Just what concrete vapor barrier?
A concrete vapor barrier is any material that stops moisture from entering a layer of concrete. Vapor barriers are employed because while fresh concrete is poured wet, it’s not meant to stay doing this. It needs to dry and then stay dry to prevent flooring problems.

If you’ve ever had a problem with a basement floor (or any concrete floor), you know the sort of damage this too much moisture could cause. Moisture enters concrete in several ways, including via the ground, from humidity in the air, and thru leaky plumbing that passes through a slab. Naturally, there’s even the moisture that has been inside the original concrete mixture.

There’s only one-way moisture leaves concrete, though, and that’s via its surface. For those who have a concrete floor that’s in continuous connection with a resource of moisture, you’re going to have issues. This is why 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 the just like an underlayment. However, you'll find underlayments that act as vapor barriers.

Vapor barrier permeability is expressed in perms.
Vapor barriers have varying degrees of permeability, expressed in perms. The higher the number, the harder permeable the material. Impermeable vapor barriers are the types which has a rating of 0.1 perm or fewer while class II vapor retarders are the type which has a rating greater 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. In this post, we are while using the term ‘vapor barrier’.

Why is an excessive amount of moisture in concrete a difficulty?
One word: adhesives. Excessive moisture in concrete is a problem since it can cause pH changes that destroy adhesives. Here’s what goes on.

As moisture makes its strategy to the counter of an layer of concrete, soluble alkalies appear for the ride and raise its surface pH above that regarding flooring adhesives. This causes the adhesives to breakdown and you get flooring failures such as swelling, bulging, or cupping.

Do you really need a vapor barrier with a concrete slab?
In short, yes. Here’s why.

There’s more often than not water underneath a structure site. It may not be at the surface, but that doesn’t mean it’s not there. This water can progress up over the soil and are available into connection with the bottom of a concrete floor via capillary action. Capillary action could be stopped by installing something known as a capillary break, a layer of crushed rock that goes relating to the subgrade as well as the slab.

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

There is also 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?
Based on the Help guide to Concrete Floor and Slab Construction published by the American Concrete Institute, a vapor retarder should not be lower than 10 mils thick. You may want a level thicker barrier though if you’re covering material with sharp angles.

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


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