Just what concrete vapor barrier?
A concrete vapor barrier is any material that stops moisture from entering a concrete slab. Vapor barriers are utilized because while fresh concrete is poured wet, it’s not supposed to stay like that. It needs to dry then stay dry to stop flooring problems.
If you’ve ever endured an issue with a basement floor (or any concrete floor), you know the type of damage that too much moisture could cause. Moisture enters concrete in a number of ways, including using the ground, from humidity via a flight, via leaky plumbing that passes through a slab. Obviously, there’s even the moisture that's within 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 a resource of moisture, you’re gonna have issues. This is why a vapor barrier under concrete is vital. Vapor barriers are a good way to maintain moisture from engaging in the concrete.
Note: A vapor barrier isn't same as an underlayment. However, you can 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 higher the number, the harder permeable the fabric. Impermeable vapor barriers are the types having a rating of 0.1 perm or fewer while class II vapor retarders are those which has a rating more than 0.1 perm and 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, we are while using term ‘vapor barrier’.
Exactly why is excessive moisture in concrete a challenge?
A word: adhesives. A lot of moisture in concrete is an issue given it might cause pH changes that destroy adhesives. Here’s what are the results.
As moisture makes its strategy to the top of the concrete slab, soluble alkalies arrive for that ride and lift its surface pH above that relating to flooring adhesives. This causes the adhesives to breakdown so you end up with flooring failures such as swelling, bulging, or cupping.
Do you really need a vapor barrier with a concrete slab?
In a word, yes. Here’s why.
There’s usually water underneath a structure site. It may not be nearby the surface, but that doesn’t mean it’s not there. This water can go up over the soil and are available into exposure to the foot of a concrete floor via capillary action. Capillary action can be stopped by installing something called a capillary break, a layer of crushed rock which goes involving the subgrade and the slab.
Capillary breaks do a passable 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 under the slab that forestalls vapor moisture from entering.
You have to a vapor barrier for liability reasons since the majority manufacturers of flooring include vapor barriers or retarders within their installation guidelines.
How thick should a plastic vapor barrier be?
According to the Guide to Concrete Floor and Slab Construction provided by the American Concrete Institute, a vapor retarder should not be less than 10 mils thick. You need a level thicker barrier though if you’re covering material with sharp angles.
Bottom line: Vapor barriers need to be sufficiently strong enough enough so they really don’t easily puncture. Whenever they do, moisture will get in and that’s what you’re attempting to keep out.
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