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What is a concrete vapor barrier?

A concrete vapor barrier is any material that stops moisture from entering a concrete slab. Vapor barriers are used because while fresh concrete is poured wet, it’s not meant to stay doing this. It needs to dry and after that stay dry to stop flooring problems.

If you’ve been able to a problem with a basement floor (or any concrete floor), you know the sort of damage that too much moisture could cause. Moisture enters concrete in a variety of ways, including through the ground, from humidity via a flight, and through leaky plumbing that goes through a slab. Needless to say, there’s also the moisture which was within 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 experience of an origin of moisture, you’re gonna have problems. This is the reason a vapor barrier under concrete is essential. Vapor barriers are a good way to hold moisture from getting into the concrete.

Note: A vapor barrier is not the identical to an underlayment. However, you will find underlayments that work as vapor barriers.

Vapor barrier permeability is expressed in perms.

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

You’ll hear people while using terms ‘vapor barrier’ and ‘vapor retarder’ interchangeably. However, in fact, they aren’t the same thing. Vapor barriers are less permeable than vapor retarders. In this post, we are while using term ‘vapor barrier’.

Why’s an excessive amount of moisture in concrete an issue?

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

As moisture makes its approach to the top of a layer of concrete, soluble alkalies come along for the ride and lift its surface pH above those of flooring adhesives. This leads to the adhesives to breakdown and you also end up having flooring failures for example swelling, bulging, or cupping.

Do you really need a vapor barrier within a concrete slab?

In short, yes. Here’s why.

There’s usually water underneath a structure site. It may not be at the surface, but that doesn’t mean it’s not there. This water can go up over the soil and are avalable into exposure to the foot of a concrete floor via capillary action. Capillary action can be stopped by using something known as a capillary break, a layer of crushed rock which goes between your subgrade and 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 concrete slab. Therefore, there must be something beneath the slab that stops vapor moisture from entering.

You might also need a vapor barrier for liability reasons because most manufacturers of flooring include vapor barriers or retarders inside their installation guidelines.

How thick should a plastic vapor barrier be?

In accordance with the Self-help guide to Concrete Floor and Slab Construction published 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 need to be strong enough so that they don’t easily puncture. When they do, moisture are certain to get in and that’s what you’re always keeping out.

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