BTW - you don't want to fill the area in your illustration with insulation. Your roof needs to 'breath' and the vents in your soffits allow air to move up the inside of your roof and remove heat and moisture.
Blocking this area off with insulation will cause mold and rot to the inside of your roof structure.
They make 'baffles' that go into that area and attach to the underside of the roof. The insulation goes 'below' the baffles and across the ceiling joists - not the underside of the roof itself. The baffles allow the movement of the air.
Later . . .
that is exactly what I was wondering about. I remember in an old house I had, we added a room with a cathedral ceiling and I remember installing these blue foam like vents that attached to the bottom of the roof sheeting. So I know what your talking about. And I was wondering if this was needed in a garage. And from what your saying that would be a yes. But in my old house there was a ridge vent in the roof and soffit vents along the bottom. Allowing for good air flow. My garage does not have a ridge vent. It has 2 metal type roof vents which i was going to box out so that they still vented to the actual room space.
I do not want to enclose the space at the joists. I dont want to loose that ceiling space for sign display. And because of the low roof angle, I would not have access to most of the created space. It would only be useful in the very center of the structure.