Exactly 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 meant to stay doing this. It requires to dry then stay dry to avoid flooring problems.

If you’ve been able to an issue with a basement floor (or any concrete floor), you understand the kind of damage this too much moisture can cause. Moisture enters concrete in a variety of ways, including via the ground, from humidity in the air, and thru leaky plumbing that passes through a slab. Needless to say, there’s even 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 connection with a source of moisture, you’re going to have issues. This is the reason a vapor barrier under concrete is essential. Vapor barriers are a great way to keep moisture from getting yourself into the concrete.

Note: A vapor barrier is not same as an underlayment. However, you’ll find underlayments that become vapor barriers.

Vapor barrier permeability is expressed in perms.

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

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

Why is excessive moisture in concrete a challenge?

A word: adhesives. A lot of moisture in concrete is an issue because it can cause pH changes that destroy adhesives. Here’s what are the results.

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

Do you really need a vapor barrier within a layer of concrete?

In short, yes. Here’s why.

There’s more often than not water underneath a structure site. It may not be near the surface, but that doesn’t mean it’s not there. This water can progress through the soil are available into connection with the base 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 that goes involving the subgrade as well as the slab.

Capillary breaks do a sufficient job of stopping water in their 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 under the slab that forestalls vapor moisture from entering.

You need to 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 Help guide to Concrete Floor and Slab Construction published by the American Concrete Institute, a vapor retarder should not be under 10 mils thick. You will need a level thicker barrier though if you’re covering material with sharp angles.

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

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