I have a question about one of my Rim Joist. I have a 4 3/4 x 1x3/4 Void. The open web truss only rest on the sill 2" and kisses the metal brick flashing,( which is loud if you tap on it) leaving this void. The 8" concrete foundation wall shares the outside patio concrete floor along with the inside truss. So after i seal the Rim joist with Green Glue acoustic sealant. What do I fill this void with? It runs the full 16' 4". I know after I fill the void and make it flush with the metal flashing (void is 1 3/4 deep) that I should add at least two 5/8 drywall with GG in between and caulk all cracks with the sealant. Im just not sure what to fill this void with. Should I use Owens Corning Polystyrene insulation board thats an 1 1/2 thick and then start adding drywall or just use layers of drywall to build up this void?

The opposite Rim Joist end sits on a 8" concrete foundation wall as well. Its in a 10" pocket up against my garage drywall so I will seal all cracks with sealant and then start layering my two Drywall panels with the GG.

The wall running parallel with the truss sits on a 8" concrete foundation wall and has a styrofoam type insulation before the outside brick. Im assuming the same drill. Seal with acoustic sealant and start adding my drywall layers.

Question on the concerts walls in the basement. I have 3 concrete walls in the studio. The concrete is one leaf. Do i need to build a 2x4 frame wall next to the concrete walls and anchor it to the floor and joist up above for the insulation to sit? Then leave a 1" or 2" gap. build the other wall with insulation and 2 layers of 5/8 drywall with Green Glue.
Is the first 2x4 wall necessary ? can I just build a 2x6" wall 2" from the concrete wall and insulate with the 2 sheets of 5/8 drywall and GG?

Thanks for any advice.

Cheap fluffy fiberglass insulation is fine