We're going to blow in fiberglass insulation to improve the exterior walls we haven't had an opportunity to upgrade—after ten years of remodeling projects we've modernized about 60% of the walls.
The walls we've stripped to studs have batts of rock wool, a black fibrous material made of "fiberized" rock and steel slag. The house is typical construction for the 1950s one-story ranch houses in the Pacific Northwest (North America) but also includes a "luxury upgrade" (for the time) of this rock wool. The markings on the facing says either R-5 or R-7 (one or the other thickness was used in various portions of the house). They are nominally 1.5 to 2 inches thick. (After removing the drywall, it is a no-brainer to remove that sickly-looking insulation and install modern R-13 fiberglass.)
We had one contractor bid $1400 for the job, but we would have to do prep (remove furniture and fixtures) and finish the job (they'll put plastic plugs in the drywall holes but we need to clean their mess, then mud, texture, and paint. I asked if they would remove the rock wool: they said "No. We blow beside it which provides extra insulation."
What are the arguments for and against removal of old insulation?