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.
4 https://www.todayshomeowner.com/how-to-deal-with-condensation-on-a-garage-floor/
5 Kanare, H.M., “Sources of Moisture”, Concrete Floors and Moisture, page 20.

 

Last updated on March 31st, 2023

28 Comments

  1. Peggy says:

    We live in Northern Florida and have noticed our front porch patio has started sweating, or very moist even when there’s no rain. What causes this, is there cause for concern and do you have insight on how to mitigate it, if so?

    • Jason Spangler says:

      Peggy:

      Thanks for the question. I would say no real need for alarm, unless it gets to a point where it is causing a safety hazard if walked on. I would GUESS, based on your location, that this is condensation forming on the surface due to high humidity in the air.

  2. Terri says:

    I am logical and methodical but I am stumped. I noticed my Bruce 21 year old Engineered flooring has begun to discolor especially if something sits on or over it for a few weeks. I have had two plumbers assess and find no leaks they suggested a moisture test be done. Yesterday a restoration company did a visual inspection and used a hand held meter and suggested I have a leak test done. A new leak test company came today and did a prong meter test into the wood floors and found all discolored areas dry except one slat that is 12’ long and 4-5” wide and 5” in the middle of the slat at a threshold between the living room and den is 24% moist. The hydrostatic leak test found No leaks. The wood floor is glued to the concrete slab floor in a humid Texas coastal city. The building is a 46 year old townhouse with 4 units built on one slab. Pier work has been done on all units with mine being the least at 8 piers, done 7 years ago on the North side of the unit. The neighboring unit on my South side has over 20 piers that are 25+ years old. We have suffered a severe drought for 2 years but when it rains it comes in buckets from one hour to days. I did not flood during Hurricane Harvey. Foundation company did a test and found a 1/4 “ drop on the south wall and a 1/2” drop a few feet from that area on the same wall. Foundation company said I do not need to do anything until there is a 3/4” drop. I don’t want to wait for more floor discoloration and damage. I react to the outgassing chemicals once the floors are moist. How do I find out what is causing the floor moisture, discoloration and fix it. I also have high humidity in the house 61%+ depending on weather and if I am home or traveling. What is causing the moist environment in my house if there are no visible leaks or water damage in pipes and from the results of the hydrostatic test. How do I know if it is a slab issue of moisture migrating thru the concrete slab? How do I find out if there is a slab crack and is that the cause. I am very concerned. Any advice is appreciated. I am your gramma’s age and need direction. Blessings and Peace

    • Jason Spangler says:

      Terri:

      Thank you for the questions. First, IF the moisture is coming through the slab, it could be very well an issue with not having a vapor retarder directly under the slab. It may not have been building with one or, due to the age of the structure, if one was used it may have broken down by now. That being said, there isn’t a guarantee that is the problem. Based on the information you provided, I would want to rule out the “easier” stuff first. You state the humidity in the house can vary. Sometimes, depending on the temperature and humidity of the air and the temperature of the surface (floor), it can cause condensation on the floor surface. Having the surface covered (box on top) MAY alter the surface temperature and make this condensation more likely. First thing you need is a device that either reads or reads and records the relative humidity and temperature in the air. Something like this Smart Logger™ | Bluetooth® Temperature & Humidity Data Logger (wagnermeters.com) but there are others. Once you have a device to measure this, you can input it into something like this Dew Point Calculator By entering the temperature and humidity in the air, it will give you a temperature result. This temperature result is the temperature at which point condensation will start to form on a surface. Once you have this temperature called “dewpoint” you can simply use a device like this Infrared Thermometer – Wagner Meters to measure the temperature of a surface that looks discolored. If that measured surface temperature is within about 5F of the calculated dewpoint, the chances that the discoloration may be at least partially caused by condensation is high.

      Example: if the temperature in your house is 70F and the relative humidity is 65%, dewpoint is calculated at about 58F. That’s pretty close actually. Minor fluctuations in any of the factors may cause an issue. If you do this and find this to potentially be an issue, you will probably need to look at your HVAC system, adding dehumidification, or just making sure you keep conditions consistent even if you aren’t home. I would rule this out first. Good luck.

  3. MELVIN says:

    I recently POURED A HOUSE SLAB 4 IN THICK 3 FT BEAMS ALL AROUND IN THE MIDDLE INSTALLED POLY BARRIER AS WELL SO WE STARTED FRAMING THE HOUSE PUT UP WALLS HAVE ROOF DECKING ON NO WINDOW AND DOORS AS OF YET BUT HOWEVER MY SLAB HAS STARTED SEATING I BEEN AROUND CONCRETE ALONG TIME AND IM PUZZLED WHT U THINK AFRAID IF I INSTALLED THE WINDOWS AND DOORS IT MAY GET WORSER

    • Jason Spangler says:

      Melvin:

      Thanks for the question. It sounds like you may have a problem with dewpoint condensation. If you measure the relative humidity of the air and the temperature of the air and plug those numbers into here Dew Point Calculator, it will tell you at what temperature the moisture in the air will condense. Then take an infrared thermometer and take the surface temperature of the concrete. Compare that temperature to the temperature calculated in the Dew Point Calculator. If they are within 5F of each other, chances are that is what’s happening. Good luck.

  4. 45 year old slab on grade on Texas gulf coast. Never had issues with sweating slab until a recent remodel. Took out large ceramic tile (18″x18″) and replaced with smaller, 8″x18″ ceramic tile. Weeks later, tiny water droplets appearing at a few grout joints. Took up some tiles and found dampness under them. Moisture would dry quickly and not return. Several tiles have been off for weeks now and never show sweating. Tested all plumbing, and it’s fine. No water intrusion from any other sources. The pH of the moisture droplets at the grout joint is off the chart high with pH test strips, (possibly unreacted grout?). Any suggestions?

    • Jason Spangler says:

      Walter:

      Thanks for the question. A 45-year-old stab probably doesn’t have an intact vapor retarder below it. Depending on the age, types of installation materials used, and installation methods, the original floor may have done a great job standing up to excess moisture in the concrete. The new installation may be a different beast. Personally, I would want some idea of what the RH% is in the concrete slab, especially in the area you currently have removed. Knowing this may lead to a solution. Good luck.

  5. Mark Bosley says:

    I have a finished basement with an area under the front porch that sweats. The area sweats mostly on the ceiling but appears along the upper portion of the walls as well. The area has a door so ventilation is not very good.
    I applied an epoxy sealer on the walls and ceiling thinking initially the water was a leak but have since determined it is sweat.
    I have another, albeit larger, area under the back porch. That area still has the 3/4” plywood in place from when the ceiling was poured. There is zero sweat visible in that area.
    I’ve considered applying foam insulation panels to the ceiling and walls in the area that sweats…Any suggestions?

    • Jason Spangler says:

      It sounds like that could be a viable option because it is probably the surface temperature of the concrete causing the condensation. I would try a small area and see what happens. Good luck.

  6. Liz says:

    Just bought a new house and the slab under the carport which is well ventilated from the front and the back sweats. What is the best solution to take care of this before someone slips and falls. Namely me.

    • Jason Spangler says:

      It sounds like there may be issues with the humidity in the air vs the temperature of the concrete which is causing condensation. Depending on the situation, you may want to look at doing some type of coating on the concrete for high traffic areas. Maybe reach out to a coating applicator for recommendations.

  7. Carol says:

    I have water on a cement floor that is most likely coming up from the soil because of no vapor retardant under the concrete. Does the floor then need to be torn up to place this retardant and then re-laid? Please advise.

    Thanks,
    Carol

    • Jason Spangler says:

      Carol:

      Thanks for the question. Depending on what you are intending to do with the space, you can usually have a liquid form of the vapor barrier applied topically so the moisture transfer is minimal and a finished floor product can be installed. Tearing out a slab and starting over is only in the very worst situations. Good luck.

  8. Jane says:

    Hello,

    We recently purchased a cottage (built 2005) The basement was unfinished just studs insulation and vapour barrier. We have closed in corner with walls for the water/pump/hot water tank etc. Recently added a stairwell because floors were not connected.
    The concrete gets dark and damp in numerous areas and sweats. The back wall where the foundation is against the ground is the worst area. We are on a slope and front of basement faces lake with patio doors.
    Would like to know what exactly is going on. Wondering if a vapor barrier was installed? The build of the cottage (so I have learned) was unsupervised. So I wonder about vapor barrier?
    Would like to put in floor. What do you suggest.
    We have purchased a air exchanger ( not installed).
    What do you think about whole home dehumidifiers? What do you think about Dricore or similar products for subfloor?
    Trying to figure out problem and what to do next!!
    Thank you.
    Jane Barker

    • Jason Spangler says:

      Jane:

      Thanks for the questions. You could have a geotechnical engineer come in and do a core sample to verify if there is a vapor retarder or not under the slab. It’s hard to know about the dehumidifier because I am not sure what the conditions in the air space are like. Besides verifying a retarder, I would also check to see what the dew point temperature of the environment is. This can be done by using a thermo-hygrometer to measure the relative humidity and temperature in the air and then plugging those numbers into something like this Dew Point Calculator (https://www.calculator.net/dew-point-calculator.html). This is the temperature, based on the current conditions, where condensation will form on a surface. Next, use an infrared thermometer to measure the temperature of the concrete surface. If that number is within about 5 degrees of the calculation, this may be the issue. Good luck.

  9. Cherise says:

    Hello, Thanks for taking questions. I would really love to hear how you would recommend mitigating my issue. We bought a 1908 Craftsman house in a rather wet little town where the water table is famously high. Many residents in town have water issues in their home, even in some new homes if engineering wasn’t very carefully considered. We got a structural engineer to look at the leaky basement. He said to put in French drains. We did–not only along one foundation wall that we had rebuilt at the same time but also through the middle, following the plumbing line and then up to the front of the house where there is a bathroom that we (to code) put in at the same time. Then we painted the flooring and walls with Dry-Lock waterproofing paint as the engineer suggested. We put the vinyl floors in with the plastic barrier underneath it in August. Well, when we tore up the section two days ago–to make sure that we had gotten all the water from a small chimney leak (which it looked like we did), we noticed that the whole concrete floor that had plastic over top of it had been actively sweating. There are parts of the basement that we left unfinished due to our own time restraints. Those have never had any sweating or water surfacing from it. What does this mean? Does this mean that there is nothing that we can put on top of the very old concrete floors? One note: the concrete that we put in over top the new plumbing had no sweating or mildew/mold–but everything else did. Please help me know what kind of finishing I could put on it. I assume that we can’t put anything on it that won’t let it breathe. But I am also afraid to put carpet over it for the same reason–I don’t want it to mildew. We do have a laser HVAC system in that is supposed to kill any mold spores. And, the WINIX filter that we put on in the room went back to blue after about an hour of the flooring pulled up (and was blue before we pulled it up–with the vinyl planking on it). Thanks for any advice. Mainly I wonder if we can safely do an epoxy finish or if that would do the same thing and make for sweating again.

    • Jason Spangler says:

      Cherise:

      Thanks for the questions. First, I’m not convinced that the concrete is “sweating” because if it were, the entire area would exhibit these symptoms. More than likely what you are experiencing is moisture vapor moving up through the concrete, from the soil, because there is no vapor retarder below the slab. Waterproofing is great, done correctly, for waterproofing, but you are dealing with vapor. I would talk with the epoxy finish manufacturer and see what they offer with regards to a moisture mitigation solution that is applied prior to the final finish. Good luck.

  10. 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
    bevrachel@gmail.com

  11. 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:

      Naod:
      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.

  12. 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

  13. Isabelle Riahi says:

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

    • Jason Spangler says:

      Isabelle:

      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.

  14. 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:

      Tyler:

      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.

      Jason

  15. Cliff Johnson says:

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

    • Jason Spangler says:

      Cliff:

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

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