Originally Posted by Moonzy1
Haha. My bad.....didn't ,ean to do that. After all this is idea sharing format.
All along I pretty much figured I'd have to build some type of sofit but I just didn't know how I was going to go about it without loosing a ton of headroom.
So based on your reply, I think I'll frame down some rails along the ductwork so that I can screw the plywood or mdf (I figured there wasn't any advantage to osb unless I'm missing something. Plywood or mdf would be stronger and more dense ...but if I'm wrong let me know) directly to the rails on the sides (and one I between the 2ducts to stay in code) such that it is just off e bottom of the ducts. Vs. framing allthe way around (underneath) the ducts which would cost me a few inches. This way I'll only be adding the thickness of the ply and one layer of 5/8 rock w gg in between. Im thinking I'll also decouple the rails where I hange them from the joists.
Yup. That's exactly what I was describing and what I did in my room.
Osb is used because it's so much cheaper than plywood or mdf. The difference in densities isn't going to make much (if any) difference with sound isolation. There's lots of people on here that use osb instead of drywall for their first layer all the way around the room. Go for the osb and enjoy the cost savings.
There's still the concern of the weight of the soffit and your decoupling method of choice. The best way would probably be to hang the top 2x2 off a dedicated piece of hat channel with an isolation clip on every joist along the span. Like I said earlier though, a three foot wide soffit is going to weigh a ton and I don't know if the clips and channel are rated to take that kind of weight. Hopefully someone who knows the specifications will chime in or you might want to pm Ted White directly to make sure what you're going to build is safe.