Here's the dilemma. If you insulate the garage, but do not insulate the wall separating the house from the garage, then you lose a lot of heat to the garage and your power bill will be higher. If you insulate the wall between them as well as the garage, then not much heat escapes from the envelope of the house and your garage won't be much warmer than the outside temperature, even though you spent money to insulate it. So, the reasoning may be: I don't want to pay to heat my garage, so why should I pay to insulate it?. If you are in an area where you would like to have some small amount of heat in the garage, you could insulate the garage and insulate the wall it shares with the house, and then have a small baseboard heater with a thermostatic control so you could keep the garage temp a little bit warmer than outside, but not do it at a huge expense. If you do decide to go that route, it would be critical that the insulated doors seal very well on all edges or you will have heat leaks to the outside that will waste a lot of energy.
You can't keep your garage toasty by simply insulating it, so you need to figure out what your goal is and whether you are willing to pay for it, both during construction and into the future. Also remember that, if you are heating the garage, every time you open it, you will have quite a bit of heat rush out through that large door opening. It's not like you're just opening your front door for a few seconds. And that heat will then need to be replenished by the system providing heat to the garage.

I have an insulated 3-car garage and my workshop occupies part of it. Our winters are fairly mild so my tools are ok without supplemental heat. I have a 1500 watt oil-filled heater that I turn on only when I want to do a project.

Does that help?

This post was edited by kudzu9 on Tue, Oct 21, 14 at 7:02