Concrete moisture test: the RH probe (ASTM F2170)

What the test measures

The test reads the relative humidity inside a concrete slab. NWFA’s installation guidelines describe it this way:

Relative humidity moisture testing is the standard test method for determining relative humidity in concrete floor slabs using in-situ probes.

NWFA Installation Guidelines, Revised © 2025, p. 43 (PDF p. 47)¹

How the test is run, in NWFA’s words, is under What a valid report looks like.

A reading means something only against a limit, and the test does not set one. The limit is in the installation document of the floor product, or of the adhesive for a glue-down floor. The table under Red flags shows how far the limits manufacturers publish spread from one floor type to the next.

What a valid report looks like

A complete report names, at minimum:

• the method, ASTM F2170;
• the date of each reading;
• where in the room each test was placed;
• the air temperature and relative humidity in the room during the test;
• each reading, test by test;
• the product or adhesive document the readings were compared with.

NWFA’s installation guidelines also set out how the test is run. Where a report says how it was run, these are the points to hold it against, quoted from NWFA:

Room conditions before the test

“Normal living conditions should be maintained for 48 hours prior to testing.”¹

Hole depth, slab drying from the top only

“If the slab is drying from top only (e.g. slab on-grade or in fluted metal decking), drill holes to 40% of the slab thickness.”¹

Hole depth, slab drying from top and bottom

“If the slab is drying from top and bottom (e.g., suspended slab), drill holes to 20% of the slab thickness.”¹

Wait before the reading

“Insert the sleeve, cap it, and allow it to acclimate for a minimum of 24 hours.”¹

Number of tests

“Conduct three tests for the first 1,000 square feet and one for every 1,000 square feet thereafter.”¹

Where the tests go

“Tests should be placed within 3 feet of each exterior wall.”¹

A steady reading

“The meter reading must not drift more than 1% relative humidity over a five-minute period.”¹

Probe calibration

“Calibration of reusable probes should be checked before every job, or if not used for 30 days.”¹

NWFA closes the procedure with “Record the data.”¹ and adds “Always follow manufacturer guidelines for testing procedure.”¹ This is NWFA’s procedure, not the floor document’s; where your product’s document describes the test, that document applies.

Red flags

NWFA’s installation guidelines are an industry guideline, not the document for your product. Where the two differ, both are shown above.

1. Any floor type: the report does not name the product document its readings were compared with. The limit belongs to the product, and products of one kind differ. In rigid core vinyl alone, manufacturers publish limits from 80 to 95 % RH across 6 documents. The limit that binds you is in your product's installation document.

2. Glue-down LVT: the report does not name the adhesive. For a glue-down floor the limit follows the adhesive. One glue-down guide, Karndean’s, gives 4 limits, one for each adhesive it names, from 90 to 99 % RH. It says why:

   The moisture limits for Karndean gluedown floors are mandated by the limits of the adhesive used.

   Karndean Gluedown Installation Guideline USA, KD80333GLO | October 2023, PDF p. 1⁸

3. Any floor type: no hole depth, no wait time, or fewer tests than the area calls for. A report that leaves these out cannot be held against NWFA’s procedure; see What a valid report looks like.

4. Hardwood: the reading is compared with NWFA’s figure alone. For engineered hardwood and solid hardwood, NWFA’s guideline and the manufacturers’ documents give different figures; both are at the foot of the table. NWFA’s sentence:

   Concrete slabs with a RH reading of more than 80% are strongly recommended to wait for further drying of the slab, or install/apply a class 1 impermeable vapor retarding membrane according to the adhesive manufacturer’s instructions, prior to installation.

   NWFA Installation Guidelines, Revised © 2025, p. 43 (PDF p. 47)¹

Questions to ask the installer

1. Which product’s installation document did you compare the readings with, and which limit in it?
2. For a glue-down floor: which adhesive will be used, and what limit does the adhesive’s document give?
3. How deep were the holes, and how long did the probes sit before the reading?
4. How many tests did you place for the floor area, and where?
5. Was the building at its normal living conditions before the test, and what were the room’s temperature and humidity during it?
6. For a hardwood floor: did you compare the readings with NWFA’s figure or with the product’s document?