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.


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


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.


@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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