Possible Condensation Issues

I am currently pricing out options for a new roof on a community pool. We have decided that a galvalume unpainted standing seam roof fits our budget and would look much better than a shingled roof. The pool building is a converted barn with a current sheet metal roof. The second floor of the barn is unoccupied with a ceiling height of 18' by 65' long. I have heard from the builders that the condensation problem should not be an issue due to the huge airspace and the fact that no HVAC is used in the space (not heated or cooled). The roof is being placed directly on the rafters with no plywood base as one does not currently exist today. He suggested a layer of Feltex slip sheet should reduce and water issues, if any. Should we be concerned that we are not applying a plywood base? If we do finish the second floor in the future, we plan to spray Icynene to the underside of the metal to insulate.

I believe that condensation will be an issue if you do not have a roof deck.