"Vinyl is waterproof, so moisture doesn't matter." That's one of the most expensive myths in flooring. The vinyl plank may be waterproof, but the adhesive under a glue-down floor isn't, and moisture trapped under any floor can cause odor, mold, adhesive breakdown, swelling of wood products and warranty denials. Moisture testing tells you what you're working with before the floor goes down.
Why concrete holds moisture
Concrete is made with water and stays damp inside for a long time after it's poured. Slabs on the ground can also pick up moisture from the soil below, especially if there's no vapor retarder under the slab, which is common in older homes. In basements, groundwater, high water tables and poor drainage add more. Even a slab that looks and feels dry on the surface can have high moisture deeper down.
Concrete test methods
Relative humidity probes (in-situ RH)
The method most flooring and adhesive manufacturers now reference is in-situ relative humidity testing, described in the standard ASTM F2170. Holes are drilled into the slab to a specified depth, sensors are placed and sealed, and after the specified equilibration time the relative humidity inside the slab is read. The test is done with the building at its normal service conditions.
The result is a percentage, and manufacturers specify maximum RH values for their products. Limits vary widely by product and adhesive, so always check the specific instructions.
Calcium chloride test
The older calcium chloride test (ASTM F1869) measures the moisture vapor emission rate from the slab surface over a set period, reported in pounds per 1,000 square feet per 24 hours. Some manufacturers still list limits for this test. It measures only the surface region of the slab and is sensitive to room conditions.
Plastic sheet test
Taping a square of plastic to the slab and checking for condensation or darkening (based on ASTM D4263) is a quick qualitative check. It can show an obvious problem, but it doesn't give a number manufacturers accept for warranty purposes.
Meters
Handheld concrete moisture meters give relative readings useful for finding wetter areas and deciding where to test. They generally don't replace the standardized tests for warranty purposes.
Testing wood subfloors
Plywood and OSB subfloors are checked with a pin-type or pinless wood moisture meter set for the right species or material. Take readings in many places, especially near exterior walls, plumbing and over crawl spaces. For wood flooring, the National Wood Flooring Association calls for the moisture content of the subfloor and the flooring to be within a few percentage points of each other, with tighter limits for wider boards. For LVP, the subfloor needs to be within the flooring manufacturer's limits and not wet.
Crawl spaces are a common source of subfloor moisture in our area. A crawl space should have a ground vapor retarder and, depending on its design, proper ventilation or conditioning.
Testing tips
- Test when the home is at normal living conditions, with heat or air conditioning running.
- Test in several locations, including near exterior walls and in the middle of the room.
- Follow the test standard's instructions for number of tests, depth and timing.
- Record the results with dates, locations and readings.
- Compare them to the specific flooring and adhesive manufacturers' limits.
What if the slab is too wet?
Options depend on the product and how high the readings are:
- Choose a product and adhesive rated for higher moisture. Some adhesives are rated for higher RH levels.
- Use a moisture mitigation system, such as a moisture-blocking epoxy or other membrane, applied before the flooring per the manufacturer's system.
- For floating floors, use a vapor barrier or an underlayment with a built-in barrier, as the manufacturer requires.
- Fix the source: improve grading and gutters, repair foundation drainage, run a dehumidifier.
- Wait, for new concrete that needs more drying time.
Seasonal moisture in Wisconsin and Minnesota
Basement slabs often read wetter in humid summers and after wet springs than in dry winters. If a basement has seasonal water issues, those need to be fixed before any flooring. Running a dehumidifier in summer helps keep basement humidity in a healthy range. See Wisconsin humidity, seasons and your floors.
Why professionals document results
When an installer tests and records moisture, it protects you. If a flooring issue ever comes up, the manufacturer will ask whether the subfloor met its requirements. Written test results with dates and locations answer that question.