How Does Moisture Move Through Concrete?
Concrete looks solid, but it holds and moves water long after it has been poured. How much moisture is in a slab, and which way it is traveling, decides whether the floor you install over it survives.
- The Dynamics of Moisture Movement Through Concrete
- Manage Concrete Moisture Appropriately
- Excess Moisture Movement and Moisture Content in Concrete
- Testing a Concrete Slab’s Moisture Before Flooring Installation
- FAQs
Understanding how moisture moves through concrete is critical to successful flooring. It helps you reduce the risk of future moisture-related flooring failure during installation. It also helps you identify and assess the cause of moisture-related issues that have already occurred.
The Dynamics of Moisture Movement Through Concrete
Despite its outward appearance, concrete isn’t solid. It’s filled with air pockets and veins. When poured, the concrete slab contains the batch water from the mix.
As it sets, water begins to evaporate, and the air pockets and veins begin to form within the slab. These veins are called “capillaries.” As concrete hardens, batch water and vapor get pulled into these air pockets. This process is called “capillary action.”
Moisture also enters the concrete slab through external sources. Most slabs are poured over a vapor retarder because moisture from the ground below will seep into concrete. Indeed, sub-slab vapor is a primary cause of moisture-related distress in concrete floors that don’t have a vapor retarder.
Concrete will also pull moisture from the air and release moisture into it. It depends on whether the air is holding more or less moisture than the slab. This is why the relative humidity (RH) in the air and within the slab is so critical.
The RH indicates how much moisture is already present relative to what can be absorbed. If the air has an RH of 30%, it’s holding 30% of the moisture it can. If the RH of the slab is higher, moisture will tend to move from the concrete into the air. The reverse is true if the RH in the slab is lower than the RH in the air.
Yet, if the air has reached its dew point, vapor can tend to concentrate on the surface of the slab. The dew point is the air temperature at which it can hold no more moisture. The dew point is determined by the temperature and RH of the air.
If the temperature of the slab is below the dew point, condensation will appear on the slab’s surface. Knowing the dew point of the environment around the concrete is vital to assessing drying times and moisture condition of the slab.
Manage Concrete Moisture Appropriately
Concrete needs water to cure properly, so moisture itself is not the enemy. Excess or uncontrolled moisture is.
Managing it comes down to three things: keeping ground moisture out with a vapor retarder, giving the slab the conditions and the time it needs to dry, and confirming with a test that it has dried before anything goes down on top of it.
Skip any of those and the damage tends to show up in the same predictable ways.
Excess Moisture Movement and Moisture Content in Concrete
Excess moisture manifests itself in various ways that result in unsightly or possibly hazardous floor conditions. These are some of the ways moisture-related distress shows up on concrete flooring:
- Osmotic blisters: Moisture pulled from the slab through an adhesive or synthetic resin gets trapped below a non-permeable floor covering. The resulting swelling is “osmotic blisters.”
- Microbial growth occurs when there’s excessive moisture in the concrete and high levels of humidity in the air.
- Microbial growth can corrode and weaken concrete. Signs of microbiologically induced deterioration (MID) include cracking and flaking on the surface.
- When moisture rises to the surface, it brings salts within the concrete with it. The moisture will eventually evaporate away, but the salts remain. They can show up as efflorescence, those white streaks and stains that appear on concrete.
- The salts also cause adhesive degradation, which can result in floor coverings separating from the slab. Flooring separation can also occur due to coating debonding (also called delamination). Rising moisture trapped underneath impermeable flooring will bring very basic chemicals with it to break down the bonding agent or laminate agent.
Testing a Concrete Slab’s Moisture Before Flooring Installation
Ensuring the concrete slab has released enough excess moisture and has reached levels stipulated by the flooring or adhesive manufacturer before installation is your best opportunity to prevent moisture-related failure.
Measuring the RH of the air is straightforward. Measuring it in the concrete slab requires using RH sensors embedded into the slab. ASTM F2170 (Standard Test Method for Determining Relative Humidity in Concrete Floor Slabs Using in situ Probes) sets out the requirements for conducting an RH test properly.
Based on decades of scientific research, the RH in situ probe is the most reliable and accurate method of measuring the RH within the slab. It’s the only concrete moisture test that measures conditions deep within the slab.
Knowing how moisture moves through concrete, it’s clear why tests that measure the surface are unreliable.
Since the ASTM F2170 standard was first published in 2002, the science on RH testing has evolved, and so has F2170. Most recently, the standard reduced the waiting period needed between inserting the RH probe and taking a reading from 3 days down to 24 hours.
The RH in situ test kits have also evolved as technology improves. For example, Wagner Meters’ most recent RH test kit, the Rapid RH® L6 Smart Sensors, use digital technology to simplify reporting and improve data integrity.
The L6 Total Reader®, a pen-sized electronic device, captures readings from each RH probe and sends them, via Bluetooth®, to the DataMaster™ L6 app that logs the data for analysis and produces emailable F2170 reports.
That removes a step where human recording errors creep in.
The L6 Smart Sensor can also be installed very early in the slab drying process and read periodically, which gives a more accurate estimate of drying time. More accurate estimates allow for tighter scheduling and less downtime.
Regardless of which kit you use, RH in situ testing is always the most accurate method for determining a concrete slab’s moisture condition.
Free Download: 4 Reasons Why Your Concrete Is Taking Forever to Dry
FAQs
Does Moisture Weaken Concrete?
Yes, excessive moisture can indeed weaken the concrete. It promotes the growth of microbes which can corrode the concrete and manifest as cracks and flaking on the surface. This phenomenon is known as microbiologically induced deterioration (MID).
What Causes Water to Seep Through Concrete?
Water can seep through the capillaries formed in concrete during the curing process. In addition, moisture from the ground below can seep into the concrete, particularly if there’s no vapor retarder.
How Do I Stop Moisture Coming Through a Concrete Floor?
To stop moisture from seeping through a concrete floor, installing a vapor retarder during the construction phase is essential. Additionally, it’s crucial to ensure the concrete has released enough excess moisture before installation. RH in situ testing, like the Rapid RH L6 Smart Sensors, can accurately determine the concrete slab’s moisture condition.
Can Water Go Through Concrete?
Water can move through concrete via capillary action and external sources such as ground moisture. Furthermore, concrete can absorb moisture from the air depending on the relative humidity.
A version of this article by Jason Spangler was previously published in Concrete Decor, October 2019.
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 September 21st, 2026
800-634-9961
We have water under the slab in our kitchen and our hardwood floor has popped up. Water readings show the concrete in the kitchen has moisture but the living room and family room appear to not have moisture (all part of the same slab). If we seal the floor in the kitchen and put down tile, will the water in the kitchen part of the slab eventually migrate and cause the hardwood in the living room to bubble up?
Hi, We recently had a the foundation floor poured for a below grade basement. A few days after the pour we had heavy rains before the plumber was able to open up the pipe rough-ins.
A lot of water seeped in between the vapor barrier that is over the gravel and the poured concrete.
Question is: Will the water now between the vapor barrier and the concrete flat-work eventually evaporate out through the concrete? Will the vapor barrier dry out?
Thank you.
Eran:
Thanks for the question. If the conditions are appropriate in the basement for the concrete itself to dry, I would expect that the moisture sitting on top of the vapor retarder will eventually make its way up. Based on stories I have heard regarding vapor retarders and slabs, if this is your worst “issue”, you are golden.
Hi i have a polished concrete pathway slab on ground that has a crack that has formed . It was poured 100mm thick on 2 layers of membrane and the mesh was chaired on 65mm chairs. The slab FFL is at the same height as the lawn and has a irrigation system 50mm from the edge of the slab . I have had the crack fixed by the concrete polisher but every time it rains you can see the crack. Do you have any suggestions
Jordan:
Thanks for the question. Honestly, besides POSSIBLY starting from scratch with the crack and probably the immediate area, my best suggestion would be to have this discussion with the polisher and see if they can come up with another suggestion. Good luck.
Yes, bacteria and microbes can migrate through a concrete slab as the concrete is porous. Microbes and bacteria will always follow the water and the transfer of water through a porous medium is known as Capillary action.
We have a sewer line leak below a commercial lease space. Multiple In-situ test RH at 96-99. Soil samples from under slab show presence of Enterococci – bacteria commonly found in human waste. Building owner doesn’t think it is a big deal as it is under slab. My question is can this bacteria migrate through the 4” concrete slab on grade and affect health of space occupants