What should we do if there is evidence that there is high Moisture coming up our Concrete Slabs?


Have scratched the idea of replacing the old carpet we have left in the house. Bedrooms and Hall with a few closets, we found moisture under some plastic storage containers in the closets and remembered the wetness under an exercise bike mate sometime ago. There doesn't seem to be any mildew or mold signs or stains. Is the assumption that the carpet has let the humidity vapor through and not caused any major problems. Even the carpet under the plastic containers did not absorb enough moisture to feel wet or soggy, just the plastic and the top of the carpet where touching the plastic seemed to be wet. I pulled up part of the carpet to inspect and there is no stains or discoloration. The tiled and laminate part of the house seems fine but have no idea what lays underneath that can be called a problem. THe carpet is old and needs to be ousted, the tile and laminate is 15 years old with the tile looking like it did when installed and the laminate normal scratches here and there and a couple of water damaged boards under the freezer from either overflow or a lot of dropped ice under the unit.
Is it easy to say to just replace the old carpet with new as long as it can breath, as long as it can let the vapor escape?


ANSWER - Carpet might be allowing the moisture to evaporate through it, but as dirty as carpets get you could develop some microbial growth in the carpet.  Adding more carpet doesn't solve or prevent problems.

As your existing tile demonstrates it holds up well.   Ceramic Porcelain tile is more resistant to moisture. It can allow moisture to evaporate through the grout joints although you could develop efflorescence staining if there is excessive moisture.

Best bet is to replace with a porcelain tile. If the moisture in the concrete slab isn't excessive you can apply a liquid applied waterproof/crack isolation membrane over the properly prepared concrete and then adhere your tile to the membrane.

To verify the Relative Humidity of the concrete slab follow test method ASTM  F2170 or perform the Calcium Chloride test ASTM F1869.  Different manufacturer's have different requirements and limitations for moisture transmission.  If the moisture is extreme then you install a moisture mitigation product over the slab.  There are a number of different types provided by the various manufacturers of tile and stone installation products.

4 thoughts on “What should we do if there is evidence that there is high Moisture coming up our Concrete Slabs?

  1. w bennett says:

    The ASTM F2170 result in the slab of my Florida home is 99% RH and we have efflorescence. We currently have vinyl plank flooring, which had issues with the moisture and is buckling and cracking after 4 years. We are replacing with porcelain tile. The options for a moisture seal seemed pretty expensive with no real guarantees, plus it seems there is always going to be moisture and not letting it escape may cause other issues (ie epoxy can pop, other options require continous seal but we can’t seal under cabinets and walls, so moisture could still get thru). I was thinking no moisture barrier would be the way to go, so the moisture can escape thru the grout lines, and we would leave it unsealed. Is this approach sound or would you always recommend a “liquid applied waterproof/crack isolation membrane”? Is installing a “moisture mitigation product over the slab” warranted in this situation and if so, is there a type you would recommend? Also, what is meant by “properly prepared concrete” ? Thank you

    • Donato Pompo says:

      There are moisture mitigation systems manufactured by the key manufacturers such as Laticrete, Mapei, and Custom Bldg Products. They are epoxy based and they say they can be used over slabs up to 100% RH. They are the most expensive and will provide the best performance. They do provide a high degree of tensile strength in terms of being resistant to hydrostatic water pressure.

      If you have a high water table that during raining times raises it can create a hydrostatic condition. So whether you use the mitigation system or just install it over the slab, excessively high hydrostatic pressure can cased debonding.

      Properly prepared concrete is where it has been scarified to remove any potential contaminates and makes the slab readily absorb water. This helps ensure a good bond to the concrete.

      You can use a liquid applied membrane

      • w bennett says:

        What type of contractor can identify definitively if we have hydrostatic pressure in our slab and offer solutions on how to mitigate that?

        • Donato Pompo says:

          Hydrostatic pressure is where water is exerting a pressure, so water should be oozing up through the concrete slab, so it should be obvious.

          There are companies who specialize in performing relative humidity testing per ASTM F2170. They drill a hole into the concrete slab in insert a probe and seal it and then measure the RH over a few days. If there is hydrostatic condition the hole that is drilled would immediately fill up with water.

          As stated above, there are epoxy moisture mitigation membranes that can be applied that tile can adhere to.

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