Managing Moisture Between the Flooring and the Concrete Slab

Water is an inherent part of the hydration process of concrete. However, allowing excess moisture to leave the slab after it’s poured is crucial to a successful flooring installation.

Once the slab is poured, the excess moisture must leave the slab in order to strengthen the concrete bond. The slab must also dry to a specified level of moisture before flooring materials can be installed on top of it. Moisture-related damage to the flooring materials is possible.
Laying Ceramic Tiles

3 Common Flooring Materials Run The Risk of Moisture-related Problems:

  1. Adhesives
    Moisture-related adhesive failures are a problematic reality in the flooring business. Recent trends towards restricting volatile organic compounds (VOCs) in flooring adhesives have increased the number of moisture-sensitive adhesives used. If the adhesive used to install the flooring does not have the correct moisture tolerance for the concrete subfloor, the entire installation can be at risk.
  2. Floating Floors
    Floating floor systems are attractive because they don’t need to be attached directly to the subfloor. Instead, the floor pieces “lock” together to become a cohesive unit that’s not as vulnerable to seasonal shifts, dimensional challenges, or other moisture-related issues. In fact, floating floors are often recommended on projects where moisture risks are high with standard attached floor systems. For floating floors, manufacturers often recommend installing a moisture barrier between the subfloor and the floating floor to prevent moisture intrusion. The difficulty, of course, is that if the moisture barrier is compromised in any way, moisture from the slab beneath can still damage the flooring or finish.
  3. Grout or Cementitious Bonds
    Excess moisture issues in a grouted tile or mosaic floor often appear as efflorescence, a whitish residue on the surface of the grout. It’s the result of water-soluble minerals getting transported to the surface of the grout with the moisture as it evaporates away. Since minerals don’t evaporate, they’re left behind on the floor surface as a visible residue. The more porous the concrete or grout, the more likely efflorescence will appear. In the majority of cases, these minerals are actually part of the concrete slab mixture. Although they can possibly be in the ground beneath the slab, and seep into the concrete if no moisture barrier was installed. If the slab was not dried to the required specs before the tile was installed, the natural moisture migration of the drying concrete will impact the grout. Remediation steps will be necessary to correct the problem. In extreme cases, excess moisture can lead to flaking or chipping of the grout which results in a complete grout or thin-set failure.

Are you seeing a theme yet? The real risk to a successful floor lies with the moisture that can accumulate within the layer between the concrete slab and the flooring itself.

Adhesives, floating floors, and grout or cementitious bonds are 3 common flooring materials that can run the risk of causing moisture-related problems. Moisture control is often one of the most crucial, yet most overlooked, elements of any floor’s success over time. Responsible moisture control (having accurate moisture measurements) starts with the concrete slab.

Make sure to use the most accurate concrete rh test from Wagner Meters.

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moisture in subfloor

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Moisture in a Concrete Subfloor

For moisture to accumulate between the concrete slab and the flooring, it needs to find its way to that middle layer. In this section, we breakdown the main ways water can get into your concrete, which causes a buildup of excess moisture, and list effective methods to prevent moisture problems from occurring.

Sources of Moisture in Concrete

The primary source of moisture in a concrete slab is the proportion of water mixed with the cement. No water source has a greater impact on the time it will take concrete to dry.

Yet you have other water sources to worry about. A variety of potential external water sources at a worksite can affect slab drying and curing.

  • Rain, snow, and sprinkler systems are culprits on a work site open to the elements. These water sources increase in danger if the grade of the grounds around the slab slope towards it. Not only is the concrete absorbing the water from above, but it’s also taking on the runoff from the areas around it.
  • The concrete slab can also absorb the groundwater below and around it. Thus, the amount of natural groundwater has a huge impact on concrete moisture conditions.
  • Unnatural water sources can also leak water. Any poor plumbing installation at the worksite creates a high risk of excess moisture. Old plumbing that’s degraded and has leaks presents the same risk.
  • Ambient conditions can also increase the concrete slab’s water content. Condensation develops on a slab that has a lower temperature and moisture level than the air’s dew point. The dew point is the temperature at which the air can hold no more moisture. You know, when dew (or condensation) starts to form. The slab will absorb some of the condensation.

    The slab will also absorb moisture from its environment when its relative humidity (RH) is below the air’s RH. Moisture wants to level out. If the air holds more moisture than the slab, as evidenced by its RH, that moisture will move to the concrete.

Rain, snow, groundwater, leaks, increasing water content, and sprinkler systems are all potential sources of free water. That is, water the concrete doesn’t need to cure. Any moisture the slab doesn’t need is moisture that can undermine your flooring installation.

What Are Causes of Excess Moisture in a Concrete Slab

Inadequate drainage around the slab exacerbates the risk of any source of moisture. In fact, the existing water source itself may not be the problem. A little rainfall or a bit of groundwater can drain away with well-designed drainage. Even minimal sources of water can pool on the concrete without adequate water lines and drains.

Excessive water can also intrude in a structure due to poor subfloor protection. Groundwater will move into the concrete if no vapor retarder sits between the ground and the subfloor.

The more likely cause for poor subfloor protection is using the wrong vapor retarder. Certain ASTM standards allow a vapor retarder to have a perm rating of 0.3 perms, which could allow up to “approximately 18 gallons of water per week in a 50,000 square foot area.” A vapor retarder with too low a perm rating won’t do the job it needs to do.

In other cases, the vapor retarder may have been sitting on the ground. It’s helpful to have a separation barrier between the ground and vapor retarder. Contractors should install the vapor retarder over granular fill to create extra separation from the groundwater.

A torn vapor retarder is another potential hazard to subfloor protection. Torn vapor retarders can occur in a reckless worksite. Hurried construction schedules create all sorts of moisture (among other) threats.

Fast-paced project plans often mean that concrete slabs don’t get the time they need to dry. For example, slabs may get power troweled to speed up preparations for flooring. The compression caused by the troweling closes off the evaporation outlets in the slab. The result is that over troweling will extend the drying time.

If the schedule doesn’t allow for that time, then adhesives or surface membranes get installed on concrete with too much moisture. A moisture-related flooring failure is practically guaranteed under such circumstances.

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

How to Avoid Excess Moisture

The best of intentions to avoid excess moisture doesn’t matter if you don’t have accurate concrete moisture testing. There are two main ways to suffer inaccurate moisture testing. The first is to choose the wrong concrete moisture test. Only the in situ RH test measures moisture below the slab’s surface. Any test measuring only surface moisture is necessarily providing inaccurate results.

The other way to get inaccurate moisture test results is to perform the in situ RH test wrong. If you don’t place enough sensors throughout the floor, you won’t get an accurate picture of the space.

ASTM F2170 requires three sensors for the first 1000 square feet and another sensor for each additional 1000 square feet. Other testing mistakes include not inserting the sensor to the proper depth.

Serious errors can even occur from simply miswriting readings on your chart. The Rapid RH® L6 sensors contain integrated data storage that automates results reporting. When the meeting takes place to decide when to install flooring, no one has to rely on paper notes.

Make sure to use the most accurate concrete rh test from Wagner Meters.

Shop Rapid RH L6

How to Prevent Moisture Problems on Concrete Floor

Moisture is an inevitable part of concrete construction. Moisture-related problems are not.

  • Keep a low water to cement ratio. The more water in the mix, the greater the chance the slab won’t get all the time it needs to dry. Try to avoid adding water to concrete that’s already mixed. That water is a new variable that makes managing timelines and moisture issues difficult.
  • Take all necessary actions if pouring concrete below grade or at a wet building site. Whether that means installing more draining lines, using pumps to dewater the site, or any other method – do it. Make sure that your water displacement methods aren’t generating run-off in the wrong directions.
  • The layers above and below the concrete slab to prevent water seepage into the slab. Start with a vapor retarder with a perm rating that reflects the needs of the space. Install it over a layer of fill. Inspect it before pouring the concrete and fix any tears it may have. If necessary, use a proper underlayment between the concrete slab and the flooring. This is especially true when using wood flooring. Installing a plywood underlayment can add extra protection, but must also be tested to ensure it’s not bringing in new moisture.
  • Give the concrete slab the time it needs to dry and cure. Review the project plan and schedule. Is enough time set aside for concrete floors to dry? No reason to start out behind the eight ball. Exert as much control as possible over the ambient conditions to accelerate the timeline. Protect the space from outside elements. If the season isn’t ideal, can you take steps to reduce swings in air temperature? Will a dehumidifier help the air absorb more moisture from the concrete? Use fans to increase airflow, which speeds up drying time.

All these methods point towards a single goal: not installing any flooring too soon. Preparatory materials like adhesives or plywood will seal up the concrete. Sealed concrete will stop releasing moisture. At that point, the slab has the moisture it will hold for the long term. If there’s excess moisture trapped in the concrete, it will eventually reveal itself in ugly and possibly dangerous ways.

Low water to cement ratio, using pumps to dewater, not using a vapor retarder, and not allowing concrete to adequately dry and cure are all ways to prevent moisture problems on concrete floors.

How to Tell If There Is Excessive Moisture in My Floor

A floor may already be showing some external signs of containing excess moisture. A floor with a white or greyish powdery stain (also called “efflorescence”) likely has excess moisture. Due to moisture moving up through the slab and then evaporating from the surface. The whitish stain is the salt left behind by evaporating water. Or you may see that the flooring installed over the concrete slab is blistering or peeling away. If a wood flooring has been installed on top of the slab, the wood may be cracking or warping. These types of flooring failures occur due to excess moisture trapped between the flooring and the concrete.

No one wants to wait until the ugly signs of excessive moisture make themselves visible. You want to know if your floor is holding too much moisture well before that.

The calcium chloride test is an older method for measuring the moisture level of concrete flooring. It’s also called the moisture vapor emission rate (MVER) test. It’s standardized as the ASTM F1869 (Standard Test Method for Measuring Moisture Vapor Emission Rate of Concrete Subfloor Using Anhydrous Calcium Chloride).

The MVER test uses the weight differential over a 72-hour period of calcium chloride salt placed on the slab’s surface. The calcium chloride, sitting under a sealed dish, absorbs the moisture evaporating from the slab. You calculate the rate of evaporation based on the weight differential.

Unfortunately, ambient conditions often corrupt MVER test results. F1869 doesn’t even allow its use on lightweight concrete. Of greater concern is what the MVER test measures. It measures moisture only at the surface of the concrete slab. It’s not testing the moisture condition that matters over the long haul. You need to know the moisture condition within the concrete.

Only the in situ RH tests for moisture below the slab’s surface. Sensors inserted into the slab measure the RH within the concrete.

And these aren’t random depths. ASTM F2170 (Standard Test Method for Determining Relative Humidity in Concrete Floor Slabs Using in situ Probes) specifies the depth based on whether the concrete is poured on grade and whether vapor retarders are used.

Rigorous scientific testing conducted at universities and laboratories has determined and validated the proper depths. At the proper depth, the RH sensor accurately reflects what the moisture condition of the slab will be once the flooring is installed.

The Rapid RH L6 returns the scientifically reliable readings you need to complete a successful flooring project. Readings that the MVER test can’t provide. Even better, the RH test can be completed in 24 hours. That’s one-third the waiting time required to conduct an MVER test.

Excess moisture is caused by a number of ways a few being: the hardwood floor is cupping or cracking, you see efflorescence or a white residue on the concrete floor, or the floor over the concrete is blistering or peeling away.

Moisture Management Requires Accurate Moisture Readings

Total Reader and Test Results FormAccurate concrete moisture measurement is only achieved with RH testing. Unlike surface-based tests like calcium chloride tests, RH testing determines the accurate moisture condition within the slab by placing probes at a strategic and proven depth. Moisture often rises through a slab from the bottom to the top in the drying process. Only testing performed at the correct depth can let you determine if the final moisture condition of the slab will be compatible with the flooring and the products used to install it.

Wagner Meters has been assisting flooring professionals for over 50 years. During these decades, we have designed some of the most accurate, innovative, and easiest RH testing sensors on the market today. The Rapid RH L6 is the newest iteration, taking advantage of 21st-century technology to simplify reporting.

All our Rapid RH sensors and test kits are based on decades of scientific research and technological advances to help each builder and flooring specialist accurately determine the correct concrete RH level for a project’s chosen flooring materials. Our innovative Total Reader® and factory-calibrated Smart Sensor design delivers quick, reliable results. The Rapid RH line of products is affordable and conforms to ASTM F2170 requirements for easy recording and reporting.

We also understand that sometimes a building project schedule means making alternate choices in adhesives or even flooring products. The Rapid RH sensors help you make informed decisions in real-time. Along with accurate, actionable testing, we’ve also compiled a one-stop list of manufacturers that provide an RH tolerance specification for their flooring products at

The truest way to protect a floor system is to ensure that all components are safe from excess moisture intrusion from any source. The Rapid RH family helps you prevent your concrete slab from being the source of a moisture-related flooring adhesive or grout failure. Don’t let moisture problems come between you and a successful flooring installation.

Last updated on October 1st, 2021