Cellulose can be installed in existing walls by drilling holes in the stud cavity. The holes are aprox. 1" in diameter. Usually a hole mid way and at the top. It's beneficial to know if you have fire stops inside the wall. The holes are then plugged on the outside if it's siding, remortered if it is brick or patched with compound if it's done from the inside. What makes cellulose the preferred method vs foam is the cellulose ability to flow into the cavity easier than sticky expanding foam. A much higher chance of success rate of filling the entire cavity and eliminating voids.
Again I ask, what is currently in the walls for insulation?
Again I ask, what is currently in the walls for insulation?