REWahoo
Give me a museum and I'll fill it. (Picasso) Give
I don't disagree with the envelope analogy, but I will say that my attached 3 car garage has insulation within the ceiling area (I also believe insulation is within the exterior walls). My garage essentially has no heating source other than 2 hot water heaters and the furnace and furnace duct work which sit within the garage, but yet the garage only runs about 5 degree cooler than the house itself. Without insulation I have to assume that garage would have a temperature differential much greater than 5 degrees so I believe it does help with my energy conservation. My garage does have its own Reznor gas heater, but it never runs (even during a CO winter) as I have the set point at 55 Degrees.
If it was me, I would probably price blowing in insulation above the garage when I "topped off" the house (even without insulated garage walls), as I have to believe it would help some. If the additonal cost was great since your starting with no insulation, I might change my mind (?).
Even though you don't use it as such, your garage was constructed to be heated. That's why it has wall and attic insulation, to retain the heat in that envelope.
Since the OP does not have a heated or air conditioned garage, there is no need to create an envelope to retain heat or cold. That's why adding attic insulation above the garage would be an unnecessary expense.