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 1-800-634-9961 or order your Rapid RH® moisture measurement system online.

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Jason Spangler

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.


  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:


      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.


  2. Sander Moraes says:


    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:


      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.



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