Just what is a concrete vapor barrier?

A concrete vapor barrier is any material that forestalls moisture from entering a concrete slab. Vapor barriers are employed because while fresh concrete is poured wet, it’s not meant to stay that way. It requires to dry after which stay dry to avoid flooring problems.

If you’ve ever endured a problem with a basement floor (or any concrete floor), you realize the level of damage that too much moisture could cause. Moisture enters concrete in a variety of ways, including through ground, from humidity via a flight, via leaky plumbing that passes through a slab. Naturally, there’s the moisture which was from 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 resource of moisture, you’re likely to have problems. For this reason a vapor barrier under concrete is crucial. Vapor barriers are a way to maintain moisture from getting into the concrete.

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

Vapor barrier permeability is expressed in perms.

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

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

Why is excessive moisture in concrete an issue?

One word: adhesives. Too much moisture in concrete is a concern given it can cause pH changes that destroy adhesives. Here’s what happens.

As moisture makes its strategy to the surface of the concrete slab, soluble alkalies come along for that ride and raise its surface pH above that regarding flooring adhesives. This causes the adhesives to breakdown and you also end up having flooring failures including swelling, bulging, or cupping.

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

In a word, yes. Here’s why.

There’s usually water underneath a building site. It may not be near the surface, but that doesn’t mean it’s not there. This water can progress up over the soil and are available into exposure to the base of a concrete floor via capillary action. Capillary action can be stopped by using something termed as a capillary break, a layer of crushed rock which goes involving the subgrade and also the slab.

Capillary breaks do an adequate 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 layer of concrete. Therefore, there needs to be something within the slab that forestalls vapor moisture from entering.

There is also 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?

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

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

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