Hot unvented cathedral roof in north Georgia, what kind of underlayment under standing seam?

I’m going to have a unvented hot roof (no attic) with closed cell on underside of deck. Do I use permeable or impermeable underlayment under the standing seam metal roof?… I’m going to have a unvented hot roof (no attic) with closed cell on underside of deck. Do I use permeable or impermeable underlayment under the standing seam metal roof?…Read MoreGreenBuildingAdvisor