Insulating under a new metal roof pole barn

Hello... I am in the process of heating and finishing the inside of an enclose pole barn building (by a well known local company). The building is 24x36'. Modern construction with 2x6 framed walls, osb and hardi siding. The roof is standing metal seam over 2x4 purlins (horizontal) on 2x6 trusses. The building is a two pitch type design, rear roof higher than the front, with the lower roof 4x pitch and the upper 3x12. There are bird blocks with vents on the gable ends. Under the metal roofing is a 1" fiberglass mat (sandwiched in vinyl) directly between the purlins and metal. The roof has no leaks and no condensation issues, and will be used as an uninhabited work shop with fully vented propane heat. My plan is to use fiberglass or rock wool type insulation with a paper vapor barrier between the horizontal purlins. That means no real venting except on the short end, 18' from the center of the span. I would basically be creating an unvented roof. Any thoughts here? Thanks

Eric, That's good insight, thanks for the reply. The space will be heated, but it's a shop, so no cooking etc. My plan would be to leave the vents at the ends of the purlins to allow some minimal venting. Would you recommend a plastic sheet between the bats and the sheetrock? What other choice for an air barrier besides with the primed/painted sheet-rock?

Whatever insulation you go with, I would definitely want to see a continuous and sealed air barrier here that has some moisture control properties to it. You want the roof to be able to dry out in one direction or the other, but not having condensation now is not reflective of what the roof may perform like once you introduce a whole bunch of moisture (via heating) into the space. That will be the true test of the system. Fibrous batts are fine, but they do Zero to stop air movement. That is why I recommend some sort of air barrier that will also stop moisture transfer here.