Hello, I live in Winnipeg MB, Canada (Zone 7, North of North Dakota).

I am insulating my garage ceiling to R22+ (standard pink fibreglass between 2×8 ceiling joists). I am also looking to insulate my garage roof to keep some of the heat from the summer out of the attic space in which I will store some things that i do not want subjected to high attic temperatures (I also expect this will help keep the garage cooler as well since the attic will be only about as hot as the outside air at most.

I intent to layer the system as follows:
1. OSB as the roof decking
2. 3.5 inches of XPS foam board
3. vertical 1x3s fastened through the foam
4. OSB over the 1x3s
5. 30lb felt
6. vertical 1x3s then horizontal 1×3 battens
7. Metal sheets

I know this seems overkill, but I am looking to ensure the top layer of OSB has airflow between it and the foam, and to ensure airflow under the metal sheets.

My intent is to keep this small attic cooler in summer, thus keeping the garage a bit cooler overall. But to be clear, the attic is unconditioned space with normal ventilation from soffits and ridge venting (so the OSB sheathing drys inwards if it ever has to dry at all) and during the winter the temperature in the attic and outside the garage will be the same so my intent has nothing to do with winter-time insulation.

Do you see any particular issues with this? Mostly I am concerned about degradation of the metal roofing but I cannot see any real buildup of moisture anywhere unless I am missing something.