I have some existing duct runs feeding my kitchen and laundry room. One 10ft long run and the other 6ft long run. They do not run parallel to the floor joist, so I cannot build a soffit like the normal way and box it in because of a door way and staircase. I’m stuck running the existing duct though the open web joist. I cannot add mass through the joist since the joist needs to flex as part of their design. I plan on using silencer boxes on the outside of the room and also inside. I’m looking for advice to solve this problem. Do I need 3 silencer boxes per run, one to go from outside live room, flex,then one inside live room then the last one to feed the upstair register? The duct in between the two live room silencer boxes will be just wrapped in insulation?