Just what is a concrete vapor barrier?
A concrete vapor barrier is any material that prevents moisture from entering a concrete slab. Vapor barriers are used because while fresh concrete flows wet, it’s not designed to stay like that. It requires to dry and after that stay dry to prevent flooring problems.
If you’ve ever endured a problem with a basement floor (or any concrete floor), you understand the type of damage that as well much moisture may cause. Moisture enters concrete in a number of ways, including through ground, from humidity in mid-air, via leaky plumbing that passes through a slab. Naturally, there’s the moisture that was in 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 connection with an origin of moisture, you’re planning to have problems. This is why a vapor barrier under concrete is crucial. Vapor barriers are a great way to keep moisture from getting yourself into the concrete.
Note: A vapor barrier is not the comparable to an underlayment. However, you'll find underlayments that act as vapor barriers.
Vapor barrier permeability is expressed in perms.
Vapor barriers have varying examples of permeability, expressed in perms. The better the number, the greater permeable the fabric. Impermeable vapor barriers are the type with a rating of 0.1 perm or less while class II vapor retarders are those using a rating higher than 0.1 perm much less than 1.0 perm.
You’ll hear people while using 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 are with all the term ‘vapor barrier’.
How come excessive moisture in concrete a challenge?
One word: adhesives. Excessive moisture in concrete is a problem as it can cause pH changes that destroy adhesives. Here’s what goes on.
As moisture makes its way to the surface of an layer of concrete, soluble alkalies arrive for the ride and lift its surface pH above that relating to flooring adhesives. This makes the adhesives to breakdown and also you end up getting flooring failures including swelling, bulging, or cupping.
Do you require a vapor barrier under a concrete slab?
In short, yes. Here’s why.
There’s almost always water underneath a building site. It might not be near the surface, but that doesn’t mean it’s not there. This water can progress through the soil and are available into exposure to the base of a concrete floor via capillary action. Capillary action could be stopped by installing something referred to as a capillary break, a layer of crushed rock which goes involving the subgrade and also the slab.
Capillary breaks do a good 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 beneath the slab that prevents 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 Concrete Floor and Slab Construction authored by the American Concrete Institute, a vapor retarder should not be under 10 mils thick. You may want an even thicker barrier though if you’re covering material with sharp angles.
Main point here: Vapor barriers must be strong enough so that they don’t easily puncture. When they do, moisture will get in and that’s what you’re trying to keep out.
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