What is a concrete vapor barrier?

A concrete vapor barrier is any material that stops moisture from entering a concrete slab. Vapor barriers are used because while fresh concrete is poured wet, it’s not meant to stay doing this. It requires to dry then stay dry to prevent flooring problems.

If you’ve ever had a problem with a basement floor (or any concrete floor), you know the kind of damage the exact same thing much moisture might cause. Moisture enters concrete in a variety of ways, including via the ground, from humidity up, and through leaky plumbing that goes through a slab. Needless to say, there’s the moisture that’s in the original concrete mixture.

There’s only one-way moisture leaves concrete, though, and that’s via its surface. If you have a concrete floor that’s in continuous contact with a source of moisture, you’re planning to have issues. That is why a vapor barrier under concrete is essential. Vapor barriers are a great way to hold moisture from getting into the concrete.

Note: A vapor barrier is not the just like an underlayment. However, you can find underlayments that act as vapor barriers.

Vapor barrier permeability is expressed in perms.

Vapor barriers have varying examples of permeability, expressed in perms. The larger the number, the greater permeable the fabric. Impermeable vapor barriers are the types having a rating of 0.1 perm or fewer while class II vapor retarders are the types which has a rating higher than 0.1 perm and less than 1.0 perm.

You’ll hear people while using terms ‘vapor barrier’ and ‘vapor retarder’ interchangeably. However, in fact, they aren’t the same. Vapor barriers are less permeable than vapor retarders. In this post, we will be using the term ‘vapor barrier’.

Why’s an excessive amount of moisture in concrete a problem?

One word: adhesives. Excessive moisture in concrete is an issue because it could cause pH changes that destroy adhesives. Here’s what happens.

As moisture makes its method to the outer lining of an layer of concrete, soluble alkalies arrive for that ride and raise its surface pH above that relating to flooring adhesives. This will cause the adhesives to breakdown and you also end up having flooring failures like swelling, bulging, or cupping.

Do you require a vapor barrier under a layer of concrete?

In a word, yes. Here’s why.

There’s almost always water underneath a building site. It might not be near the surface, but that doesn’t mean it’s not there. This water can go up with the soil and come into contact with the base of a concrete floor via capillary action. Capillary action can be stopped by installing something referred to as a capillary break, a layer of crushed rock which goes between the subgrade along with the slab.

Capillary breaks do a good job of stopping water in the liquid state from reaching a slab. However, they can’t stop water in vapor form from reaching and entering a concrete slab. Therefore, there ought to be something underneath the slab that stops vapor moisture from entering.

You need to a vapor barrier for liability reasons since most manufacturers of flooring include vapor barriers or retarders in their installation guidelines.

How thick should a plastic vapor barrier be?

According to the Self-help guide to Concrete Floor and Slab Construction authored by the American Concrete Institute, a vapor retarder should not be lower than 10 mils thick. You will need a good thicker barrier though if you’re covering material with sharp angles.

Main point here: Vapor barriers need to be sufficiently strong enough enough so that they don’t easily puncture. If they do, moisture are certain to get in and that’s what you’re continuing to keep out.

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