Insulation, Radiant and Vapor Barriers - Vapour/air barrier question

The difference between the two, and their respective purposes are clear to me, but something has got me wondering. Is vapour, in its true state, (not water), normaly carried/transmitted through air movement? If a material is an effective air barrier, would it not do well to slow or stop vapour transmission? I ask this with open cell spray foams as my specific material in mind.

I am by no means a physicist, so I take no offence to someone steering me in the right direction if I'm way off base on this.

Thanks,
Glenn
Winnipeg

As with most of us in the building trades, we all have our own opinions. Below is a link I like, especially when it starts out with the premise "Vapor barriers are also a cold climate artifact that have diffused into other climates more from ignorance than need. The history of cold climate vapor barriers itself is a story based more on personalities than physics." It is interesting reading.

BSD-106: Understanding Vapor Barriers — (http://www.buildingscience.com/documents/digests/bsd-106-understanding-vapor-barriers)

hny
Bud

Open cell spray foam is not a vapor barrier or air. Closed cell is both.

Open cell spray foam is not a vapor barrier or air. Closed cell is both.

When speaking with the various companies I have, I've recieved conflicting info aabout that, while none have claimed open cel is a vapour barrier, some have indicated that it IS an air barrier... and others have indicated that applied to a certain thickness, "can act as a vapour retarder" which is what has caused my confussion.