Question.: There are so many different vapor barriers (vapor retarders) to chose from. How do I know which one to buy and use? What's the best material for a crawlspace?
Answer.: TRUTH 1: No two water vapor retarders are created equal. The vapor retarder you select should be manufactured from high-grade raw materials that are consistent in quality, unlike common construction grades of 4, 6 or 10 mil polyethylene. Construction grade films (known as C&A or visqueen) are typically produced with very low grades of polyethylene resin and a very high percentage of "post consumer" recycled materials. The numerous heat cycles that occur in reprocessing of recycled materials, combined with the varied types of resin used, cause inconsistencies in physical strength and permeability. While this commodity film serves a purpose in temporary construction and agricultural applications, it is not designed to provide ongoing protection against unwanted moisture. Film characteristics such as low strength and poor resistance to decay should be of major concern.
TRUTH 2: A long-term vapor retarder must maintain life long integrity by resisting decay, attack by moisture, organisms in the soil and chemicals. As mentioned above, beware of vapor retarders manufactured with "post consumer" recycled resins (C&A Poly or Visqueen) which can degrade from chemicals in the soil. Paper laminates can degrade from moisture attack.
TRUTH 3: Moisture problems associated with a vapor retarder installed under the concrete slab-on-grade are often traced to punctured or torn water vapor retarders. Damage due to construction traffic during installation, can be detrimental to the performance of the vapor retarder. Physical characteristics such as high puncture resistance and tensile strength, along with low-moisture vapor permeability, are vital attributes of a vapor retarder. These physical characteristics are well quantified in ASTM E-1745 and are outlined in this guide.
TRUTH 4: When specifying a water vapor retarder, designed for use in under concrete slab applications, insist the supplier meets the newest and most stringent ASTM standard, ASTM E-1745 for "Water Vapor Retarders Used in Contact with Soil or Granular Fill Under Concrete Slabs". It assures minimum values are met regarding tensile strength and puncture resistance, along with the maximum allowable water vapor permeance. ASTM E-1745 separates these properties into three performance classes: Class A, B & C.