When it comes to moisture testing in concrete slabs, two terms often get confused: Relative Humidity (RH) and Moisture Content (MC). While they may seem interchangeable, they’re measuring very different things—and understanding the distinction is critical for flooring professionals, builders, and facility managers alike.
Let’s break down the difference and explain why it matters for long-term floor performance.
Moisture Content: A Surface-Level Snapshot
Moisture content refers to the percentage of water present in the concrete at the time of measurement, typically measured using surface meters. While this may seem useful, it only reveals conditions near the top of the slab—not what’s happening deeper inside.
Limitations:
- Surface readings fluctuate with ambient humidity
- Doesn’t reflect moisture migrating up from the subgrade
- Cannot predict future moisture behavior
Surface meters may offer convenience, but they’re not enough to ensure slab readiness. That’s why many experts caution against relying on them exclusively. For a deeper perspective, see Why a Concrete Moisture Meter Isn’t the Final Decision.
Relative Humidity: The Deeper Truth
Relative Humidity testing—particularly in accordance with ASTM F2170—measures the internal conditions of the slab using in-slab sensors placed at a specific depth. RH provides a far more accurate indication of how moisture will behave once flooring is installed.
Advantages:
- Reflects equilibrium moisture inside the slab
- Predicts long-term behavior of the floor system
- Backed by industry standards and flooring manufacturers
RH testing is especially critical for systems that trap moisture (like vinyl, rubber, or glued-down materials), as it shows how moisture will rise and concentrate over time.
Why the Distinction Matters
Confusing RH and MC can lead to major missteps:
- Flooring installed over a slab that “feels dry” may still fail due to trapped vapor
- Moisture-related warranty claims may be denied if the wrong method was used
- Remediation costs can far exceed the cost of proper testing
In short, surface dryness is not the same as slab readiness.
What the Standards Say
The flooring industry overwhelmingly favors RH testing for its reliability and repeatability. ASTM F2170 has become the benchmark for assessing in-slab moisture, and many flooring manufacturers require RH readings to fall below specific thresholds (e.g., 75% or 85%) before installation.
Want real-world advice on executing RH tests? Check out 3 Tips for Concrete Moisture Testing in the Real World.
Final Thought
Moisture content might tell you what’s happening on the surface—but relative humidity reveals the truth below. If you’re making flooring decisions based only on surface meters, you could be standing on shaky ground. Go deeper. Trust RH.