I have a 20x20ft garage that is attached to the house. 2 walls are attached to the house(which are insulated). The 2 garage doors are insulated, the ceiling and walls are drywall. Above the garage there are bedrooms. The builder lowered the ceiling in the garage. There is a 3ft distance from the 2×4 garage ceiling to the bedroom floors (which are insulated). The house has soffit along the entire front. I would assume there is lots of venting.

I have installed a mr. Heater radiant heater in the garage to cut the cold in the winter for working out there. I would like to insulate the area. I was going to cut an access hatch to blow in cellulose to insulate the ceiling. To insulate the walls I was going to drill 4″ holes at the top of each cavity and blow the insulation down the walls. I checked through a ceiling plug and there is no vapour barrier. I do not want to remove the drywall. Do I need a vapour barrier on the ceiling? Or is by blowing the insulation in the ceiling and walls cause an issue?

Thanks for the help. Hope this made sense. I live in Toronto, Ontario, Canada.