What is an MVB or moisture-vapor test?

It’s a check of how much moisture is moving up through a concrete slab before a coating goes down. Common methods are a calcium-chloride kit, in-slab relative-humidity probes, or a taped plastic-sheet check. The result tells us whether the slab needs a moisture-barrier primer first.

Can you coat a concrete floor that has moisture problems?

Yes — but only after the moisture is accounted for. We install a moisture-vapor-barrier primer so the vapor can’t push the coating off, then build the system on top. Coating a wet slab without that step is the fast track to a peeling floor.

How long does moisture testing take?

The slab tests themselves are quick to set up, but the most accurate methods need to sit for a period (often 24–72 hours) to read true conditions. We build that into the schedule rather than guessing.

Process · Apr 2026 · 5 min read

Moisture vapor and your slab: what an MVB test tells you before you coat

A slab can look bone-dry and still be quietly pushing moisture up from the ground. That vapor is one of the most common reasons coatings fail — and a simple test catches it before a drop of product goes down.

GlossWorks17 years of Saskatchewan concrete

What is a moisture-vapor (MVB) test?

It’s a measurement of how much moisture is rising through your concrete before we coat it. A slab sitting on soil constantly wicks ground moisture upward as vapor. If we seal a coating over the top without checking, that vapor has nowhere to go — and it pushes the coating off from underneath. The test tells us whether the slab needs a moisture barrier first.

Concrete is porous, not waterproof. Even a slab that has been dry to the touch for years can be moving a surprising amount of vapor, especially in spring or after a wet stretch. You can’t see it, which is exactly why it catches DIY and bargain jobs out.

Why moisture lifts a coating

A coating bonds to the top surface of the concrete. When vapor pressure builds underneath it, a few things happen:

Blisters and bubbles form as vapor collects under the film.
The bond breaks from below, so the coating delaminates in sheets even though the top looks fine.
Trapped moisture can carry alkalinity that attacks certain coatings chemically over time.

How we test

There’s no single magic number, so we use established methods: a calcium-chloride kit that measures how much moisture collects over a set period, in-slab relative-humidity probes for a reading from inside the concrete, and sometimes a simple taped plastic-sheet check as a first look. The right method depends on the slab and the system going on it.

What we do if the slab is wet

If the readings are high, we don’t cancel the job — we install a moisture-vapor-barrier primer designed to hold that vapor back, then build the coating on top of it. It’s an extra step and an extra material cost, but it’s far cheaper than redoing a floor that lifts in a year.

The slab will tell you the truth if you test it. The expensive mistake is assuming, then coating over the answer.

The takeaway

Moisture testing is one of those steps you never see in the finished floor — which is why corner-cutting crews skip it. When you’re comparing quotes, ask whether moisture testing is included and how they handle a slab that reads high. A coating is only as good as the slab it’s bonded to, and moisture is what decides that bond.

Thinking about a coating?

We’ll give you a free, written quote — and tell you straight whether your slab is a good candidate.

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