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What is a crawl space vapor barrier? Do I need one?

crawl space vapor barrier

If your home or business is on a crawl space with a dirt floor, a properly installed vapor barrier can be the solution to a number of common problems. You see, dirt or sand floors emit moisture or water vapors year-round. This is especially the case if you have a high water table. These vapors rise with normal air currents in you crawl space and eventually wind up clinging to your wood floor joists, sub-flooring and insulation.

A crawl space vapor barrier is a layer of plastic installed over the dirt floor of a crawl space. A vapor barrier controls crawl space moisture levels by slowing or preventing the evaporation of ground moisture into the crawl space air.


Signs of Excessive Crawl Space Moisture:

  • Standing water or mud after a heavy rain
  • Efflorescence on crawl space walls
    • Efflorescence is a crystaline deposit on surfaces of masonry, stucco or concrete. It is the result of salt deposits from water exposure.
  • Unpleasant musty odors
  • Pealing paint on exterior of home
  • Insects and other pest infestations
    • Insects are attracted to the moist environment coupled with all the available food.
      • Feeds on mold growth, wood rot, and various other dead material that can be found within the crawl space.

Problems Caused by Moisture Build up:

  • Mold development:
    • Elevated moisture levels in the dank crawlspace make a great environment for mold growth.
    • If mold growth in the crawl space is neglected and the water problem is unaddressed, the mold growth can easily spread up into the upper levels of the property.
    • It is important to control humidity levels to prevent mold growth.
  • Water Damage:
    • Water vapors may seem harmless at first, but given time, excessive water vapors can develop into major water issues.
    • Major water issues lead to mold development and structural damage to the property, which renders the home unsafe.
  • Wood Decay:
    • With time, water vapors absorbed into the building materials in your crawl space will begin to destroy the structural integrity of the building materials.
    • Wood decay ruins the homes support system and it lures insects to feed on the rotting material, leaving you with an unsafe and insect infested home.
  • Low hanging insulation that can no longer properly insulate your crawl space.
    • Moisture compromises the structural properties of insulation.
    • Water vapors can force insulation to drop, leaving it useless for its intended purpose.

crawlspace vapor barrier installation

How should my NJ Crawl Space Vapor Barrier be Installed?

  • A properly installed vapor barrier should be thick enough (10mm at a minimum) to prevent damage from people (and animals) crawling on it.
  • Plumbers, electricians and even the cable company carries tools that are capable of ripping your valuable vapor barrier, so be sure to tell them to be careful when crawling around down there.
  • “Properly installed” means that the floor should be excavated if necessary to ensure it is as flat as possible.
  • Once the vapor barrier has been measured and cut, all the seams should be attached using a spray on adhesive that resists moisture. This should be reinforced by taping the seams with a sturdy, moisture resistant tape.
  • If your NJ crawl space is prone to flooding, then your vapor barrier should be perforated in several areas to allow water to drain into the soil. The small drain holes may seem counter-productive, but they are necessary and will not allow enough moisture to cause a problem.
  • If your foundation walls are seeping or sweating, then you may want to cover them too. In that case, your vapor barrier would extend up the walls and piers high enough to cover any block or masonry.
  • Ultimately, you’ll want to have the walls inspected by a qualified contractor to make sure they are in good shape – and to make sure you don’t need a more aggressive solution like a perimeter drain.

Depending on the natural environment of the are your home is in, you might need more than just a vapor barrier to moisture in your crawl space. In areas that experience higher humidity levels, like New Jersey, you should consider installing a dehumidifier in addition to the vapor barrier to ensure controlled moisture levels.

In addition, your should make it a habit to regularly check your crawl space for any maintenance opportunities and signs of problems. If your pipes and sewer lines travel through your crawl space, check them regularly to ensure they are in good condition and not at risk for leakage. Noticing minor problems early on is vital to preventing more severe and costlier problems further down the road.

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