Crawl space vapor retarder

So I am currently taking the attic, insulation, ventilation and interior course. In a lot of the courses I take I keep coming across that all crawlspaces should have a vapor retarder installed on dirt floors. My question is I currently have a double vented crawlspace with a gravel floor that is sloped to a drain pipe that leads outside. Should that require a vapor retarder? It seems kind of counter intuitive to put a vapor retarder over a system like that. Should a vapor retarder be installed in a vented crawl space. Thanks for all your suggestions and recommendations.

Yes. Even though it is vented, higher humidity could still exist and cause issues. I had a vented crawl on my last house with 3/4" gravel over the soil and no vapor barrier. Eventually the gravel looked like green carpet in spots.

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Yes, the moisture could be coming from below also, in which case the barrier would stop the moisture from reaching the wood above, too.

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Welcome to our forum, Brandon!..Enjoy! :grinning_face_with_smiling_eyes:

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Yes, in addition to what was already said. Here are some of my thoughts.

There is a difference between 3/4” aggregate rock and a “gravel” material. To me gravel is a road base material that has a certain amount of fines incorporated into the aggregate to create a road surface or a base to a road surface. Do you have pictures of the material in your crawl?

When constructing a building whether the crawlspace will have a concrete floor or be encapsulated. 4” of aggregate rock is recommended for a capillary break. 3/4” is good, some may recommend a specific gradation for the aggregate to meet. All depends on what sources are available locally. We have loads of angular rock from the mountains.

The main purpose of a vapor retarder is obvious because of its name. But it should be a certain mil thickness. The American Concrete Institute’s Guide for Concrete Floor and Slab Construction recommends that the thickness of the vapor retarder be at least 10 mils .

Added benefits of a properly encapsulated crawl space include but may not be limited to:

  1. Clean area for storage.
  2. Helps with radon mitigation systems.
  3. Helps provide a barrier for insect and/or rodents.
  4. Helps with heating/cooling.
  5. Better overall air quality in your house.

Hope this helps.

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@afrydenlund That’s good stuff!

I would enjoy a house on a crawl with modern encapsulation to include proper conditioning. I do not care for a concrete slab. My son and I replaced all my cast iron pipe this weekend in my crawl. I won’t say it was easy but I will never have to cut concrete to solve a plumbing issue. Plus, a wood floor structure is just easier on my old bones to walk on.

Anyway, I got that out of the way as part of my multi step plan to DIY my encapsulation. I think I am going 12 mil with a few extra layers along the path to my furnace.

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