Oh, it cant be permanently, because I'm renting. I'm building something like two walls, the normal wall and then the wall I will be making, and between those two walls, I'm leaving soem space to throw some isolation. But I don't know what to throw in there that is cheap. Cheap because I will be needing a lot.
But those rocks you speak of, what are they ?
Sheetrock, Drywall, https://homereference.net/alternatives-to-drywall/ whatever you cover your walls with. Adding another layer of whatever was my suggestion. It sounds like my idea won't work too well for you.
Not really, because I see that it is a permanent product. Unless ... I can put it in my inside wall maybe. The inside walls are the ones I can remove.