Dehumidification and Drying Concrete: Hurrying Up the Wait

Hurry and Wait

In flooring applications, either finished concrete or applied flooring, there is always a “hurry up and wait” element – wanting to move ahead but knowing that rushing too quickly may result in a flooring failure or flooring problem. Meeting concrete and flooring specifications can seem to be a luxury that is not always available to the contractor on a tight schedule.

Dehumidification is one of the processes often used to try to encourage a concrete slab to dry more quickly in order to reach the installation point. The basic concept is simple: reduce the dew point of the air surrounding the slab so that more of the latent moisture within the slab can evaporate out through the slab surface. In the building industry, there are several common approaches to dehumidification: condensation dehumidification, heating (or drying) dehumidification, and desiccant drying.

Desiccant-based dehumidifiers use a chemical attraction to remove moisture from the air. Humid air is moved across a desiccant material that binds and holds the moisture. A hot air stream is then used to release the moisture from the desiccant and vent it away from the slab’s environment. The dried desiccant then absorbs more moisture and releases it again through heating in a continuous process. This process is not influenced by external weather so can be used year round as long as the concrete slab can be isolated from weather conditions.

Dehumidification Tubes

The condensation process uses cooling-based dehumidifiers which cool air, effectively dropping the dew point so that moisture can be collected and drawn away. By contrast, dehumidification uses heated air to raise the dew point of the surrounding air, allowing it to absorb more moisture from the slab surface. The saturated air is then circulated, either through a collection system or to the outside air where it releases the excess moisture as it cools. It should be noted here that most installed HVAC systems are not capable of removing the amount of moisture emitted through the drying process of a concrete slab. While it may be possible to raise or lower the ambient temperatures through the HVAC system, dehumidification must actively remove the excess moisture from the structure. If the moisture is simply moved around within the walls, excess moisture can cause mold growth or equipment deterioration over time.

The real key to successful dehumidification is to have the concrete slab completely enclosed so that the moisture removed during the process is not re-introduced to the slab’s environment. Either the slab must be enclosed in service-ready indoor conditions, or a vapor barrier must be erected around it for outdoor conditions. Accurate relative humidity (RH) testing lets you accurately track the movement of moisture out of the slab. Once the slab has reached the desired RH levels and the finish or flooring is applied, the concrete will be optimally able to sustain a quality finish that will last for years.

Wagner Meters’ Rapid RH® moisture testing system can help you easily and accurately test the RH so you can track the moisture out of the slab during the dehumidification process. Since 2002, ASTM-F2170 in-situ RH testing has been replacing calcium chloride as the preferred method for concrete moisture determination.

Call Wagner today at (541) 291-5123 or order your Rapid RH® moisture measurement system online.

Jason has 20+ years’ experience in sales and sales management in a spectrum of industries and has successfully launched a variety of products to the market, including the original Rapid RH® concrete moisture tests. He currently works with Wagner Meters as our Rapid RH® product sales manager.

14 Comments

  1. Kevin says:

    I have a slab that won’t dry. Our slab leak presented on May 3rd – we had dehumidifiers, fans and a vapor barrier for 6-7 days before we decided to pull the floors up. Then we tried them on top of the slab for at least another 7-10 days, and nothing changed. Then we had the floors “peeled” of the adhesive layer on top of the slab to “let it breath.”
    Now, almost a full month after that, our slab still wont dry. We’ve been running fans on the floor and the ac 24/7 for last 3 weeks. We were told to let air in, cool outside air and ac. We are hesitant to use another vapor barrier and use dehumidifiers and heat again as it could add damage to our newly remodeled downstairs.
    Now, we want to drill 2 16″ holes in our slab to see what is causing these two spots to remain. Mind you, that our entire water system under the slab has been cut off and our house has been repiiped. Yet, our slab won’t dry. What should we do?

    • Jason Spangler says:

      Kevin:

      If I am understanding, correctly, what you are presenting, you had a plumbing issue BELOW the slab and the moisture made its way INTO the slab and up to the surface, affecting the finished floor? Based on this, it would be a pretty safe bet to assume that you don’t have an intact vapor retarder below the slab. With that, you could apply some type of GOOD quality vapor retarder on top of the slab, expediting your ability to reinstall the floor and, also, mitigating against potentials like this in the future. Keep in mind, not all vapor retarders are equal and if the claims seem too good to be true, they probably are. One thing you never state is how high the moisture levels are within the slab. It is critical to know this so you know how to remedy it.

      Jason

  2. Sander Moraes says:

    Jason

    We had some water damage, had to rip out our laminate flooring. Measured our moisture level which ended up being 98%. Been running dehumidifiers, ac is down, fans blowing, however after 3 weeks it seems we have gone nowhere. Is there any other options that can help us?

    • Jason Spangler says:

      Sander:

      Thanks for the question. You don’t say how much moisture or for how long, but it can take a while to naturally dry out a slab after a flooding incident. Depending on the type of flooring, there may be acceptable installation methods for higher RH applications. Contact the flooring manufacturer and see if they have any recommendations.

      Thanks,

      Jason

  3. Bethany New says:

    Hi. We have a sunken living room and water issues. During a hurricane water bubbled up from below the wood floors and we had to tear everything out. Engineer came out and told us to fill it in. We used tar around the edges and poured 6 inches of concrete. Two weeks post pour our concrete moisture reading is 19. We are running two dehumidifiers 24/7 but we live in very humid climate. How long do you think it is going to take for floors to dry enough to install engineered wood floors? Do you have recommendations for moisture barrier?

    • Jason Spangler says:

      Bethany:

      Thanks for the comment. Unfortunately, 19 doesn’t mean anything to me in relationship to industry accepted measuring standards for concrete and flooring materials. What I can tell you though is the rule of thumb in relation to drying of a slab is this: it will take approximately 30 days for every one inch of slab thickness to reach a level close to being dry enough for that wood floor. On the barrier, I don’t recommend products, but I will tell you, in most cases, you get what you pay for. There are some very good wood glues out there that may work fine. Good Luck.

      Jason

  4. Jean says:

    So, you’re saying for a 6″ sunken living room floor, it would take 180 days before we can put a new floor in? Is that true everywhere? I know of builders that are done start to finish with houses in 3-4 months. We want to raise a living room floor and have been told it’s better to use concrete than to raise it with wood. Is that right? We can’t wait 6 months!

    • Jason Spangler says:

      Jean:

      Thanks for the questions. Yes, the information states 30 days for every inch of slab thickness, once the environmental conditions are conducive for drying. So what that means is that in a lot of cases, it takes longer than that. The level of “dryness” is also dependent on the type/breathability of the flooring being used. Wood flooring being glued to a slab is potentially more moisture sensitive than say a breathable carpet. There are also various adhesives and product that minimize the amount of moisture that can escape for the slab, allowing for products to be installed on the slab faster. I would consult your flooring contractor for more in-depth information on acceptable products. Good luck.

      Jason

  5. Ramon says:

    I poured a slab outside my house and didn’t use any vapor barrier. It’s been 2 months and the slab has a big spot. I dryer using a torch and still shows moisture in the slab. Is there anything gong I can do to remove the dark spot from the slab?

    • Jason Spangler says:

      Ramon:

      Thanks for the question. There are many times that concrete may have color variations, some of which can be moisture, but other times can be caused by various other inconsistencies in the material. If the inconsistency is that bad, you may think about some kind of topical color coating to make the slab look uniform. Good luck.

  6. Kai Drekmeier says:

    Hi Jason – Thanks for this helpful article.

    Here is my basic question: How much moisture should I expect as my concrete basement cures over its first year?

    Background: We installed a full concrete basement in April as part of a major renovation/rebuild. The space (pre-concrete) was bone dry (almost zero moisture in the dirt). We have a good sealer/vapor barrier + french drains etc., so don’t believe any water is getting in (plus it hasn’t rained – we’re in California) . . . but the basement was quite damp and some of our things got a bit moldy. I just bought a good de-humidifier and it is pulling about 2 gallons of water out of the space each day. Is this normal? It is a 1400 foot basement with a fairly thick slab and walls, with basically no place for the water to go but to evaporate into the space.

    Thanks in advance for your reply.

    Kai
    Oakland, CA

    • Jason Spangler says:

      Kai:

      Thanks for the questions. Assuming there is a vapor barrier directly below the slab, keeping ground moisture from penetrating the slab, then all you are dealing with is construction water. A rule of thumb says that once there is constant heat/air/humidity in the ambient space, it will take approximately 30 days for each inch of slab thickness to reach a relative humidity, in the slab, of around 85% to 90%. I hope this help. I hope this helps.

      Thanks,

      Jason

  7. SERGIO CUQUERELLA says:

    Hi Jason… Thanks a lot for the writing and for all these answers.

    We installed a WarmUp floor heating system in a very small amount of self-leveling concrete(1/4″ thick). Underneath this layer there is a concrete slab covered in plywood and a thin layer of paper that was left from removing some old linoleum, I would assume there is also some glue between paper and wood.
    On top of this, we are trying to install a terrazzo tile floor. The installation instruction from the tile manufacture are very precise and indicate a max humidity in the slab of 4%, that sounds quite low to me and hard to achieve when I read numbers online, a lot of people say with 8% humidity its OK.

    Anyways, I have a Hydrometer probe and keep logging numbers from 5 spots on our bathroom floor, it quite small only 5×5 feet. It has come to 10% in the best spot and 12% in the worst, seems it won’t go under that. We have only been 3 days using the bathroom fan and Kitchen fan to move air, I tried a portable heater but somewhere I read against it and removed it. Mind we live in BC, Canada and it’s already getting cold and damp here… not the best season but here we are…

    We also have an underground parking under our slab, and no Idea what kind of water barrier was used, we could not use any in our side because of the electric heat mat. So I wonder if we are actually pulling moisture out of our underground and will never come to the right numbers. Also, we can not isolate the bathroom since its the only one in our apartment and we use it on a regular basis.

    Any idea of what the best way would be to deal with this situation?

    • Jason Spangler says:

      Sergio:

      Thanks for the questions. Unfortunately, I am not familiar with the moisture scale you are referencing. It most definitely isn’t RH% in the substrate as most finish products would specify. You would usually see 75%RH + in those situations. As far as drying, the heat and air movement probably is a good thing. The way to verify is to measure the ambient RH% before heat and then measure it after heat. As long as the heat is “dry” and the moisture in the air isn’t stagnating, then it is probably helping. The other thing to keep in mind is that most self-levelers, especially at a ¼” thickness, are designed to dry very quickly. Many people won’t even measure moisture in the leveler, they measure moisture in the substrate below, prior to the leveler.

      Thanks,
      Jason

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