spot- the center of the slab. If that's the case, something like Dricore will solve the condensation problem. I'm fairly certain that foam tiles can on on top of the Dricore. (Laminate flooring can.)

A Test:
If some floor tiles in the gets wet area can be pulled up to expose concrete, try this trick:
A) With the tiles removed, let the floor completely dry.

B) Seal a piece of clear plastic, such as kitchen wrap, on an area. Be sure to tightly seal the entire perimeter to the floor.
It is ideal if you can get the plastic to be in contact with the floor. This will keep the plastic at floor temperature.
(Item C-3 below is the reason to have the plastic touching the floor.)
Sometimes it's hard to get tape to stick to concrete. Carpet tape works, but is hard to remove without shredding.
Duct Seal or temporary caulk work very well. Duct seal is much cheaper & has no VOCs. (About 1.50 USD per pound)

C) After a few days, look at the plastic.
1) Is there water under the plastic? Yes = Water is coming from the concrete
2) Is there no water under or on top of the plastic? Either whatever event causes the water (rain, perhaps) did not
happen yet.
3) Is there water only on top of the plastic? Yes = Humidity condensing from the room, as explained earlier is
why the floor is wet

If it turns out that the water is coming up through the slab, there are mitigations that can be done. Some dig outside, some put a perimeter drain around the room, leading to a sump. There are lots of methods.

You definitely know your stuff, thank you! I started with just leaving the gym mats up for a couple days and saw no liquids seeping up from the flooring. I’ll go through the steps you suggested from there.
I started with just leaving the gym mats up for a couple days and saw no liquids seeping up from the flooring.
That may be really good news!

Perhaps, if it won't interfere with using the room, leave those mats up until a day or two after a heavy rain. If no water then shows up, my guess is that the water is condensation of humidity from inside the room. (More conclusive will be to do the plastic on the floor idea from above.)

To better explain my thoughts on condensation:
In warm weather the coldest area of the floor will be farthest from the walls. If the floor temperature below the dew point, humidity in the room will condense in the cold spot. (Glass of ice water versus glass of warm water. One gets condensation & one doesn't.)

The reason is that the perimeter has footings extending below the floor. The warm ground will conduct heat to the footings & the footings will conduct to the concrete floor. Therefore, the perimeter is warmer than the center where water will condense. (In winter it's the opposite. Colder perimeter & warmer center)

Maybe your solution is as simple as ventilating the room or a dehumidifier.

Here's Hoping!
If the problem persists you might want to think about just placing the mats where you will be using them the most and leaving some open areas for room air to keep the floor from condensing. They make tapered edge strips that snap into the mat sections to prevent tripping.
Following along the lines of Bud16415's suggestion, if the area isn't too large perhaps this will work for you:
I use 9.5 mm thick Elephant Bark from Rubber-Cal and roll it up when I'm done napping- Oops! I meant "exercising".
Yeah, foam padding can trap water vapor and cause pooling. Concrete floors can let moisture through, even if you don’t see it. Try using a vapor barrier under the foam to prevent this.