I use half of my two car garage as a garage and the other half as a gym. I am lucky enough that when the house was built they routed AC ducts to the garage but since the thermostat is inside (and I am thankful for that) it can still get pretty warm in the summertime (Texas).
To help it stay cooler I was thinking about adding an insulation kit to my garage doors, but I was unsure if it would actually be effective. The walls all have panels up so I can't tell for sure if there is insulation in them but from the looks of it, it was being used as a room in the past as carpet had been laid down at some point so I assume it would be insulated.
One other note, the garage door doesn't see a direct sun hit. The doors are closer to lining up north to south than east to west.
Does putting insulation on a garage door make a noticeable difference in maintaining cooler temperatures in the summer or are they still too leaky?