How to Control Moisture Coming Up Through the Concrete Floor?
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.
Here is what you’ll learn in this article to have a successful installation:
- 3 Common Flooring Materials Run The Risk of Moisture-related Problems
- Moisture in a Concrete Subfloor
- Sources of Moisture in Concrete
- What Are Causes of Excess Moisture in a Concrete Slab
- How to Avoid Excess Subfloor Moisture
- How to Prevent Subfloor Moisture Issues
- How to Tell If There Is Excessive Moisture in My Subfloor
- Moisture Management Requires Accurate Moisture Readings
Once the slab is poured, the excess moisture must be left to strengthen the concrete bond. The slab must also dry to a specified moisture level before installing flooring materials. Moisture-related damage to the flooring materials is possible.
3 Common Flooring Materials Run The Risk of Moisture-related Problems:
Moisture-related adhesive failures are a problematic reality in the flooring business. Recent trends toward 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 less vulnerable to seasonal shifts, dimensional challenges, or other moisture-related issues. In fact, floating floors are often recommended on projects with high moisture risks 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 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 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 were not dried to the required specs before the tile was installed, the natural moisture migration of the drying concrete would impact the grout.
Remediation steps will be necessary to correct the problem. In extreme cases, excess moisture can lead to flaking or chipping the grout, resulting 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 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.
Moisture in a Concrete Subfloor
It must reach that middle layer for moisture to accumulate between the concrete slab and the flooring. In this section, we break down 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 Floors
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. Potential external water sources at a worksite can affect slab drying and curing.
1. Weather and Elements
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 greatly impacts concrete moisture conditions.
3. Unnatural Sources
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.
4. Ambient Conditions
Ambient conditions can also increase the concrete slab’s water content. Condensation develops on a slab with 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, it 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 some groundwater can drain away with well-designed drainage. Even minimal water sources can pool on the concrete without adequate water lines and drains.
Excessive water can also intrude into a structure due to poor subfloor protection. Groundwater will move into the concrete if no vapor retarder sits between the ground and the subfloor.
1. Using the Wrong Vapor Retarder
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.
The vapor retarder may have been sitting on the ground in other cases. It’s helpful to have a separation barrier between the ground and the vapor retarder. Contractors should install the vapor retarder over granular fill to create extra separation from the groundwater.
2. Damaged Vapor Retarder
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.
3. Not Enough Time to Dry
Fast-paced project plans often mean that concrete slabs don’t have the time 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 that time, 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 Subfloor Moisture
To effectively avoid excess subfloor moisture, it is crucial to conduct accurate concrete moisture testing. Inaccurate moisture testing can occur in two primary ways.
1. Choose the Correct Moisture Test
Selecting an inappropriate concrete moisture test can lead to inaccurate results. The in situ relative humidity (RH) test, which measures moisture beneath the slab’s surface, is the most reliable method. Tests that only measure surface moisture may yield incorrect data.
3. Use the Correct Amount of RH Sensors
Improper execution of the in situ RH test can also result in inaccurate results. To obtain a comprehensive understanding of the area, ensure an adequate number of sensors are placed throughout the floor. According to ASTM F2170 guidelines, three sensors are required for the first 1000 square feet, with an additional sensor needed for every subsequent 1000 square feet. Other testing mistakes, such as incorrect sensor depth insertion, can also impact accuracy.
Even minor errors like misreporting readings on a chart can lead to serious consequences. Wagner Meters’ Rapid RH® L6 sensors feature integrated data storage for automated results reporting, eliminating the need for paper notes during decision-making meetings about flooring installation.
Always use the most accurate concrete RH test from Wagner Meters to ensure you avoid excess subfloor moisture.
How to Prevent Subfloor Moisture Issues
Moisture is an inevitable part of concrete construction. Moisture-related problems are not.
1. 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. Water is a new variable that makes managing timelines and moisture issues difficult.
2. Pour Concrete Below Grade
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. Ensure your water displacement methods aren’t generating run-off in the wrong directions.
3. Prevent Water Seepage
The layers above and below the concrete slab 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. Use a proper underlayment between the concrete slab and the flooring if necessary. This is especially true when using wood flooring. Installing a plywood underlayment can add extra protection, but it must also be tested to ensure it’s not bringing in new moisture.
4. Give it Time to Dry
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 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 dry and cure adequately are all ways to prevent moisture problems on concrete floors.
How to Tell If There Is Excessive Moisture in My Subfloor
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 wood flooring has been installed on top of the slab, the wood may be cracking or warping.
These 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 are visible. You want to know if your floor is holding too much moisture well before that.
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
Accurate 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 during drying. 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 today. The Rapid RH L6 is the newest iteration, using 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.
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 www.rhspec.com.
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.
Last updated on August 4th, 2023