What is a concrete vapor barrier?

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

If you’ve ever had a problem with a basement floor (or any concrete floor), you realize the sort of damage that too much moisture may cause. Moisture enters concrete in many different ways, including through the ground, from humidity via a flight, and through leaky plumbing that goes through a slab. Needless to say, there’s the moisture that was within 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 an origin of moisture, you’re going to have issues. That is why a vapor barrier under concrete is vital. Vapor barriers are a way to maintain moisture from stepping into the concrete.

Note: A vapor barrier isn’t same as an underlayment. However, you can find underlayments that become vapor barriers.

Vapor barrier permeability is expressed in perms.

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

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

Exactly why is an excessive amount of moisture in concrete a problem?

One word: adhesives. Too much moisture in concrete is a problem since it may cause pH changes that destroy adhesives. Here’s how are you affected.

As moisture makes its strategy to the top of a concrete slab, soluble alkalies show up for the ride and raise its surface pH above that of flooring adhesives. This leads to the adhesives to breakdown and you get flooring failures for example swelling, bulging, or cupping.

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

In short, yes. Here’s why.

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

Capillary breaks do a sufficient job of stopping water 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 should be something underneath the slab that forestalls vapor moisture from entering.

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

How thick should a plastic vapor barrier be?

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

Important thing: Vapor barriers have to be strong enough so that they don’t easily puncture. Whenever they do, moisture will get in and that’s what you’re continuing to keep out.

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