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Vapor Barrier in Sub-Structural Crawl Spaces


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Washington State Energy Code [502.1.6]; WSEC Builder's Field Guide, Page 2-3:

Ground Cover. Six-mil black polyethylene or approved equal must be laid over the ground within all crawlspaces.

The ground cover:

  • Must extend to the foundation wall
  • Seams must be lapped 12 inches. (See Figures 2.1 and 4.6)
  • May be omitted if a minimum of 3-1/2 inch concrete slab is poured in the crawlspace
That should do it for you. Go here to download the code text and a copy of the Builder's Field Guide.



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Thank you much fellas for the references. They are helpful, (well, kind of).

I'm finding a lot of poorly installed vapor barriers - especially from drainage contractors who are installing sump pumps, perf. drains etc. They "warranty" their work for the life of the building and assure the homeowner that their work will pass all inspections.

The standards I've called for years are: 1)black plastic should lay flat on the soil, provide 100% coverage of all the soil and shouldn't ride up on to the foundation stem wall or wrap around the bases of wooden posts 2) the seams should overlap at least 6", 12" is better 3) the plastic should be secured at the corners with bricks or rocks. I believe these standards were derived from our state pest association - I really can't remember now!

What I mostly find in new homes and newly installed drain systems are vapor barriers are being bunched up in the corners, riding up on to the foundation stem walls, and wrapped around the wooden post bases - obviosly just sloppy cuz noone wants to work down there or think that anyone will see it or care.

Just trying to cite a specific reference. The references given aren't as specific about installation as, in my opinion, they should be.

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We have a lot of lake properties that have been converted from seasonal to full time homes. Most of these are built on a crawlspace with a dirt floor. In the last two years I have found only one home that had a vapor barrier that was properly installed. Most had no vapor barrior at all and some had the painters light weight plastic all curled up and thrown around. I find that you really take your life in your hands when entering these enviroments.

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