Moisture intrusion topic

My questions is in regards to adding a vapor diffusion retarder especially with the slap on grade.
If using vinyl, or some other heavy mill plastic lying on the ground obviously separating the ground from the underside of the decking, wouldn’t that be a cause of concern where any kind of moisture buildup laying on top of the retarded material could like to still water buildup which could draw mosquitos, grow mold etc…?

Is this not a cause for concern?

Wrong term in this case. Think permeability.
Class I vapor retarders (0.1 perms or less):
Class II vapor retarders (greater than 0.1 perms and less than or equal to 1.0 perms):
Class III vapor retarders (greater than 1.0 perms and less than or equal to 10 perms):
Vapor retarders can help control moisture in:

  • Basements
  • Ceilings
  • Crawlspaces
  • Floors
  • Slab-on-grade foundations
  • Walls

A vapour barrier is a plastic membrane laid under the slab to improve its performance against rising damp. It is also called a ‘moisture barrier’ because it stops the moisture from the soil migrating into the concrete slab.

You lost me here. Are we talking about what is under the slab?

The vapor barrier between the slab and the aggregate has no air gaps for water or moisture to enter. It protects the underside of the slab from moisture moving up.

This link provides an good explanation of what and why.

So to add some more insight, my son in Idaho recently bought a house and when the inspector came to his house, he looked at the crawl space under his house as he has no basement. The inspector noted that by code and properly installed was a thick polymil barrier separating the ground from the crawl space framing.

Moisture is going to collect on top of that, it has to, which leads to my question about stagnant water laying on top of the polymil.

How would moisture get on top?

  1. Moisture intrusion because of a foundation problem
  2. Component leaking such as AC or plumbing
  3. Condensation due to lack of proper ventilation

If you maintain your home properly, none of the above will occur and the vapor barrier will due its job, keeping humidity levels low in the crawlspace cavity.

A proper vapor barrier and adequate ventilation will prevent elevated moisture issues which will lead to wood destroying organisms or wood destroying insects as well mold.

Put down the vapor barrier and take care of your crawl. You will be healthier for it.

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