Im in southeast pennsylvania which i have mapped to be a zone 4 and i'm looking to insulating an existing flat roof. Roxul R30 batts will give me good coverage for the existing space which doesn't have any insulation at all. The space between the finished ceiling and the actual roof is almost 2 feet so the R30 batts will leave plenty of ventilation room left.

I plan on cutting more access holes in the closet spaces and then laying the batts within the space. My question is about vapor barrier. Some say that because it is an asphalt roof, it already counts as a vapor barrier but if there are roof vents, the moisture will equal the outside so i must still need a vapor barrier. I was planning on getting 6 mil plastic sheeting and laying that down to act as the vapor barrier and laying the batts on top but it doesnt smeem like much of a vapor barrier to me if the edges arent sealed.

Is this n acceptable vapor barrier solution? If so, I'm not sure how I would seal the edges.