I'm planning on stripping the outside walls right to the studs in an old cabin in cold/very cold climate. It will be heated once a week in winter. There is no vapor barrier right now in the walls. Is it OK to install polyethylene on the backside of the studs, then adding fiberglass mats, finishing with Tyvek, furring and wooden exterior. Usually the vapor barrier is installed on the INSIDE of the studs, not on the outside. Any molding risks ?