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Just what is a concrete vapor barrier?

A concrete vapor barrier is any material that stops moisture from entering a layer of concrete. Vapor barriers are widely-used because while fresh concrete flows wet, it’s not likely to stay like that. It has to dry and after that stay dry in order to avoid flooring problems.

If you’ve had a problem with a basement floor (or any concrete floor), you already know the sort of damage this too much moisture might cause. Moisture enters concrete in many different ways, including via the ground, from humidity up, via leaky plumbing that passes through a slab. Obviously, there’s also the moisture that was inside the original concrete mixture.

There’s only one-way moisture leaves concrete, though, and that’s via its surface. In case you have a concrete floor that’s in continuous exposure to a source of moisture, you’re going to have problems. This is the reason a vapor barrier under concrete is essential. Vapor barriers are a great way to hold moisture from engaging in the concrete.

Note: A vapor barrier is not the just like an underlayment. However, there are underlayments that behave as vapor barriers.

Vapor barrier permeability is expressed in perms.

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

You’ll hear people using the terms ‘vapor barrier’ and ‘vapor retarder’ interchangeably. However, strictly speaking, they aren’t a similar thing. Vapor barriers are less permeable than vapor retarders. In this article, we are using the term ‘vapor barrier’.

Exactly why is too much moisture in concrete a challenge?

One word: adhesives. An excessive amount of moisture in concrete is a dilemma as it can cause pH changes that destroy adhesives. Here’s what goes on.

As moisture makes its approach to the surface of the concrete slab, soluble alkalies come along for the ride and lift its surface pH above those of flooring adhesives. This will cause the adhesives to breakdown and also you end up with flooring failures for example swelling, bulging, or cupping.

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

In short, yes. Here’s why.

There’s usually water underneath a structure site. It might not be nearby the surface, but that doesn’t mean it’s not there. This water can move up through the soil and are avalable into connection with the base of a concrete floor via capillary action. Capillary action might be stopped by using something called a capillary break, a layer of crushed rock which goes involving the subgrade and the slab.

Capillary breaks do a sufficient job of stopping water rolling around in its liquid state from reaching a slab. However, they can’t stop water in vapor form from reaching and entering a concrete slab. Therefore, there must be something underneath the slab that prevents vapor moisture from entering.

There is also a vapor barrier for liability reasons since most manufacturers of flooring include vapor barriers or retarders of their installation guidelines.

How thick should a plastic vapor barrier be?

According to the Guide to Concrete Floor and Slab Construction created by the American Concrete Institute, a vapor retarder mustn’t be lower than 10 mils thick. You need a good thicker barrier though if you’re covering material with sharp angles.

Bottom line: Vapor barriers should be sufficiently strong enough so they don’t easily puncture. If they do, moisture will get in and that’s what you’re trying to keep out.

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