Concrete

Subfloor Moisture Testing: Calcium Chloride vs RH Probes

In-situ RH probes (ASTM F2170) vs calcium chloride (ASTM F1869): what each measures, how to run them, and why you can't convert one to the other.

Updated · 4 min read

Most flooring and adhesive failures on concrete trace back to moisture. Before you install, run the test your flooring and adhesive manufacturers name in their instructions, and compare the result to the stricter of the two limits.

Quick answer

For concrete, in-situ relative humidity probes (ASTM F2170) are what most current installation instructions call for.[1] Calcium chloride (ASTM F1869) still shows up on older specs and some products.[2] They measure different things, and there is no conversion between them. For wood subfloors, use a wood moisture meter and compare the subfloor to the flooring.

Side-by-side

The two concrete tests answer different questions, which is why the results can't be swapped for each other.

RH probes, ASTM F2170[1] Calcium chloride, ASTM F1869[2]
What it measures Relative humidity (%) inside the slab at a set depth Moisture vapor emission rate from the surface, in lb per 1,000 sq ft per 24 hr
Where 40% of slab depth if the slab dries from one side (slab on grade); 20% if it dries from both sides The top of the slab only
Time Probes equilibrate at least 24 hours before a documentable reading Dish is sealed for 60 to 72 hours
How many 3 tests for the first 1,000 sq ft, plus 1 for each additional 1,000 sq ft Per the standard and the manufacturer's spec
Limits Set by the flooring/adhesive manufacturer, e.g. 75%, 80%, 85% Set by the manufacturer; 3 lb is a commonly seen older limit
Not suited for Anything the manufacturer doesn't accept it for Lightweight aggregate or gypsum toppings; slabs with coatings, sealers, patch or leveler on them

A calcium chloride result only tells you about the slab surface on the day you tested, and it may not predict how the slab behaves later.[2] RH at depth predicts what the slab will do once it's covered, which is why most manufacturers moved to it.

Before you test

Test only after the building is running the way it will once the floor is down.

  • The building should be enclosed and the HVAC running. F2170 requires the slab and the air above it to be at service conditions for at least 48 hours before testing.[1]
  • Record ambient temperature and RH at each test location. A thermo-hygrometer makes this quick.
  • For calcium chloride, test bare concrete. Old adhesive, sealers and patch have to come off the test area first.

Running an RH test (F2170)

The RH probe test takes a day of waiting, but the setup itself is quick if you follow the steps in order.

  1. Mark test locations: 3 for the first 1,000 sq ft, 1 more per additional 1,000 sq ft. Include a spot near an exterior wall and one in the middle of the floor, not just near the door.
  2. Drill to 40% of slab thickness (slab on grade) or 20% (elevated slab drying from both sides).[1]
  3. Clean the hole, insert the liner or sensor and seal it as the probe maker directs.
  4. Wait at least 24 hours, then read and record RH and temperature.[1]
  5. Sensors need current NIST-traceable calibration.[1]

Reading the results

A number on its own doesn't pass or fail a slab. It has to be compared against the right limits.

  • Compare every reading, not the average, to the flooring limit and the adhesive limit. The lower one governs.
  • If you're over, you have three options: more drying time with the HVAC running, a moisture-tolerant adhesive rated for your number, or a mitigation system rated for your number. Get the manufacturer's approval in writing either way.
  • Keep the readings, locations and dates in the job file. A warranty claim will ask for exactly that.

Wood subfloors

For plywood or OSB, take readings with a pin or pinless wood moisture meter and compare the subfloor to the flooring. The rule of thumb: the subfloor can run up to 4 points of moisture content above solid strip flooring (under 3" wide), and up to 2 points above solid plank (3" and wider).[3] Take at least 20 readings in the first 1,000 sq ft, then 4 more for every additional 100 sq ft.[3] See Acclimating wood flooring and checking subfloor moisture.

Common mistakes

Most moisture-related failures trace back to a test that was run too early, in the wrong place, or against the wrong limit.

  • Testing before the HVAC is on. The numbers mean nothing until the space is at service conditions.
  • Using a surface meter as the spec test. Handheld concrete meters are good for finding wet areas and choosing where to test. They don't replace F2170 or F1869 when the instructions call for them.
  • Converting between CaCl and RH. No published ASTM conversion exists.[1][2]
  • Checking only the flooring limit. The adhesive often has the tighter number.

Tools for testing

These are the testing tools we carry for concrete and wood subfloors.

References

  1. ASTM F2170: Relative humidity in concrete slabs using in-situ probes
  2. ASTM F1869: Moisture vapor emission rate using anhydrous calcium chloride
  3. NWFA: Wood Flooring Installation Guidelines

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