What is "rock wool"?
#1
Some of the DIY books I've browsed talk about "rock wool". What is this? Is it still used, and if so, when is it used instead of fiberglass? It doesn't seem like they are talking about that killer asbestos.
#2
First of all was asbestos a very useful product with a lot of bad press--but for the people who don't know what it is or how to deal with it--its best they stay away from it. "Rock wool" that I think you are refering to is mineral wool or "brown death" as we call it in the construction industry. I am not sure of its use in residential construction and personally have not seen it in any of the old houses I have owned. But in the comercial side we use it as a fire stop between floors, above walls, and around door frames etc. It has no insulating value that I know of and does require respiratory protection to be used by the installer unlike fiberglass.
#4
I have no idea what the R-value of minerial wool is--but 3-4 inches of any kind of insulation is not enough for an attic! The minerial wool (which I think is the same stuff you people are calling rock wool) we use in commercial building is spec for fire proofing and has no R-value listed on the package. Like I said earlier I have not seen this stuff used in residential--but I have only owned several older homes--which either had no insulation or a little (6") of fiberglass in the attic.