In Southern California, during new construction with a finished garage, the cost of adding insulation at this point should be low.
But if the space will not have any active cooling or heating elements (outside of a tankless water heater and, perhaps, a cat), is there any value in insulating it?
Everyone loves going from a cool house in to a hot garage or vice a versa. Would insulating it make any difference? Would it not get as hot in the summer or as cold in the winter?
Seems to me that it would retain whatever temperature it have longer (whether hot or cold) with the insulation than without, and one could "reset" the temperature of the garage by opening the door for a few minutes to the outside (for example, open it in the cool morning in the summer hoping to better maintain that during the day). Just not sure if it would be enough to notice or bother with.
Historically the garage has mostly been closed vs opened several times a day for cars. Whether that will be the case at the new house is up for debate.
Best Answer
Oh, yes
It's a cornerstone of passive solar design.
We spent $2.4M on a large building we intend to never heat. We insulated it to the nines. Here's what that does.
Extremes of temperature are moderated by building's thermal mass*, which (with the insulation) helps it resist changes in temperature. This affects
Condensation, which is significantly reduced or eliminated. This makes life much easier for the contents of the building.
If you don't have enough thermal mass*, you can add some by installing large barrels of water or antifreeze.
* ”thermal mass" is the terminology used in building design, but it's actually a misnomer. Mass/weight does not store heat. Atoms do. And atoms vary in mass dramatically from 1 for hydrogen to 207 for lead. But an atom's heat storage ability is about the same, 20-30 j/mol/degK, most 24-26 j/mol/degK. Different materials contain different mixes of atoms, so their heat capacity by mass is all over the map. Water is ideal.