What is the difference between a vapor barrier and a vapor retarder?
According to industry experts, a vapor barrier is defined as having a permeance of 0.01 Perms (grains/[ft2*hr*inHg]) or less when tested in accordance with ASTM E 1745 Section 7.**
A vapor retarder is classified by ASTM E 1745 Table 1 as a plastic material having a permeance of less than 0.1 perms (grains/[hour * ft2 * in. Hg]) as tested according to ASTM E 1745 Section 7**.
**Section 7 of ASTM E 1745 specifies the testing conditions and allows for either of the following test methods: ASTM F1249 or ASTM E96.
Though it is true that many people in the construction industry will use these terms synonymously, there is a very big difference between the two. A vapor barrier needs to have a low enough permeance to mimic the permeance of a floor covering. Otherwise the potential exists for the buildup of relative humidity/dew point within the slab/under the floor covering.