External Influences on Internal Concrete Moisture Condition

There are several external factors that influence the moisture condition of a concrete slab. They are:

  • Time
  • Vapor retarders
  • Bond breaking agents
  • Heating, ventilation, and air-conditioning (HVAC)

Concrete slab construction is an exercise in convergence. Builders apply several methods of facilitating the successful curing, drying, and hydration of concrete slabs. However, those processes and building materials can distort moisture readings. While this can be troublesome, contractors and consumers can help to avoid flooring failure by simply understanding the external forces that sway moisture measurement.

A volume could be written on this topic, but let’s keep it simple: concrete slabs require sufficient time to dry and cure (two separate processes). Moisture vapor rises from the bottom of a slab to evaporate at the surface. Ambient conditions conducive to proper drying affect overall drying time. The most crucial point is this: concrete slabs require sufficient time to cure and sufficient time to dry. Rule of thumb is that it will take approximately 30 days for every one inch of slab thickness to dry once the environment is conducive for drying.

Vapor Retarders

The Time FactorBuilders often install moisture vapor retarders before pouring a concrete slab. Put simply, vapor retarders establish a seal which impedes external moisture vapor from migrating into the slab from the ground. This focuses the drying process on the excess water in the concrete mix ONLY and takes away the potential for the ingress of seasonal moisture when done properly with the correct retarder.

Bond-breaking Agents

Building materials manufacturers make products with bonding agents that can deter timely concrete slab completion. Common curing, sealing, or bond-breaking compounds can inhibit the slab’s release of moisture vapor. This, in essence, can delay the length of time necessary for the slab to dry to a level necessary to install finished floor products and/or coatings. Doing any type of surface moisture testing on a slab with any products of this nature, without first grinding them off, will produce inaccurate results. This is one of the reasons why relative humidity (RH) testing, especially with the Wagner Meters’ Rapid RH®, has become so popular. Since the RH probe is installed down in the concrete, no surface preparation is necessary to get accurate results.

Heating, Ventilation, and Air Conditioning (HVAC)

Commercial HVAC Unit

Obviously, doors and windows seal in RH, and that alters moisture readings. However, the same is true for HVAC systems. Activated HVAC systems may create comfy ambient conditions for humans; moisture assessments are most accurate when ambient conditions reflect the normal operating environment of the facility. If HVAC systems are not activated, make sure to bring ambient RH conditions into environmental compliance before assessing concrete slab moisture condition.

Knowledgeable consumers and contractors can use the tools of the times to ensure accurate concrete slab moisture readings. A slab well done is a job well done.

Free Download – 7 Things You May Not Know about Concrete Slabs

Last updated on October 1st, 2021


  1. Timothy says:

    Great and informative write-up. Very well explained.

  2. Eric says:

    Excellent write up. Concrete is a great material and know how its impacted by the environment can definitely help with sound installation

  3. Sawyer says:

    Great article! I did not know about the effect of it before reading your content. Thanks for the informative post.

  4. Georgia Miller says:

    Didn’t know the effect of it, until I read your article. Thank you.

  5. John Belk says:

    This is great information. People don’t realize everything that can effect the concrete.

  6. Jason Spangler says:


    This is a great point and not a question I can answer definitively. My opinion though would be that the accurate depth is not what is going to consistently vary, but the length of time necessary for that additional moisture in the blotter layer to make its way into the concrete, prior to taking moisture readings, is what would be critical. Obviously, there would not be much moisture moving until service conditions were met. Meaning, that especially with this type of slab construction, service conditions on for a duration of time, prior to measurement, would be critical. Just my two cents.


  7. Evan Adams says:

    What about slab design? (blotter layers) Doesn’t that pool of water in the wet sand under the slab change the RH? I’ve often thought such slabs should have a different depth of equilibrium.

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