I recently discovered that our garage ceiling has no insulation in it. The house is a raised ranch with three bedrooms and two bathrooms above it. Although the garage has a register for heat(no return), at best it is kept to 50 -55 F in the winter. That end of the house is always hot in summer, colder in winter. I am sure insulating is a good idea, but it is a finished ceiling over the garage (5/8 in. drywall). The question is do you blow-in insulation and hope for adequate coverage, or do you rip out the drywall and insulate with fiberglass bats, then replacing the drywall?
If you have plenty of space it might be easier to simply leave the existing ceiling as is and drop down 4 inches another ceiling, placing batts between the old and the new.