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Is an additional layer of sheetrock allowed? because this https://www.greengluecompany.com/products/noiseproofing-compound stuff in between an addition layer of sheet rock, covered with that foam works wonders.

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.