You need to put insulation in the ceiling. Any other attempts to insulate without insulation above is futile since the majority of heat is going to be lost up there.

I would just hire a company to come out an blow insulation up there. I just had my one car garage done for $650 and they had to build an air vent along the soffit on one side. It was really worth the pros doing it and me not having to take hours or days to learn how to lay insulation properly on exterior walls.

In Cali I'm sure its probably pricier but when I priced it out to pay someone to do it was about as much as it would be for me to buy materials.