Is the Concrete Slab Ready for You to Install Underlayment?
Choosing the right underlayment and installing it correctly can make or break a flooring project. Doing it right starts with making sure the concrete slab is ready.
For tile installations, leveling of the concrete floor is critical. The “lippage” caused by height differences between adjoining tiles can cause tile damage and even safety issues.
Underlayment is a broad topic. To keep it simple, this article doesn’t cover installation over wood subfloors and gives only a few examples of underlayments and their uses. Also, the terms “underlayment” and “subfloor” are sometimes used interchangeably in the flooring industry.
In this article, “underlayment” always means a layer between the concrete subfloor and the floor covering (the surface layer).
Underlayment Examples
Underlayment serves many purposes, from protecting the floor covering from moisture and movement in the concrete subfloor to meeting sound insulation requirements in multi-story buildings. Floor covering manufacturers usually specify any underlayment their products require.
Always check the floor covering manufacturer’s specifications and warranty to make sure the underlayment you choose is approved for that product.
Wood Flooring
For wood flooring installed over concrete, the National Wood Flooring Association (NWFA) recommends an “impermeable vapor retarder with a perm rating of less than or equal to 0.15.” Examples include 6-mil construction-grade polyethylene film, double felt, chemical retarders, or a urethane membrane.
For details, see the NWFA Installation Guidelines, which are available for purchase at www.nwfa.org.
For glue-down and floating floors, you may need a pourable, self-leveling underlayment to correct minor imperfections in the slab surface.
For floating wood floors, manufacturers sometimes recommend padded underlayment.
For floating wood floors, manufacturers sometimes recommend padded underlayment. Its smooth surface lets the planks move as they expand and contract with seasonal humidity changes. Padded underlayment can also help reduce footfall noise and add thermal insulation. Examples include felt, foam, rubber, cork, and rubber-cork rolled goods. Some padded underlayments for wood floors include a vapor retarder layer.
Ceramic, Porcelain, and Stone Tile
For tile, leveling the concrete floor is critical. “Lippage,” or height differences between adjoining tiles, can damage tile and even create safety issues. Mortar beds were the preferred leveling underlayment for tile for many years, but pourable, self-leveling underlayments are now more common because of construction time constraints.
Carpet
As with other floor coverings, you may need a pourable, self-leveling underlayment to level and finish the slab surface. A carpet pad may also be required. Padding can provide cushioning, noise reduction, and thermal insulation.
Noise Reduction
In multi-story buildings, you may need acoustic underlayment, sound-absorbing carpet padding, or other products to reduce impact noise, such as footsteps, and airborne noise, such as voices and music.
Concrete Slab Preparation
Before installing underlayment, make sure the concrete slab is clean, free of contaminants, and dry.
Testing Concrete Moisture
To be sure the concrete slab is dry enough, first check the underlayment and floor covering manufacturers’ specifications for their concrete moisture limits.
Then test to make sure the slab’s moisture doesn’t exceed those limits. Manufacturers usually give limits for both moisture vapor emission rate (MVER) testing and in-situ relative humidity (RH) testing.
Calcium chloride (CaCl) testing, which measures the moisture vapor emission rate (MVER), has been used in the United States for a long time. But more and more flooring professionals now prefer in-situ RH testing.
MVER testing measures only the moisture near the surface of the slab before the flooring is installed. In-situ RH testing measures moisture within the slab, which moves toward the surface and stays there once a low-permeability floor covering is installed.
That’s why many flooring experts consider in-situ RH testing a better way to tell whether excess moisture in the slab could cause the flooring to fail.
In-Situ RH Testing Tools
The recognized standard for in-situ RH testing is ASTM F2170, and many floor covering and underlayment manufacturers now require F2170-compliant testing. You can buy a copy of the standard at store.astm.org.
In situ RH testing measures moisture within the concrete slab, which travels to the surface and remains there once a non-permeable material is installed.
When choosing an in-situ RH testing product, look for a complete reading, recording, and reporting system designed to follow ASTM F2170. For example, you’ll need probes that measure RH and temperature at each test site and an instrument that measures the ambient temperature and RH in the room.
ASTM F2170 calls for three tests for the first 1,000 square feet and at least one more test for each additional 1,000 square feet. You also need to record ambient temperature and RH during testing. An ambient monitor also helps you confirm that the slab and the space above it have been at service conditions for at least 48 hours before testing, as the standard requires.
The product should include probes and extenders that make it easy to install them at the correct depth. Per ASTM F2170, that’s 40% of the slab’s thickness for a slab drying from one side and 20% for a slab drying from two sides.
It should also include accessories that make it easy to thoroughly clean each drilled hole, since leftover dust can cause false readings.
ASTM F2170 requires each probe’s calibration to be checked within 30 days before use. The easiest way to meet this requirement is with single-use probes that come with calibration certificates from the manufacturer.
Free Download – 4 Reasons Why Your Concrete Is Taking Forever to Dry
When selecting an in-situ RH testing product, think about how easy it will be to take readings and meet the ASTM F2170 reporting requirements. Look for a product that records and stores test-site and ambient readings while you’re away from the job site.
Better yet, choose one that sends the data directly to your mobile device when you return to the job site, with free mobile apps that make reports easy to create. A system with these features makes recording and reporting faster, more accurate, and more secure.
How can you be sure the slab is dry enough to install underlayment? Confirm the maximum moisture limit for the flooring system, and follow ASTM F2170 for in-situ RH testing when applicable. The easiest way to do that is to choose a testing system designed specifically to meet the standard.
For more information, call us at (541) 291-5123.
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 October 21st, 2026
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