Insulation..... packed or air space ?

I have red the posts on insulation. But before I spend 600$ more i figured i would ask.... The air space in my ceiling is 13" ..the top is an outside roof with rolled roofing and plywood. the bottom will be 2 layers of 5/8 drywall. My Question... will 6" of fluffy stuff touching the bottom be enough. and if i spend 600$ more to fill the whole air space will it make that much difference.???? Thank you, Steve

The first 6" gets you the most; with ceilings, gravity helps by keeping the insulation in contact with the ceiling panels, which improves damping (therefore isolation) I'd save the extra $$$ for more drywall/caulking or a new toy... Steve

Thank you So much ! steve