How to Measure High Moisture in Concrete

We once looked at a garage floor down in Florida for a client. The local contractor there showed us a simple piece of plastic he had taped to the floor to prove it was dry, but my husband Tim knew better. While it is one method of checking moisture, a piece of taped plastic doesn’t tell you the whole story of what’s happening inside the slab.

Determining whether or not moisture is present does not tell you if it’s high. In order to do that, you must find the relative humidity (RH) or the moisture vapor emission rate. The RH measures the moisture that is sitting inside the slab, while a vapor emission rate tells you how much moisture is passing through it. The plastic sheet test falls under the vapor emission test category, but it does not provide a gauge on the amount of moisture. There are other, more scientific test methods that can be used for an accurate gauge.

Understanding moisture in concrete

When I talk about high moisture, I don’t mean the concrete is physically wet to the touch. It’s not like a freshly poured, wet slab. Instead, we are talking about relative humidity (RH) and vapor emission.

Think of it like a humid day. You can feel the moisture in the air, but you aren’t necessarily in a dripping wet environment. Your concrete feels the same way. In areas like Michiana, we find that most slabs are above standard moisture levels, even if they were poured years ago.

Why your concrete is a hard sponge

Whether you can see it or not, moisture is constantly moving through your slab. You should think of concrete as a hard sponge with millions of tiny holes.

1. Moisture from the ground moves up through the slab.
2. Vapor barriers (plastic sheeting under the concrete) are helpful when installed correctly. The key word being WHEN!
3. Moisture only gets trapped when you cover it with something that doesn’t breathe, like an epoxy coating.

What happens when you ignore the moisture

If I apply a standard epoxy primer over a slab with high moisture, it’s a gamble. Most traditional primers can only handle up to 75 RH. If your concrete is higher than that, the moisture gets blocked and builds up pressure.

This leads to osmotic blistering. The moisture literally pushes the coating off the slab. On a solid color or metallic style floor, you’ll see bubbles that look like the bumps on a toad. They are usually about the size of a quarter, though they can vary. If you have a chip style floor, these bubbles are harder to see, but the damage is still happening underneath.

4 ways to measure moisture

There are four main ways we test for moisture. Some are easy for you to do at home, while others require professional tools.

1. The plastic sheet test
   • Tape an 18 by 18 inch piece of plastic to the floor.
   • Leave it for 24 to 48 hours.
   • If the concrete is dark or the plastic is damp, you have moisture.
   • This is the least scientific method because it doesn’t tell you how much moisture is there, just that it exists.
2. The calcium chloride test
   • Place a small dish of calcium chloride under a plastic dome for 60 to 72 hours.
   • Weigh the dish before and after.
   • The weight difference tells you how much moisture is moving through the slab.
3. Moisture testing meters
   • Tools like a Tramex meter are placed on the surface.
   • They give a reading from 0 to 6 percent.
   • Anything above 4.5 percent is considered high.
4. Relative humidity (RH) probes
   • I consider this the most reliable method.
   • You drill a hole into the concrete and insert a probe.
   • It measures the humidity inside the slab rather than just on the surface.
   • Anything between 76 to 100 RH is considered high.

What if my concrete has high moisture?

If your test results come back high (over 75 RH is considered high), you have a few options.

• Use a moisture mitigating primer. These cost more and are applied thicker, but some can tolerate up to 100 RH.
• Use a cement urethane primer. This is an upgraded, hardy option that is self leveling and a nominal 1/8” thick. It can handle high moisture and higher levels of contaminants in the concrete slab.
• Choose a breathable coating. These allow moisture to pass through, though they aren’t quite as tough as traditional epoxy.
• Polished concrete. If you don’t want to risk a coating, polishing doesn’t seal the surface, so the concrete can breathe naturally.

How we do things differently

Many coating companies know moisture is an issue, but they don’t always talk about it. If you check their warranties, you’ll often find that high moisture failure is excluded.

I prefer to test upfront or simply choose a high moisture primer right from the get go. It’s one way to decrease risk and provide a better product for the long term.

In closing

Don’t assume that because your concrete is old, it must be dry. I’ve seen slabs that are decades old that still test high in moisture. Defaulting to a standard primer or skipping the testing phase is a risk. If your floor bubbles, the solution is grinding it off and redoing it with a moisture mitigating primer or breathable coating.