I would not waste the insulation on an outside wall unless your garage door was getting insulation. Drywalling has benefits though. First you need to use fire. If you choose to use spray foam or batt insulation, you will need new drywall at the end of the project to cover the.
There is a feet of space between the plaster ceiling and drywall. I wanted to place or slide fiberglass insulation in the ceiling from the top in attic. He also insulated some of my ceilings which included drywall repair and did some.
To avoid mildew, the insulation needs to dry for at least two days before it is covered with drywall. Use drywall clips as needed to hold drywall in place. Find the best insulated access doors for drywall based on what customers said.
Existing finished walls are insulated all the time. You can use fibrous types, ( cellulose, fiberglass, or rockwool), a part Spray Foam or rigid sheets. May insulation inspection wall hole drywall. If you live in an older home, the walls may or may not have insulation in them.