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Moisture is the most common reason concrete sealers and floor coatings fail. Too much moisture moving up through a slab can make epoxy blister, acrylics turn white, and flooring adhesives let go. Testing before you seal, coat or install flooring tells you whether the slab is ready.
Why Concrete Holds Moisture
Water is part of every concrete mix, and new slabs can take weeks or months to dry out. Slabs on grade, especially basements without a vapor barrier, can also wick moisture up from the ground indefinitely. A certain amount of moisture is normal. The goal of testing is to find out whether there is more than your sealer, coating or flooring can tolerate.
Method 1: Plastic Sheet Test (Quick Check)
This simple test (based on ASTM D4263) gives a yes/no indication of moisture.
- Tape an 18″ x 18″ sheet of clear plastic tightly to the floor on all four edges.
- Leave it for at least 16 hours (24 is better).
- Lift it and check. Condensation under the plastic or a darkened slab means moisture is coming through.
It’s quick and free, but it isn’t quantitative. Use it to decide whether more precise testing is needed.
Method 2: Calcium Chloride Test
The calcium chloride test (ASTM F1869) measures the moisture vapor emission rate (MVER), the pounds of moisture released per 1,000 sq. ft. over 24 hours. You seal a dish of calcium chloride under a plastic dome on the slab for about three days, then weigh it to see how much moisture it absorbed. Kits are inexpensive and widely available.
Method 3: Relative Humidity (RH) Probes
The most accurate method (ASTM F2170) measures relative humidity inside the slab. You drill a small hole to a set depth, insert a probe sleeve, let it equalize, and take a reading. Most flooring and coating manufacturers now specify a maximum RH reading, so this is the test to use before installing an epoxy coating or flooring.
What to Do If the Concrete Is Too Wet
- Give new concrete more time to dry, with heat and air movement if indoors.
- Fix drainage, grading and gutters that send water toward the slab.
- Use a coating system rated for higher moisture, such as a moisture-mitigating epoxy primer.
- For basements, a penetrating water repellent or densifier can reduce dampness. See best sealer for basement waterproofing.
Always compare your results with the limits on the product’s technical data sheet; acceptable moisture levels vary by product.
Frequently Asked Questions
Do I need to test moisture before sealing outdoor concrete?
Usually not with breathable penetrating sealers, as long as the surface is dry. Testing matters most before film-forming coatings and indoor flooring.
Can a sealer stop moisture from coming through a slab?
Penetrating sealers reduce water absorption, but they don’t stop vapor pressure from below. High-moisture slabs need moisture mitigation, not just a sealer.
