My plan is to stuff the cavities between the joists with fiberglass batting, I assume a higher R rating will be better? After that, I'd like to run rubber strips or something along the bottoms of the joists, then screw 2 layers of drywall to it with a layer of 'green glue' in between. I know it won't be silent upstairs, but this would be an acceptable level of soundproofing for my purposes.
My concern is that the joists are not standard 2x12's or whatever, they're I-joists - 2x3's on the top/botton, and what looks like a 10" or so strip of Norboard in between. I was told a year ago not to hang weight from the bottom of I-joists, because while they're strong when you push on them, they are NOT strong when you PULL on them.
So, by my guesstimates and calculations - lets say a sheet of drywall is 40 lbs. 2 sheets thick is 80 lbs every 4x8 section, which is 2.5 lbs per sq. ft. That's a total of 750 lbs if drywall on the whole thing. All that batting is going to add weight too, not sure how much, maybe 100 lbs?
Will this be safe? Any alternatives?