Hey Dan
I’m not exactly an expert on sound proofing. But I think for your purposes of just dampening the sound from showering that any r-13 or 15 insulation would do the job
I know that Rockwool advertises their insulation as be exception with this
https://www.homedepot.com/p/ROCKWOOL-Safe-n-Sound-3-in-x-15-1-4-in-x-47-in-Soundproofing-Stone-Wool-Insulation-1-Bag-RXSS31525/202531875
I’d imagine a Local depot would carry this
thanks
steve
