Attached and underground garages can be a source of energy waste and discomfort in the living areas, due to heat loss - or unwanted heat gains, in hot weather.
To prevent it, in new construction and remodeling projects, you should define your home’s thermal boundary - and where and what to insulate.
It's not uncommon for temperatures inside garages to soar to 100º F/38º C, during the summer, or to drop to freezing levels, in the winter. And that's a major problem when the garage is used for ends other than car parking and storage.
Insulating the common wall/Ceiling
Your garage isn't a part of your home's thermal boundary; you are just using it for storage and parking; you don't want to air-condition it.
If this is your case, you do not need to insulate the ceiling and the exterior walls of the garage; only the wall(s) between the garage and the living space...
The insulation materials and the procedures associated with their installation are those used for the home's shell. See: Recommended Levels of Insulation and Insulation Materials are Rated in R-values.
Insulating the exterior walls of the garage, and the garage door
You want to heat/air-condition your attached garage; you are going to use it for purposes other than parking and storage.
In this case the attached garage should become part of the thermal boundary (drawing at left).
You will have to insulate the external walls of the garage (and its ceiling), and install an energy-efficient garage door, and consider high levels of insulation and airtightness.
Car parking garages:
» Insulate the wall/ceiling between the garage and the living space.
» Insulate the exterior walls of the garage;
» Use an energy-efficient garage door.
» Consider it as a basement, for insulation purposes.
A common doubt: should you insulate the wall between the garage and the living space?
Answer: it may not be necessary; but since garage doors are difficult to seal and have a small insulation-value, it may be advantageous to also insulate that wall.
Underground and Tuck-Under garages
The conditions involving underground garages are very similar to those of basements; they can be considered as basements for insulation and sealing purposes.
See: Basement Guide