Dehumidification and Drying Concrete: Hurrying Up the 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.
Hurry and Wait
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.


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Last updated on June 8th, 2021

34 Comments

  1. Robert Workman says:

    This is an outstanding post that’s filled with so many useful nuggets. Thank you for being so detailed on dehumidification and drying concrete: hurrying up the wait.

  2. Oliver says:

    Thanks for sharing the tips. I am using this to make others understand

  3. Victoria says:

    Hi Jason. We had a concrete and screed mixed poured on top of our ufh (35mm worth). Our builder recommended we wait one month before putting our new carpets down, but we are four days away from that mark and we can still see dark patches at the surface level – compared to other, light grey blotched areas. I did a moisture test (using an apparatus I bought online) but I can only measure the surface level which is coming up at 1.0%, even on the darker spots. We own a basement flat and have not been living in the property, so the windows are not open all day to air anything out. What would you suggest we do to speed up the drying process?

    • Jason Spangler says:

      Victoria:

      Thanks for the question. The best conditions for drying will be consistent temperatures, air movement across the surface, and low humidity in the air. So getting the ufh going, fans moving air, and potentially some type of dehumidification would be your best bet. Good luck.

  4. Bradford Hall says:

    Hi Sir,

    I am acting as an appraiser for a residential policy holder that had a water leak on 3/1/20. The opposing appraiser had the slab tested on 3/4/21 ( a year later) as he hypothesized there was another leak. Leak detection indicated there was no additional leak. I do water mitigation as well, but have only dried a slab once in my fourteen year career, probably 12 years ago. I recall this taking around 2 months to dry. With a desiccant, floor mat system, and air movement how long do you suppose drying would take? Also, I imagine its similar to other materials in that the longer a concrete slab has been wet, the longer it takes to dry, is that correct in this case?

    Thank you in advance for your time!

    Brad Hall

    • Jason Spangler says:

      Brad:

      Thanks for the questions. The answer to you first question is, it depends. How thick is the slab? Was the water from the top down or the bottom up? How wet is it? How dry does it need to be? If the leak was from below, is the “wetness” in an isolated area? If not, is there no vapor barrier below the slab? In general, if you are looking at a finite amount of moisture to dry from a slab, it will take 30 days for every 1” of slab thickness to reach around 85% RH, internally. Yes, you are correct that the longer the water sits on a slab, the more it will absorb, and the wetter the entire mass will become. Good luck.

  5. Stephanie Wright says:

    Hi,

    My company is in the process of building a new temporary building.

    Currently the concrete has been laid but the walls are not up yet, because of this water is accumulating on the concrete.

    Do you know of anyway that we can remove or absorb the water ?

    Thank you

  6. JS says:

    1 month ago had a leak in the cinderblock from water main. A hole was put in the outside wall to repair. It was covered by us temporarily pending insurance etc.
    We initially had a mitigation company treat the area and it dried but rained afterward and wet again we believe. Mitigation refused to redry due to insurance payment problems.
    We bought a 50 pint dehumidifier and it is removing water. The area is getting smaller.
    Humidity in room down to 28 we live in Florida.
    It still has areas in the 90s. Is this a normal speed?

  7. John Bruckner says:

    We had a slab leak surface 12/21 & repaired on 12/22, pinhole in 3/4″ cold water line, below our grade level slab confined to one room. Not sure how long the leak had been present before the water showed up under the carpet. We have removed all carpet and padding from the room and are using dehumidifier, heater and fan to attempt to dry slab. On a moisture meter the slab in most of room reads 100% moisture level. First question is would there be any way to better expedite the drying process and if not is there a way to gauge how long it might take to get the moisture level to a point where sealer could be applied? The restoration plan is to install a moisture proof underlayment below a vinyl plank flooring surface.

    • Jason Spangler says:

      John:

      Thanks for the questions. First, the flooring industry, in general, doesn’t recognize surface moisture meters for making installation decisions. With this, it makes it hard for me to interpret your 100% reading. I would consult the flooring manufacturer that is being installed and find out what type of moisture testing is required to maintain a warrantied installation. Once you have done this testing, then you can gauge how much drying needs to be done. As far as how long, I cant give you a definitive, but I can say it sounds as if you are taking the right corrective steps. Good luck.

  8. Howard Edwards says:

    I live in a basement…I have mine on day and night alt 50%…my question is…is it healthy…does the dehumidifier will rid of water of your body also? I notice that my skin is getting dryer and dryer every time.

  9. Hi Jason,
    We have a slab we are trying to dry for a wood floor installer. He would like MC to be /b/ 3-3.5 on a concrete moisture encounter meter (0-6 on scale). Slab was exposed to water damage for 3-5 days. It is contained in one room. We tented the slab and used a desiccant to dry. There was no movement in the MC of the slab. We removed the moisture barrier and installed infra red heat lamps coupled with dehumidification. MC went down a full point on our concrete encounter meter but has plateaued. We are on day 20 of drying. This past weekend, I turned off the heat lamps for two days (dh still operating with decent grain depression). After turning off the lamps, we gained a point to point and a half (4-4.5) of drying but the second day after the lamps were turned off, the MC went back up. I take it as positive info that moisture is moving in the slab. Is there any thing different we could do to push it over the top to obtain the dryness the installer is seeking?

    • Jason Spangler says:

      George:

      Thanks for the question. Its sounds like you are doing quite a bit. I would be curious to know what the RH% in the tent is during application of heat lamps and then during that 48 hours after you turned them off. Anyway, the only additional thing I could recommend would be a high level of air movement across the slab although that may be difficult with tenting. Good luck.

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