Vapor barrier over insulation

We are getting to the stage of our passive solar straw bale house build where we will be soon adding insulation.  We did go the route of venting our cathedral ceilings (thanks to the help of many from Permies  )  Should mention it is a metal roof and we will be using lime plaster. We have some rockwool we will be using, but our main source will be sheeps wool insulation. We live in North Dakota zone 3/4, so vapor barrier I guess is a must with our weather. but I CANNOT for the life of me come to terms with the idea of using PLASTIC all over our ceiling!?!? its not very green and all i can picture is it CAUSING mold and moisture problems on the insulation side? surely there has got to be a better solution?  

please tell me I am overthinking this part or that maybe there is a straight forward answer? i have tried to research on this as much as I can but it really isn't easy finding answers when EVERYONE has their own two cents that seem to contradict each other.  At least with this group I know there are many like minded people that are willing to collaborate with each other on their ideas to come up with a solution..

Thanks to all who take the time to read this!

I think you are going to have to consider some type of "thermal break" . A common problem with metal structure were the temperature differential causes condensate to gather on the inside of the structure. Skoolie conversions of buses to dwellings a prime example. Many products out there most expensive and not so green but many of the skoolie folks have addressed that issue and in green ways.