A Summerlin homeowner told me his slab could not possibly have a moisture problem because it had not rained in two months and the desert is dry. The relative humidity probe in his slab read 89 percent. Both statements were true, and that is the part that trips people up.
Where the water comes from in a dry climate
Slab moisture is not about rainfall. It comes from three sources, and only one of them is weather.
First, the mix water. Concrete only chemically binds about half the water it is batched with. The rest has to leave by evaporation, and it can only leave through the top if the slab was poured over a vapor retarder.
Second, the ground. Even in the Mojave the soil a few feet down holds moisture, and irrigation around a house puts more there. Vapor moves from the higher concentration below to the drier air above, continuously, for as long as the building stands.
Third, wetting after the fact — a plumbing leak, a landscape drip line, a poorly drained patio.
Ambient dryness actually increases the vapor drive through a slab, because it steepens the gradient. A slab in Las Vegas can pass more vapor than the same slab in a humid climate.
The two numbers and what they actually measure
MVER — moisture vapor emission rate, from an anhydrous calcium chloride test, expressed in pounds per 1,000 square feet per 24 hours. A dish of desiccant sits under a sealed dome for 60 to 72 hours and gets weighed. It measures what is coming out of the top half-inch or so right now, under the room’s current conditions.
Internal RH, from a probe in a hole drilled to 40 percent of slab depth on a slab drying from one side, left sealed for 72 hours. It measures the moisture the whole slab will eventually equilibrate to.
They answer different questions. MVER is a snapshot of the surface; RH is a forecast for the assembly. A slab can pass a calcium chloride test in a cool, dry, air-conditioned week and fail badly once the building is occupied and the surface is covered.
That is why most manufacturers now write their warranties against internal RH. Typical thresholds: 75 percent for many resilient flooring adhesives, 80 to 85 percent for many epoxies, and up to 95 percent for systems specifically formulated for high vapor drive.
How readings get misread
- Testing too soon after grinding. Surface prep opens pores and briefly raises MVER. Wait 24 hours.
- Testing outside service conditions. The standard calls for the building to be at service temperature and humidity for 48 hours first. Testing in an unconditioned shell gives a number that means nothing.
- Too few tests. Three tests for the first 1,000 square feet, one per additional 1,000. A single test in the middle of a room misses the perimeter and the areas over wet ground.
- Treating a pass as permanent. A number is valid for the conditions at the time. If the slab gets wet again — irrigation change, plumbing leak — the number is void.
What to do with a high number
Either use a moisture-mitigation primer rated above your reading, at $2 to $5 per square foot, or choose a breathable finish — polished concrete, penetrating densifier and stain, or a cementitious overlay — that lets vapor keep moving. Testing costs $100 to $200 per location and takes three days, which is the cheapest insurance in the whole flooring budget. Any bid for Las Vegas epoxy flooring should name the test method, the threshold and what happens to the price if the slab reads high, because “we’ve never had a problem” is not a moisture reading.
Frequently asked questions
Can I test a slab myself? Calcium chloride kits are inexpensive and the procedure is straightforward, but the weighing needs a scale accurate to 0.1 gram and the conditioning requirements are strict. RH probes are more forgiving to use and more meaningful. Either way, follow the conditioning rules or the number is decorative.
Does a vapor retarder under the slab solve this? Largely, if it is a 10 to 15 mil sheet, properly lapped and sealed, and the concrete is placed directly on it. Most slabs poured before the 2000s do not have one, and many that do have it punctured or covered with a sand layer that holds water.
My slab tested fine but the floor still failed. Why? The usual causes are testing before service conditions were established, too few test locations, or a new water source after testing. Adhesive failure with a chalky white residue is the signature of alkaline moisture regardless of what the paperwork said.