Moisture in Crawl Space Is Preventable With Sheet Plastic

Crawl Space
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Moisture is part of the life when it comes to houses; it cannot be entirely avoided.  But one way to retard crawlspace moisture is with a very simple and rock-bottom cheap project:  laying ordinary sheet plastic in your crawl space.

Water Is Everywhere

Water determines if you can finish your basement.  It determines what kind of below-grade flooring you install. It affects kitchen and bathroom walls, floors, and especially ceilings.

 Insulation is installed vapor-barrier-inward in response to the huge build-up of internal moisture.

But in no other place is this more apparent than in the crawl space.  Because the crawl space has contact with the ground--it literally is dirt--it should be no surprise that moisture will develop here.

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Crawl Space Moisture Problems

It is difficult to overstate the problems associated with crawl space moisture.  Reasons why excessive moisture is a bad thing:

  1. Mold, Mildew:  The main problem is mold, fungi, and mildew. Remediating mold-related problems is expensive. While in some cases mold problems are blown all out of proportion, as a direct byproduct of crawl space moisture it will begin blackening floor cavity insulation and structural elements.  Because there is no light and only minimal ventilation, the problem never improves on its own.  
  1. Rot:  Structural elements of your house--joists, sills, posts, beams--are made of wood.  Being an organic material, wood will begin to rot when it comes into contact with water.  
  2. Creatures:  Animals of all types are drawn to water and can infest your home.  This includes rats, carpenter ants, and termites.
  1. Resale:  Even if moisture did not create those problems above, its presence will turn away home buyers.  When you try to sell your house, the property inspector will explore the crawl space and note the presence of water on the report.  It does not matter if you have contentedly lived in your home like this for 20 years--the buyer will want the problem fixed or demand a credit.

What a Vapor Barrier Will Do For You (and Not Do)

To fix this problem, you will lay out a vapor barrier of sheet plastic in your crawl space.  More recently, the U.S. Department of Energy has revised this popular term and now calls it a "vapor diffusion barrier."  This is more accurate since you cannot seal off 100% of the moisture migration.  Rather, the plastic slows down the process.

Sheet plastic will cure normal soil moisture.  "Normal" means water that comes in the form of invisible vapor, and does not include pooling of water from below or from above.  

  • Water From Below:  When large amounts of water pool up in the crawl space, you may have a ground water problem or a problem with rainwater intrusion.  In this case, you should hire a crawlspace company.  They will dig a trench around the perimeter, add drain pipe, cover with drain gravel, and add a sump pump.
  • Water From Above:  Water leaking from above must be fixed before you put down vapor barrier.  Water may be coming from pipes leading to the bathroom and kitchen.

Vapor Barrier: Ground Plastic to Retard Ground Moisture

  • Find Your Crawl Space Entrance:  Crawl spaces often have entry hatches located in closet floors.  If your floor has wall-to-wall carpeting, look for a square seam in the carpeting that indicates a hatch door.  Or you may find the entrance outside, on the side of the crawl space wall.
  • Dry It Out:  Begin with a crawl space that is as dry as possible.  
  • Clean:  Crawl spaces are notoriously messy, as construction workers often toss discarded building materials in this space.  Remove all debris, especially anything sharp, as it will puncture the plastic.
  • Roll It Out:  Lay down 6-mil polyethylene plastic over the entire crawl space. Overlap the seams by no less than 1 to 2 feet.  
  • Lap Against Columns:  You will encounter internal supports such as columns and piers.  Cut the plastic as needed and lay it about 6 inches up each column or pier.
  • Lay Against Walls:  Bring the plastic about 1 foot up the crawl space walls and tape it.
  • Tape:  Tape all seams down with mesh tape or with mastic.