BSI-073: Macbeth Does Vapor Barriers (Double, Double Toil and Trouble)
Source: buildingscience.com/Joseph Lstiburek

Life was simple when I grew up in Canada. Winters were long and cold, we had no air conditioning, walls dried to both the outside and the inside and the Toronto Maple Leafs would win Stanley Cups. That all changed when we started putting plastic on the inside of walls and began to put impermeable insulating sheathing on the outside. We started to have double vapor barriers. Shakespeare was right in "that Scottish play2." Double vapor barriers meant "double toil and trouble." With double vapor barriers if a wall got wet it would not be able to dry to either the outside or the inside. 
In the old days assemblies worked just fine with no vapor barriers on either side. That approach still works today, but for reasons that are difficult for me to understand it is not used as often as it should be or could be. We looked at this in "BSI-071: Joni Mitchell, Water and Walls" and we will look at it again here
Hear more from Joe Lstiburek, Ph.D., P.E., ASHRAE Fellow, at the SprayFoam Convention & Expo, Jan. 28, 8-10 a.m.