What is the difference between a vapor retarder and an air barrier?
A vapor retarder's mission is to control the entry of water vapor, or various gases into the buildings they are protecting. Air barriers role is to control the movement of air through the building assembly. There are some instances where an air barrier system may have a rubber membrane on the exterior of a masonry wall installed which will make for an effective air barrier and have it the characteristics of a vapor retarder.
Permeability to water vapor is the key physical property which distinguishes vapor retarders from other materials. Polyethylene liners or any material which retard water vapor to pass through them are considered impermeable. The beauty of polyethylene is that it is a non-hygroscopic material. It does not change its permeability no matter what the relative humidity is doing.