I do believe that extra insulation in any part of a garage that is attached to a heated or cooled habitable space will affect the rate of heat gain or loss of the habitable space. How one measures the worth of that has many variables, not just the actual dollars that may be saved on heating or cooling the habitable space.
We built our home in 1985 and it has the typical attached 2 car garage. Not so typical for its time are the 2x6 wall framing (instead of 2x4) and blown in place cellulose insulation. The garage walls were insulated and the ceiling was drywalled like the house and R36 insulation in both the garage and house ceilings/attic space. The steel overhead door was only a steel panel when installed. I fitted the panels with foam insulation panels cut to fit. That helped slow the cooling off and the heating up some. But, in winter for example one could feel the cold radiating off the steel panels parts that were folded over to the inside.
The garage is my workshop and a stand-alone 2 car garage was built later to park vehicles in.
So we are now into cold weather so let’s look at what I have noticed so far. The garage was never as cold as the exterior temperatures in winter. If I did not heat the garage it was still warmer than the outside temperatures, though it could get quite chilly. Heat has been via wood stove with a kerosene heater used when it was not so cold outside; spring, fall.
With the new R18.8 garage door I have yet to see the temperature in the garage drop below 55 F. That is with night temperatures frequently in the 25 F to 28 F range. I can get that temperature up into the high 60’s almost too easily. I have to relearn how big a fire to make in the wood stove. I have grossly overheated several times. I am learning. I think I might be able to heat the garage/workshop with a small heat pump instead. I have an excess of solar electric energy from the changes made to more efficient appliances heating and cooling for the home, as well as led’s instead of incandescents. Our actual KWh used is now less than it was when the house was built. And now it is our power, not the power company’s.
The garage with the new door does not cool off as fast. It does not get as cold as the outside air.
I believe we all know that insulation slows the movement of heat. We should know that heat also always moves from the warm side to the cool side.
Not recognized by many is the fact that heat moves, or transfers, at a greater rate when the differential between the hot side and the cool side is greater. So a warmer garage slows the rate of the house cooling.
So, #1, the new garage door keeps the heat inside the garage better. #2, with the garage being warmer the rate of heat loss from the house is slowed down.
Whether or not I would get a cash payback on the cost of the new door is going to be hard to measure dollarwise in our situation. But I have a workplace with a more stable temperature, I will not have to cut wood, split it, haul it, make and keep a fire going, and then haul and dispose of ashes, etc.
I know that the time, effort and money spent starting a decade ago to replace windows and increase the house insulation has paid off in comfort.