Great question! Many factors
come into play with this....
If your garage is insulated, closing the door will
help conserve energy, particularly if there is a bedroom located above
the garage. However, if the garage is not insulated and is facing South,
the heat gain can be intense! Therefore, leaving the door open will allow for
air flow through it, resulting in potentially
lower ambient temperatures.
Remember to air seal all holes, gaps and openings that could potentially create pathways to the home! This could include garage door hardware, electrical, plumbing and duct penetrations. Another good idea is to make sure there is solid weather-stripping on any entry doors to the home.
Stay cool!