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Crawl Space Vapor Barriers- 4 mil, 6 mil, Class 1....Help!

Posted by The Plastic Sheeting Gurus! on Tue, Jun 21, 2022 @ 10:54

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Crawl Space Vapor Barriers...Your Questions Answered

  • What are crawl space vapor barriers? A crawl space vapor barrier is a specific type of plastic sheeting/ membrane that works to prevent the infiltration of water vapor from passing through to the surface above.
  • Crawl Space Vapor Barriers key considerations:  Permeability of water vapor, tensile strength of the film, and puncture resistance. All three matter to do the right job.
  • Is thickness the only thing to consider when selecting a crawl space vapor barrier? Absolutely NOT! To truly prevent water vapor from passing through the plastic sheeting/ vapor retarder, it must be high-quality plastic  sheeting that is not made from recylcled plastic. This plastic is called "virgin plastic" or "engineered plastic". Next you want to get a "Class I Vapor Retarder". This means that the plastic sheeting/ liner has been tested in a laboratory for ensure that a maximum of 0.1 perms of moisture to pass through. 
  • Does "Tensile Strength" matter? YES! Will the liner resist gettingpunctured during installation? Tensile strength is measured in pounds (force) per inch, and is tested by applying stress to the material that simulates some of the loads a vapor retarder will face during installation. If the liner gets punctured it's compromised.
  • Does "Puncture Resistance" matter? Puncture resistance addresses how well the vapor retarder material withstands impact with blunt objects. Vapor retarders are subjected to impact by a rounded falling dart, which is intended to roughly simulate some of the hazards of construction.
  • Class A, B or C..what is that?  These classifications have to do with the permeability, tensile strength and puncture resistance. 
    • Class A - Maximum of 0.1 perms, minimum 45 lb/in tensile strength, 2200 grams puncture resistance.
    • Class B - Maximum of 0.1 perms, minimum 30 lb/in tensile strength, 1700 grams puncture resistance.
    • Class C - Maximum of 0.1 perms, minimum 13.6 lb/in tensile strength, 475 grams puncture resistance. 
  • How to Choose a crawl spce vapor barrier thickness.  We recommend that you consider more than just the thickness of a crawl space liner when making your decision. Remember, the key ingredients for a properly functioning crawl space liner is- Permeability,  Tensile Strength and Puncture Resistance.
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