Rafter insulation in small shed

[MerlinsBeard]

I have a small 10x16' shed with 7' walls and a cathedral style ceiling with dormer and 2x4" rafters. Thinking of insulating to install a mini-split down the road. Trying to determine the cost/benefit of different insulation. I do have a ridge vent and smartvent installed on eave under the shingles for airflow that I do want to maintain, but 2x4' rafters is not much space.

Not adverse to spending money if the quality is worth the upgrade, but don't want to spend unnecessarily. Not intending for anyone to live in the shed, but do want to try to make it a nice environment for spending time in the shed. The main question is whether I need to extend the rafter depth as that's holding up when I can pursue getting electric run in the ceiling. Any suggestions are appreciated.

 

[Syberia]

For a shed, I've used foil-faced foam board between the rafters sealed with foil tape at any gaps and spray foam around the edges. Friction holds it up until the spray foam cures and secures it into place.