Existing duct salution Help!

[JLDrumStudio]

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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?

[Gregwor]

I would build a 1" MDF box around the duct and have it sit on top of the bottom 2x4. You might have to do this in little chunks of MDF. Basically just make sure your surface density is never compromised and that it is air tight. Stuff insulation in where you can. The weight should not hinder the function of the joist as it would be no different than if you had installed drywall onto the bottom side of the bottom 2x4. Certainly not a fun or fast job but I'm not sure how else you'll box it in and keep it air tight.

Greg

[JLDrumStudio]

Thanks Greg

[Soundman2020]

There's another option here, and it is one of the very, very few places where I consider MLV to be useful in acoustics....

Wrap your duct in a thick layer of fairly low density insulation, such as pink fluffy, or something similar. Make it as thick as you can, hopefully a couple of inches, but at least one inch. Then wrap that with MLV (1 pound at least, preferable 2 pound). Seal up the MVL very carefully, first with a broad strip of vinyl glue where it wraps over itself, then duct tape over that, and finally wrap it all around with string, or something like that, to ensure that it can't ever come apart. Not too tight: you don't want to compress the insulation much, not to cause the string to cut into it from being too tight. Just enough to keep it together.

This should give you reasonable isolation on that duct. I've used this technique myself in my own home, to quiet a sewer pipe from a bathroom right above my TV room, and the result was very good. Before I did that, we had very unpleasant water noises from that pipe, plus sounds from the TV room making it back up into the bathroom. Now it is dead quite, in both directions.

Yeah, it's a lot of work, and as Greg said, it's a pain to get in there between all those framing pieces to do it, but if you work carefully it can be quite effective.

Like I said, this is one of the half-dozen (or less) places where I think MLV is justifiable: in this case, because it is high mass and also very flexible, so you can wrap it around curved objects. It is also limp mass, which helps quite a bit, as it is self-damping. It's just not nice to work with, tears easily, and is expensive, but for this type of problem, it works.

- Stuart -

[JLDrumStudio]

Thanks Stuart. That’s an option I might just use. I found Soundsulate 2lb 4’ x 25’ for $225.00. On eBay. Being flexible this might be a good option. This hurdle has been driving me crazy. If I go over to the next brace I can fit 2.25 x 12” duct and hug the subfloor after the 2 5/8 GB with green glue. I’m still debating on inside out ceiling or tucking new ceiling joist in between existing joist. The 2.25 x 12 duct will allow space for new ceiling joist to set up in there. I only have 7’ 7” to bottom of the joist you see in the pics. Hurdles and decisions lol. One at a time I guess.
One more thing, is there any advantage of a silencer box feeding right under the upstairs register? At that point the duct will run parallel to the joist and easy enough to either then, box it in like normal or run the 2.25 x 12 duct into a silencer box right before the upstairs register. Just a thought.

Ok off to add mass to subfloor and caulk my butt off. Lol

[JLDrumStudio]

So am I understanding this right, I do not need silencer boxes on existing duct as long as it does not enter the inner leaf? Any special way penitrating the outer leaf with the existing duct?

[Gregwor]

Any penetration through your inner or outer leaf needs a silencer box.

Greg