What is the best way to insulate my garage walls that are already sheetrocked? Is is it possible to cut a hole in the top and drop loose material down the cavity? Thanking you in advance. George
In a garage, the walls that are typically drywalled (sheetrocked) are those between the garage and the interior of your house. The reason is that the drywall plays an important role in creating fire resistance between those spaces. If those are the walls you are referring to, they may very well already be insulated. An easy way to check to to pull off an outlet or light switch cover and try and look into the wall cavity around the electrical box.
If the wall does in fact have drywall and is not insulated, then adding blown-in insulation would be the best option. Typically a hole that is about 1 1/2" is drilled near the top of the wall cavity. Insulation is blown in and then the hole is sealed and spackled.