How should I insulate my garage roof if I want to use the garage as a workshop?
Well, it's a good idea but why not care about re-sale &amp; do it right by drywalling everything &amp; install some outlets &amp; add more lighting? The R-11 is okay, but I'd even go with at least foam boards on top of the rafter ties. Hot air does go up &amp; in the summer without vents it will radiate down.

Absolutely put in gable vents or at the very least drill 200 small holes in a rectangular pattern in each gable...angled down if drilling from the inside or angled up if drilling from the outside, to keep water out.

Finally, you'll want to insulate the garage door with foam board &amp; make sure the entire door's perimeter is sealed. What I'm saying is that 1 major weak point will ruin the idea of ever being comfortable.