I had a few ideas in mind to keep our newborn from waking up our light sleeping toddler sleeping in the adjacent room. I'm open to new suggestions. :-)
1) Blow cellulose from above (unfinished area). I don't know if there is a firewall in the interior wall though so can't tell if the blown in will be blocked by it
2) Rigid foam boards, R-5 stuff available in my hardware store in the newborn's room, painted in something matching the room.
3) Egg crate foam stuck to the wall (seems very unattractive)