Concrete slabs may look solid, but they’re constantly interacting with moisture. That’s why accurate moisture testing is critical before installing any flooring system. But when it comes to methods, two options dominate the industry: calcium chloride testing and relative humidity (RH) testing.
So which one should you trust—and when? Let’s unpack the differences so you can make the right call for your next project.
Calcium Chloride Testing: The Classic Approach
Also known as the MVER (Moisture Vapor Emission Rate) test, calcium chloride testing involves placing a small dish of desiccant (anhydrous calcium chloride) under a sealed dome on the surface of a concrete slab. After 60–72 hours, the dish is weighed to determine how much moisture it absorbed.
Pros:
- Simple and low-cost
- Easy to understand
- Can reveal surface moisture emissions
Cons:
- Only measures surface-level moisture
- Doesn’t account for subsurface vapor pressure
- Affected by ambient humidity and recent HVAC changes
This method was widely used for decades, but has lost ground due to reliability concerns—especially when flooring adhesives or coatings are involved.
RH Testing: The Industry Standard
Relative Humidity (RH) testing, especially using in-situ probes, measures moisture deeper within the slab—typically at 40% depth in accordance with ASTM F2170. This provides a more accurate representation of how the slab will behave once sealed by flooring.
Pros:
- Measures internal slab conditions
- Aligns with modern flooring warranties
- Predicts long-term performance
Cons:
- Requires specialized probes and calibrated sensors
- Slightly higher setup cost
- Needs 24+ hours of equilibration time
Because it mimics the real-world scenario of trapping moisture under a finished floor, RH testing has become the gold standard for slab readiness.
Want a practical field-level perspective on RH testing? See 3 Tips for Concrete Moisture Testing in the Real World.
Side-by-Side Comparison
| Factor | Calcium Chloride | RH Testing (ASTM F2170) |
|---|---|---|
| Measures surface only | ✅ Yes | ❌ No |
| Measures internal slab | ❌ No | ✅ Yes |
| Aligns with warranties | ❌ Rarely | ✅ Commonly |
| Fast results | ✅ 60–72 hrs | ✅ Within 24 hrs |
| Long-term predictability | ❌ Limited | ✅ Strong |
| Preferred by pros | ⚠️ Outdated | ✅ Standard of care |
Which Method Should You Use?
If you’re installing resilient flooring, coatings, or anything vapor-sensitive, RH testing is the superior choice—period. While calcium chloride can still offer value as a surface indicator in specific retrofit or demo contexts, it should never be the sole method used for critical flooring decisions.
Final Thought
Moisture testing isn’t just a checkbox—it’s a decision-making tool. When the stakes include warranties, occupant health, and long-term flooring integrity, only one method tells the whole story. Choose depth. Choose RH.