Exterior walls
Walls that face the sun take in the most heat.
Ceiling / attic
The attic above can get extremely hot in summer.
House wall
Keeps garage heat out of the house.
Radiant barriers
Ask whether a radiant barrier makes sense for your roof.
FAQ
Is my garage already insulated?
Many garages have little or no wall insulation, especially on exterior walls and above the ceiling. A pro can check.
What about the wall shared with the house?
Insulating the shared wall and ceiling can also help keep garage heat out of the rooms next to it.
Is it worth it if I'm only venting the garage?
It can still help by reducing heat gain. Ask the pro how it fits with your plans.