The Dangers of Concrete That Sweats!

It Opens Up Pandora’s MAD Box…

Rough Finished Concrete FloorConcrete can sweat! Are you aware that this has been well-documented? Can the problem of concrete condensation really open Pandora’s “MAD” box? What is Pandora’s “MAD” box and how do we prevent this from happening? A lot of unanswered questions…

What Causes Concrete to Sweat

Concrete sweating, also known as sweating slab syndrome (SSS), refers to condensation that develops on the surface of the concrete. It is directly related to the dew point. “If the surface of a floor slab is colder than the dew point temperature of the ambient air above the slab, moisture will condense on the surface of the slab. This condition, commonly called “sweating,” typically occurs when warm, moist air flows into a building that has relatively cool floors.”1

“A second type of dew point condensation can occur if the concrete has a high moisture content and the surface is strongly cooled. For example, a floor slab with a resilient floor covering and no vapor retarder below the slab can have relative humidity in the mid-to-upper 90% range. If the HVAC system is set to cool the interior of the building just a few degrees below the ambient floor temperature, condensation on a concrete slab can occur in the upper region of the concrete, just below the floor covering. The presence of liquid water then dissolves compounds in the concrete that can raise the pH and begin to attack the adhesive and floor covering.”2

From this expert source, it is clear to see that concrete sweating can become a major problem…but open Pandora’s “MAD” box? The phrase sounds ominous enough…should we look inside? What is “MAD”?

  • M = Mold Growth
  • A = Alkali Attack
  • D = Dangerous Slick Conditions

Mold Growth

Mold is a predominant factor in airborne allergies and pulmonary problems.
Any condition that supports mold growth and its associated problems should be avoided at all cost. A sweaty slab left unchecked is a perfect environment for mold spores to begin multiplying.

The building industry continues to push towards methods that promote a tighter “building envelope” (a good thing). Within those guidelines, proper care must be taken to ensure that…

  • Buildings have proper ventilation systems in place.* This is very important as most moisture conditions that exist in a building are created by the occupants. For example; showering, laundry, cooking, etc. are primary causes of moisture within a home.
  • Vapor retarders (or barriers) are in place between native soil and below-grade concrete…according to ASTM standards and local building codes. This is critical to preventing sweating slab syndrome.*
  • Insulation is placed in strategic areas, according to ASTM standards and local building codes. Insulation plays an important role in maintaining a proper thermal break between native soil and any type of concrete foundation…between the internally conditioned climate and the external atmosphere. Whether it is a slab-on-grade foundation or below-grade basement walls…proper insulation is a key factor in preventing concrete from sweating.*

Minimizing any chance for water to condense on concrete…minimizes the opportunity for mold growth to begin.

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Alkali Attack

The alkali-silica reaction (ASR), more commonly known as “concrete cancer”, is a swelling reaction that occurs over time in concrete between the highly alkaline cement paste and the reactive non-crystalline (amorphous) silica found in many common aggregates, given sufficient moisture. ASR can lead to serious cracking in concrete, resulting in critical structural problems that can even force the demolition of a particular structure.3

“A vapor barrier may not have been installed under the slab when the concrete was poured. This can allow moisture from the ground to penetrate up through the concrete, resulting in damp conditions. If there are cracks in the slab and the drainage is poor, it may even cause ground water to seep up through the cracks and puddle on the floor.”4

Efflorescence“Liquid water diffusing upward through a concrete floor slab can carry alkalis in a high pH solution that can attack floor finishes.”5 Over a period of time, alkali attack can cause permanent damage to the concrete itself. This can result in costly repairs.

Dangerous Slick Conditions

Moisture that accumulates on any smooth surface can be an undeniable slipping hazard. Moisture that accumulates on sealed concrete creates an even more dangerously slick surface…while the damp environment that is so prevalent with the growth of mold can predispose concrete surfaces to slick, dangerous conditions. More evidence of the dangers of sweating concrete.

For concrete professionals…best practices dictate that careful planning prior to beginning your concrete project, and incorporating preventative measures to avoid any future moisture challenges provide the best solutions.

For consumers…if you suspect that you have a concrete moisture problem, retain a qualified concrete consultant to determine if you have a moisture problem and provide you with solutions to correct the problem.

Concrete can sweat…but don’t let it make you “MAD”.

*Always check with local building authorities as well as ASTM standards, to verify current codes and best practices.

1 Kanare, H.M., “Sources of Moisture”, Concrete Floors and Moisture, page 15.
2 Kanare, H.M., “Sources of Moisture”, Concrete Floors and Moisture, page 15.
3Alkali–silica reaction in concrete.” Understanding Cement. Archived from the original on 10 August 2007. Retrieved 2007-08-11.
5 Kanare, H.M., “Sources of Moisture”, Concrete Floors and Moisture, page 20.


Last updated on June 8th, 2021


  1. beverly rachel says:

    Hi! I’m looking at buying a house built around the 1950’s. The current owner explained there’s no carpet throughout because the concrete floor sweats. My question is, should I run from buying this house?
    Thank you in advance!
    Beverly Rachel

  2. Naod says:

    I have one question. We are constructing 3B+G+21 building and we are currently on the construction of 5th floor. What I am seeing is located under the 2nd floor slab. Which is like sweating under and has a darker black color with a different odour. I have recognized that there hair-cracks. What can be the cause and the solution? Thankyou

    • Jason Spangler says:

      Thanks for the question. If it’s sweating on the bottom side I would rule out condensation by taking the bottom temperature and compare it to the dew point calculation based on the temperature and humidity of the environment on the first floor. This may be the problem and it may solve itself when the HVAC units are turned on. Good luck.

  3. Gary Johnson says:

    Jason I have an old machine shed that is always sweating, the building is probably 80+ years old, so of course I didn’t pour the floor, my other outbuildings don’t have any issues, so I’m kinda surprised my shop is Thanks,Gary

  4. Isabelle Riahi says:

    Hello Jason,
    What would be a better sealer please? There are so many to choose from! Thanks!

    • Jason Spangler says:


      Thanks for the question. Unfortunately, I can’t recommend a specific brand of product, but I can tell you that the epoxy sealers seem to be the best. Good luck.

  5. Tyler Olson says:

    Jason – we have LVT plank flooring over the basement slab. After about a year, minerals and water started creeping up between the plank seams. The planks were pulled up and the slab was sealed with a concrete sealant. It worked for about 6 months and now more mineral deposits are showing up in the seams. What would you suggest to use to seal that portion of concrete under the vinyl planks? Its a slab that is 18 years old and had no problems until we finished the basement 2 years ago and put flooring over the slab. Thank you.

    • Jason Spangler says:


      Thanks for the question. You will probably need to either go to a better “sealer”, one that is good to 100% relative humidity in the concrete, is warranted in a basement setting, works on a slab with no vapor retarder (due to age, odds are pretty high if yours had one, it may not still be intact), and works up to a 14 pH. Most of the mineral deposits are salts moving to the surface of the concrete. The other option is to go to a more breathable flooring product. I hope this helps.


  6. Cliff Johnson says:

    Is there a solution to a slab that has been subject to an “Alkali Attack”

    • Jason Spangler says:


      Thanks for the question. I would need to better understand the specific problem you are outlining. How is it presenting?

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