Venting an unvented low pitch roof… why?

My 67 y/o mid-century modern, Seattle  home (Zone 4) needs a new roof, and I’m proposing adding insulation under a new standing seam metal roof… but I have questions. The attached… My 67 y/o mid-century modern, Seattle  home (Zone 4) needs a new roof, and I’m proposing adding insulation under a new standing seam metal roof… but I have questions. The attached…Read MoreGreenBuildingAdvisor