Concrete Moisture Testing for Epoxy: RH vs. CaCl (ASTM F2170 Explained)
- 2 hours ago
- 2 min read
Installing epoxy on concrete requires more than surface grinding — you must know how much moisture is inside the slab. Excess moisture is one of the top causes of epoxy failures, leading to bubbling, peeling, whitening, and complete delamination. Two industry‑standard tests dominate the field: Relative Humidity (RH) testing and Calcium Chloride (CaCl) testing. This guide explains both methods, including how they work and which one gives the most reliable results according to ASTM F2170.
Why Moisture Testing Matters for Epoxy Floors
Concrete naturally releases moisture as vapor. When sealed under epoxy, that vapor becomes trapped, creating pressure that pushes the coating off. Moisture testing helps installers:
Avoid epoxy failure
Prevent bubbling and blistering
Comply with manufacturer requirements
Validate moisture conditions before installation
Skipping moisture testing is one of the most expensive mistakes epoxy installers make.
RH testing measures the moisture inside the concrete slab, not just on the surface. Installers drill small holes into the slab and insert RH probes. After 24 hours, the probe shows the internal humidity percentage.
Why Pros Prefer RH Testing
Gives deeper, more accurate readings
Less affected by weather conditions
Matches how coatings actually behave after installation
Required by many epoxy manufacturers
RH testing is considered the most reliable moisture test for epoxy flooring.
Calcium Chloride Testing (CaCl): Surface‑Level Moisture Reading
The CaCl test measures how much moisture vapor moves from the concrete surface into a sealed container of calcium chloride over 60–72 hours. The result is expressed as Moisture Vapor Emission Rate (MVER).
Pros of CaCl Testing
Simple and affordable
Useful for surface moisture screening
Good for older slabs
Limitations
Only measures surface moisture
Weather and HVAC conditions affect results
Less accurate for thick or dense concrete
Because of these limitations, CaCl tests are usually paired with RH testing for a full moisture profile.
Which Test Should You Use?
For epoxy flooring, the industry recommends:
RH Testing (ASTM F2170) → Best for accuracy
CaCl Testing (ASTM F1869) → Good supplemental test
Using both → Ideal for borderline slabs
Proper testing protects the installer, the client, and the final coating.
Protect Your Epoxy Flooring Business
Moisture testing reduces installation risks — but insurance protects your entire business. For epoxy installers and concrete coating contractors, Wexford Insurance offers customized liability coverage. Get a fast, no‑obligation quote today.
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