| Method | Standard | Time | Measures |
|---|---|---|---|
| Calcium chloride | ASTM F1869 | 60-72 hours | Surface emission, lbs / 1,000 sq ft / 24 hr |
| In-situ relative humidity | ASTM F2170 | 72 hours after probe set | Internal slab RH, % |
| Plastic sheet test | ASTM D4263 | 16 hours | Pass/fail indication only |
What the numbers mean
- Under 3 lbs / 1,000 sq ft / 24 hr: most coatings are safe
- 3 to 5 lbs: moisture-tolerant primer recommended
- Over 5 lbs: moisture-mitigating epoxy primer required
- Under 75% internal RH: safe for most systems
- 75-85% RH: mitigation primer required
- Over 85% RH: full mitigation system, and reconsider the coating choice
Why in-situ RH is the better test
The calcium chloride test only samples the top half inch of the slab, and it is easily skewed by ambient humidity and by a slab surface that happens to be dry that week. In-situ RH probes drilled to 40 percent of slab depth measure what the concrete will actually release over time once you seal the top.
Slabs most likely to fail a test
- Poured without a vapor barrier beneath: common in homes built before the 1980s
- Below grade or partially below grade
- Downhill of a slope, or with poor exterior drainage against the foundation
- Less than 60 days old
- In humid Gulf Coast and subtropical climates year-round
- Slabs with existing efflorescence, dark patches, or persistent damp spots
What mitigation involves
A 100% solids moisture-mitigating epoxy primer applied over a properly ground slab creates a vapor-resistant membrane rated to hold back high emission rates. It adds roughly $1 to $2 per square foot and usually a cure period before the finish system goes down, which is one of the legitimate reasons a job becomes a two-day install.
The verdict
Never accept a coating quote that does not address moisture. The test is cheap, the mitigation is affordable, and the failure is a full teardown.
Frequently asked questions
- How do you test concrete for moisture before coating?
- The two reliable methods are calcium chloride testing per ASTM F1869 and in-situ relative humidity probes per ASTM F2170. A plastic sheet test only gives a rough pass-or-fail indication.
- What moisture level is safe for floor coatings?
- Under 3 lbs per 1,000 sq ft per 24 hours, or under 75 percent internal relative humidity, is generally safe for most systems.
- What happens if moisture is too high?
- Water vapor pressure pushes the coating off the slab, causing blisters and delamination. A moisture-mitigating primer is required above the safe threshold.
- Is in-situ RH better than calcium chloride?
- Yes. RH probes measure moisture at 40 percent of slab depth, which reflects what the slab will release over time. Calcium chloride only samples the surface and is easily skewed by weather.
- Which slabs are most likely to fail a moisture test?
- Slabs poured without a vapor barrier, below-grade slabs, new concrete under 60 days, and slabs in humid Gulf Coast or subtropical climates.