I am insulating my house, going with R38 Fiberglass batts (single faced) in the attic. I will have a decent amount of Mineral wool 1.5" thick and 7" thick that ranges from R5 to R30 respectively. I don't think R-38 in my zone 5 is super sufficient in terms of cutting energy costs but I do not have the money to buy more fiberglass or cellulose to add to the top. Consequently I am thinking about adding my leftover mineral wool to the attic on top of the fiberglass. It would not be enough to cover the entire attic floor though. I also don't know if it would interfere with future insulation? The brand is owens corning and it's rock fiber just like the Roxul stuff.
The other option I have is to use it in my interior bathroom and bedroom walls for sound deadening.