Insulating a garage - from the outside or the inside?
I don't think you can do this.  Even 1" of spray foam will stop most of the heat going in or out of the garage by conduction through the walls.  

What it won't stop is heat being transferred by cold or hot air coming in around the door.  In a well insulated house, it is vitally important to control draughts; highly insulated houses have heat-recovery units to bring fresh air into the house without losing the heat.  The problem is that garage doors are not designed to be draught proof - so you will have massive heat loss/gain around the door.