Moisture vapor transmission
Also called: MVT, moisture vapor emission, MVER, slab moisture, plastic sheet test, calcium chloride test, ASTM F1869, ASTM F2170
Water vapor moving up through a concrete slab from the ground beneath it. Under a floor coating it has nowhere to go, builds pressure, and lifts the coating in blisters or sheets. Tested with a taped plastic sheet or a calcium-chloride test; a mitigating primer handles slabs that fail.
Concrete is porous, and a slab poured on damp ground without a vapor barrier — common in older garages and basements — wicks moisture from the soil and releases it into the air above as vapor. Uncoated, that’s harmless; the slab breathes. Coated, the vapor collects under the film, pressure builds, and the coating separates from the concrete in blisters that grow into peeled sheets, usually within the first year. Adhesion was never the problem; the slab pushed the floor off.
The homeowner test is the plastic sheet: tape a 2×2 ft square of clear polyethylene to the slab on all four sides and leave it 24 hours. Condensation on the underside or a darkened patch of concrete means vapor is coming through. Do it in three spots, in the season you’ll coat. The professional tests are the calcium-chloride test (ASTM F1869), which weighs a desiccant dish sealed to the floor for 72 hours and reports pounds of moisture per 1,000 sq ft per 24 hours, and the in-situ relative-humidity probe (ASTM F2170) drilled into the slab; coating manufacturers publish the limits their products tolerate.
A slab that fails gets a moisture-mitigating primer — a specialized epoxy rated for high vapor emission — at $1–$2 per square foot before the coating, or a different floor. It’s the single most common exclusion in coating warranties, which is why a contractor’s quote should include the test and a DIY project should start with the plastic sheet.