Concrete Moisture Testing: Why It's Non-Negotiable

We've touched on moisture testing in several articles, but it deserves its own deep dive because moisture is the #1 cause of coating failure in Florida. If your installer isn't testing for moisture, walk away.

The Science

Concrete is like a rigid sponge — it's full of microscopic pores and capillaries. Water moves through these pathways via capillary action and vapor diffusion. In Florida, where the water table is often just a few feet below the surface, moisture is constantly migrating upward through the slab.

This moisture vapor creates hydrostatic pressure at the concrete-coating interface. When this pressure exceeds the coating's adhesion strength, the coating lifts off. It's physics, not bad luck.

The Two Standard Tests

ASTM F1869 — Calcium Chloride Test

A dish of anhydrous calcium chloride (a powerful desiccant) is sealed to the concrete surface for 60-72 hours. The weight gain of the calcium chloride tells us the moisture vapor emission rate (MVER).

Acceptable levels:

• Most epoxy systems: < 3 lbs/1,000 sq ft/24 hours
• Some moisture-tolerant systems: < 5 lbs

Limitations: Only measures surface moisture, not internal slab conditions. Can give misleading results if the concrete was recently sealed or cleaned.

ASTM F2170 — Relative Humidity Test (Preferred)

Small holes are drilled into the slab at 40% of the slab depth. Calibrated RH probes are inserted and allowed to equilibrate for at least 72 hours. The probes measure the internal relative humidity of the concrete.

Acceptable levels:

• Most epoxy systems: < 75% RH
• Moisture-mitigating primers: can handle up to 95-99% RH

Advantages: More accurate, measures internal conditions, less affected by surface treatments.

What Happens If Moisture Is Too High?

You have three options:

1. Wait — Allow the slab to dry out (can take weeks or months; not practical for occupied buildings)
2. Apply a moisture mitigation system — Specialized epoxy primers designed to block vapor transmission
3. Install a vapor barrier — A membrane system that creates an impermeable barrier between the slab and coating

We prefer option 2 or 3, depending on the severity of the moisture issue. Our approach to moisture management is one of the key differences between our installations and cut-rate competitors. Read our comparison in epoxy vs polyurea.

Red Flags

If a flooring contractor:

• Doesn't mention moisture testing → 🚩
• Uses only the "plastic sheet test" (duct-taping plastic to the floor) → 🚩 (This is a screening test, not a quantitative measurement)
• Says "concrete looks dry, we're fine" → 🚩🚩🚩
• Doesn't own moisture testing equipment → 🚩🚩🚩🚩

There are multiple threads on Reddit from homeowners who learned this lesson the hard way.

Don't risk it. Contact us and we'll test your slab properly.