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How to Install a Vapor Barrier in a Crawlspace

This article will show you how to add 6 mil plastic sheeting on the ground and insulate the walls in your crawlspace. We use foil-faced rigid insulation to keep the space under the house dry. The plastic and the insulation will eliminate any moisture problems you have in the crawlspace, such as water droplets collecting on the concrete walls and pipes.

By the DIY experts of The Family Handyman Magazine:May 2005

How to Install a Vapor Barrier in a Crawlspace

This article will show you how to add 6 mil plastic sheeting on the ground and insulate the walls in your crawlspace. We use foil-faced rigid insulation to keep the space under the house dry. The plastic and the insulation will eliminate any moisture problems you have in the crawlspace, such as water droplets collecting on the concrete walls and pipes.

By the DIY experts of The Family Handyman Magazine: May 2005

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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Required Tools for this Project

Have the necessary tools for this DIY project lined up before you start—you’ll save time and frustration.

    • Hammer
    • Tape measure
    • Caulk gun
    • Straightedge
    • Utility knife

Required Materials for this Project

Avoid last-minute shopping trips by having all your materials ready ahead of time. Here's a list.

    • Rigid insulation
    • Treated 1x4s
    • Caulk
    • 1-1/2 in. rigid insulation
    • 6-mil plastic
    • Tape
    • Landscape fabric stakes

Comments from DIY Community Members

Share what's on your mind and see what other DIYers are thinking about.

1 - 6 of 6 comments
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February 02, 9:48 PM [GMT -5]

We had moisture problems in the summer. I took digital pictures & one showed water streaming off a vent. We have now sealed up all the vents (as recommended), replaced our older unit, replaced all the ductwork (they now have an open vent or two in the crawlspace), wore protective masks & etc and scrubbed off mold with tea tree oil & reapplied the tea tree oil as
a preventive for further mold & have had a vapor barrier installed on the foundation floor which was overlapped & sealed with Mastic. After research, we decided to seal the brick & masonary foundation walls with Radon Seal (masonary) and LastiSeal (brick) & hope we won't need further insulation on the walls. We figured if it prevented water from coming in basement walls it should prevent moisture from coming from the masonary & bricks. Hope this helps.

July 03, 11:03 AM [GMT -5]

not yet done. very helpful

March 27, 8:19 PM [GMT -5]

Working on insulation at the moment, then come back with vapor barrier. My crawlspace has a sump pump, and moisture runs by block foundation to sump pump, so not sure system will work for me. Thinking of using 1 x 4s attached below vents, and keeping the space vented.

Any suggestions on screws to use to hold the vapor barrier and 1 by 4s?

February 14, 12:53 PM [GMT -5]

For the most thorough website to answer all your questions pertaining to crawlspaces visit http://crawlspaceinfo.com This is the most helpful site that i have found.

August 03, 4:18 PM [GMT -5]

I understand that the ISO boards are great in R per inch, (what is shown against the concrete) but that they can wick up water. For that reason would extruded polystyrene be a better choice?

June 18, 9:47 PM [GMT -5]

My contractor says that the ground level under the crawl space should be ABOVE the outside ground level. Is this correct? The photo clearly shows the the crawl space ground level is BELOW the outside ground level.

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How to Install a Vapor Barrier in a Crawlspace

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