Roof venting for cathedral type ceiling

Hey all, I need to insulate and fix up a little outbuilding with low joists and make it as simple as possible. It is of simple construction and doesn't have eaves or rakes. If you look at the attachment, picture #1 shows the basic construction. Its walls are structural brick and offers very little insulation. The rafters are open and also have no insulation. So the plan right now is to frame and insulate the walls, then for the ceiling I can add spacers between the sheathing and insulation for airflow. Then I can add a ridge vent. Here's my question: Since it doesn't have soffits, can I create a protected vent in the fascia board and expect the same effect? Or does the air need to come up underneath and into a proper soffit vent to work properly? I might add a proper soffit in the future, but just wondering if this will be effective in the meantime? Thanks for any feedback.