1d (edited) • Systems & Techniques
Understanding Concrete Moisture The Right Way
💦 Moisture is one of the top causes of coating failures, and most installers still misunderstand how concrete actually behaves. People rely on surface readings, seasonal changes, or “rule of thumb” methods that have nothing to do with what’s happening inside the slab. Here’s the simple, accurate, installer-focused truth.
Concrete Is a Sponge 🧽
Concrete literally acts like a sponge.
• It holds more water than the cement needs
• The top dries first because it’s exposed to air
• The bottom stays wet because it sits on soil that’s basically 100% humidity
This creates a moisture gradient from top to bottom.
Dry on the surface. Wet underneath.
Understanding that one concept unlocks everything else.
Why a Slab Can Look Dry Today and Fail Tomorrow
The ground under a slab never runs out of moisture.
If the vapor retarder is missing or compromised, the slab constantly absorbs water from the soil and releases it upward.
When you apply a low-permeance system (epoxy, polyaspartic, urethane, MVB, etc.):
• You block evaporation
• Moisture gets trapped
• The slab rebalances toward the humidity of the soil
• And failures begin — bubbles, blisters, darkening, outgassing, mapping
This is why floors fail after installation, not before.
Why Vapor Retarders Matter
ASTM requires vapor retarders under moisture-sensitive flooring for one reason:
To stop ground moisture from entering the slab.
Even with a vapor retarder, you still must verify the internal RH before sealing anything. Never assume. Always test.
Why RH Testing at 40% Depth Is the Gold Standard
Surface readings and pin meters only tell you what’s happening at the top — which is always the driest part of the slab.
ASTM F2170 in-situ RH testing measures humidity inside the slab at 40% depth because:
• It predicts the RH the surface will rise to once sealed
• It shows the real conditions where failures occur
• Manufacturers base their limits on this number
• It removes guesswork and protects your installs
This is why 40% RH testing is the trusted standard.
Is RH Just a “Snapshot in Time”?
No.
Internal RH stays extremely stable when:
• The vapor retarder is intact
• The slab isn’t being externally wetted
• Indoor conditions are normal
Seasonal swings change the surface.
They do NOT change the interior unless something is wrong at a foundational level.
This is why surface readings mislead installers over and over again.
Bottom Line
When you understand the moisture gradient and rely on the correct testing methods:
• Moisture becomes predictable
• Failures become preventable
• Your work becomes more durable
• And you protect your long-term reputation
RH testing at depth isn’t optional — it’s essential.
If you need help interpreting readings or deciding whether a mitigation system is truly needed, I’m here to help you get it right.
Looking to learn more? Moisture Control Systems Course Coming Soon.
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Understanding Concrete Moisture The Right Way
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