Relative humidity testing measures moisture inside a concrete slab using probes set into small holes drilled in the floor. The standard method is ASTM F2170. Readings show how much moisture may move up into a floor covering or coating. Manufacturers set maximum readings their products can tolerate.
What a Concrete RH Test Tells You
Concrete is usually wetter in the middle than on top, a bit like a loaf of bread that’s dry on the crust and soft inside. An RH test reaches past the crust. The installer drills small holes, typically 40% of the way into a slab that dries from one side, and sets a probe in each. Official readings require a 24-hour wait so the hole can settle.
Because it measures deeper than other accepted concrete moisture tests, it gives a better idea of how the slab will behave once it’s covered. Surface tools, like a moisture meter or the sealed-dish test, mainly read the top layer.
What to Ask Your Installer About RH Testing
The holes are small and get patched, and the readings help decide which finish and primer will last on your floor. Ask:
- Will you test the slab’s internal humidity or only the surface?
- How many test spots, and where? Edges and damp-looking areas matter.
- What reading does the coating or primer you’re using allow?
- If the number is high, will you use a moisture-blocking primer or suggest a different finish?
For an overview of all the methods, see moisture testing.
RH Testing FAQ
What is the best moisture test for concrete?
In-slab relative humidity testing is often called the most accurate picture of a slab’s future moisture because it reads below the surface. Installers may still use a quick meter scan first to find wet spots worth testing.
What is an acceptable moisture reading for concrete?
There’s no single number. Each coating, primer, and flooring product lists its own maximum, and the test result is compared to that limit.