Concrete is tough so it doesn’t matter what is beneath a slab of it, right?
Concrete has a property of being porous to water in the vapor phase.
If you don’t think about that, you can inadvertently cause mold to grow under a finished floor.
The structure’s occupants may not become ill, but air quality will be affected and the occupiable space will not be a healthy one.
6 mil plastic (0.006 inch thick) is commonly used as a vapor barrier.
Here is an example of a recent project in my world:
Does the vapor barrier need to be continuously sealed to the vapor barrier on either side of it? No – simply get it as close as possible. With vapor control, 99% continuous is 99% effective.
With bulk water control, if I were 99% effective on my 15,000 sq ft building, I’d have a 150 square foot hole in my roof – no good at all!
But vapor is different.
Watch the video below to learn some of the finer points about vapor, slabs, and tying a slab into an existing slab.
Let me know what questions you may have.
Until next time,
Dr. Lee Newton