While it may be possible to get by without installing a poly vapor retarder, it is important to be sure the type and thickness of rigid insulation being used meets the perm rating for a vapor retarder. Check the perm rating with the specific manufacturer that you are using. Also, my experience has been that some building inspectors still just won't accept it. They see the installation as problematic(difficulty taping/sealing of seams) and don't feel it will properly do the job of vapor retarder. Make sure they accept it before getting too far.
