Exactly what is a concrete vapor barrier?
A concrete vapor barrier is any material that forestalls moisture from entering a concrete slab. Vapor barriers are used because while fresh concrete is poured wet, it’s not designed to stay this way. It must dry then stay dry to stop flooring problems.
If you’ve ever had a problem with a basement floor (or any concrete floor), you understand the sort of damage that too much moisture can cause. Moisture enters concrete in many different ways, including through ground, from humidity up, and thru leaky plumbing that goes through a slab. Of course, there’s the moisture which was in 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 contact with a source of moisture, you’re planning to have issues. This is why a vapor barrier under concrete is essential. Vapor barriers are a great way to keep moisture from getting into the concrete.
Note: A vapor barrier isn't same as an underlayment. However, there are underlayments that behave 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 more permeable the fabric. Impermeable vapor barriers are the ones using a rating of 0.1 perm or fewer while class II vapor retarders are the ones using a rating in excess of 0.1 perm and fewer than 1.0 perm.
You’ll hear people using the terms ‘vapor barrier’ and ‘vapor retarder’ interchangeably. However, strictly speaking, they aren’t the same thing. Vapor barriers are less permeable than vapor retarders. On this page, i will be while using the term ‘vapor barrier’.
How come an excessive amount of moisture in concrete a problem?
A word: adhesives. Excessive moisture in concrete is a dilemma given it can cause pH changes that destroy adhesives. Here’s how are you affected.
As moisture makes its approach to the surface of your layer of concrete, soluble alkalies appear to the ride and raise its surface pH above those of flooring adhesives. This causes the adhesives to breakdown and you also end up with flooring failures like swelling, bulging, or cupping.
Do you want a vapor barrier within 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, however that doesn’t mean it’s not there. This water can move up with the soil and are avalable into experience of the base of a concrete floor via capillary action. Capillary action may be stopped by installing something known as a capillary break, a layer of crushed rock which goes relating to the subgrade and also the slab.
Capillary breaks do a sufficient 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 concrete slab. Therefore, there should be something under the slab that prevents vapor moisture from entering.
You have to a vapor barrier for liability reasons because most manufacturers of flooring include vapor barriers or retarders of their installation guidelines.
How thick should a plastic vapor barrier be?
In line with the Self-help guide to Concrete Floor and Slab Construction published by the American Concrete Institute, a vapor retarder really should not be below 10 mils thick. You may want a much thicker barrier though if you’re covering material with sharp angles.
Bottom line: Vapor barriers must be sufficiently strong enough enough so they don’t easily puncture. When they do, moisture is certain to get in and that’s what you’re attempting to keep out.
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