How to Install a Vapor Barrier in a Crawlspace

Use plastic sheeting to control moisture in your crawlspace.

Damp crawlspace solution

If you have water collecting on the pipes and concrete walls in your crawlspace, then you're experiencing a common problem. The ventilation openings were designed to flush out moisture from crawlspaces, but they often don't do the job, except in dry climates. In fact, during humid summer conditions, they often make matters worse. The humid air carries moisture that condenses on the cool walls, the pipes and even the underside of floors.

The best solution is to completely close up the vents (or omit them in new construction) and control crawlspace moisture in other ways. The photo illustrates several key concepts. However, local and regional conditions vary and the exact details of this system may not work in every crawlspace. Make sure to get approval from your local building inspector before taking any steps.

  1. Create good drainage around your home to keep rainwater from flowing in under your house. Sloping the soil away from your home at about 1/2 in. per foot is usually the best thing you can do to reduce crawlspace moisture.
  2. Cover the dirt crawlspace with a plastic moisture barrier. Overlap any seams and tape them. And bring the plastic about 6 in. up the wall and fasten it there. Stake the plastic down with landscape fabric stakes to keep it in place. Chances are that someone will have to slide around in the crawlspace later to fix a pipe or run a new cable. A few holes won't make any difference in performance, and they'll drain puddles if a heavy rain or leaky pipe leaves water on the plastic.
  3. Cover exposed foundation walls with 1-1/2 in. of rigid, moisture-proof insulation. This not only insulates the walls but also keeps rising dampness in the concrete from getting into the crawlspace. We used Thermax brand (available through lumberyards) because it doesn't require an additional fire barrier. Other types may require a drywall covering.
  4. Insulate the rim joists. We used rigid insulation and caulked it in place to stop air leaks.
  5. Cap the interior of the foundation wall with a 6-mil layer of plastic and galvanized flashing both to block moisture and to serve as a termite barrier. There is no sure solution for termites; building codes may require different details for termite control in your region.

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