Rock wool insulation. Nasty stuff to work with. Wonderful sound absorption. It makes fiberglass insulation feel like velvet. Musts for installation: dust mask, long sleeves, hat, glasses, gloves, a shower afterwards.

I used Roxul's product. The green bags in this picture are the rock wool.

Here is the end result. The product I bought was 3" thick, so I installed two layers in by 6" wall cavities.

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