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Vapor retarders are sheet materials used below slab-on-ground floors to limit the amount of moisture that migrates into and upward through concrete slabs. If unchecked, moisture can cause floor adhesive, covering and coating failures, growth of mold and mildew, offensive odors, and unwanted increases to interior humidity levels. For health reasons, most local building codes require the installation of a vapor retarder below on-ground concrete slabs in inhabited spaces. To protect human health and to minimize moisture-related flooring failures, all concrete floors in contact with the ground must have a vapor retarder below the slab, regardless of the depth of the water table.

Retarder or barrier?
Vapor retarders are often called vapor barriers. However, vapor retarders and barriers are not the same. Vapor retarders, as the name implies, only retard the migration of moisture; whereas, vapor barriers essentially stop the ingress of moisture. According to ASTM E 1745-09, vapor retarders must have a permeance rating (water vapor transmission rate) less than 0.1 perms. By definition, 1 perm equals 1 grain per ft2 per hour inch of mercury pressure differential. A permeance rating of 0.1 perms is equivalent to an approximately moisture vapor emission rate (MVER) of 0.142 pounds per 1000 ft2 per 24 hours for a slab exposed to a 70° F and 50 percent relative humidity (RH) environment at the top and 50° F and 100 percent RH at the bottom. A MVER of 0.142 pounds per 1000 ft2 per 24 hours is considerably less than the maximum MVER of 3.0 pounds per 1000 ft2 per 24 hours typically specified by manufacturers of moisture-sensitive flooring.

True vapor barriers should have a permeance rating of 0.00 perms; however, it is generally accepted in the industry that materials with a permeance rating of 0.01 perms or less are vapor barriers. Converting 0.01 perms to an equivalent MVER, as above, yields a value of 0.014 pounds per 1000 ft2 per 24 hours. Although both MVER values are small, there is a difference between vapor retarders and barriers. Traditionally, vapor retarders are more frequently used below ordinary on-ground floor slabs than vapor barriers.

Selecting a vapor retarder
ASTM E 1745-09 defines three classes of vapor retarders. Classes have the same permeance but different strength properties. The current edition, approved February 15, 2009, shows 0.1 perms as the maximum water vapor permeance for all three classes of vapor retarders. Previous editions (1997 and 2004) had a 0.3 perm limit. As shown in Table 1, a Class A vapor retarder will be more resistant to tearing and puncturing than a Class B or C.

Thicknesses of vapor retarders satisfying the requirements of ASTM E 1745-09 typically vary between 10 mils (0.010 inch) and 15 mils. Increased thicknesses offer increased resistance to moisture transmission and provide additional durability during and after construction. When heavy equipment such as ready mix trucks, laser screeds or concrete buggies will be operating on the vapor retarder, use a 15 mil thickness.