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Just what concrete vapor barrier?

A concrete vapor barrier is any material that stops moisture from entering a concrete slab. Vapor barriers are employed because while fresh concrete is poured wet, it’s not designed to stay like that. It must dry and then stay dry to avoid flooring problems.

If you’ve had an issue with a basement floor (or any concrete floor), you know the sort of damage that too much moisture could cause. Moisture enters concrete in many different ways, including through the ground, from humidity in mid-air, and thru leaky plumbing that goes through a slab. Naturally, there’s the moisture that has been within the original concrete mixture.

There’s only one-way moisture leaves concrete, though, and that’s via its surface. When you have a concrete floor that’s in continuous exposure to a resource of moisture, you’re planning to have issues. For this reason a vapor barrier under concrete is vital. Vapor barriers are a good way to hold moisture from getting into the concrete.

Note: A vapor barrier is not 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 numbers of permeability, expressed in perms. The better the number, greater permeable the information. Impermeable vapor barriers are those using a rating of 0.1 perm or fewer while class II vapor retarders are those which has a rating more than 0.1 perm and much less than 1.0 perm.

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

Why is excessive moisture in concrete a challenge?

One word: adhesives. A lot of moisture in concrete is a problem as it can cause pH changes that destroy adhesives. Here’s how are you affected.

As moisture makes its approach to the top of your concrete slab, soluble alkalies appear for that ride and lift its surface pH above those of flooring adhesives. This causes the adhesives to breakdown and also you get flooring failures including swelling, bulging, or cupping.

Do you really need a vapor barrier within concrete slab?

In a word, yes. Here’s why.

There’s usually water underneath a structure site. It may not be at the surface, however that doesn’t mean it’s not there. This water can go up through the soil are available into connection with the base of a concrete floor via capillary action. Capillary action may be stopped by installing something termed as a capillary break, a layer of crushed rock that goes between the subgrade along with 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 needs to be something under the slab that stops vapor moisture from entering.

You might also need a vapor barrier for liability reasons since the majority of 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 provided by the American Concrete Institute, a vapor retarder shouldn’t be under 10 mils thick. You may want a good thicker barrier though if you’re covering material with sharp angles.

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

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