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LegGodt 2nd March 2012 08:32 PM

Connecting an HVAC return in a room-within-a-room design
What is the best way to connect an HVAC return vent to keep it decoupled in a room-within-a-room design? I will have a 1.5 inch gap beween the original wall that will have a DIY baffle behind it and the new wall.

I already know that rigid ducting will allow for transfer of sound, but what about flex ducting (which I already happen to have)? It's going to be 1.5 inches from the inside of the studs or 8.5 inches from the outside of the studs that I need to connect the original wall to the new wall with the vent for my rehearsal room.

jwl 2nd March 2012 09:30 PM

You should get Rod's book, he talks about this at length in there.

LegGodt 2nd March 2012 10:41 PM

I have his book and I can't find anything specific about this question.

I'm talking the last step. Baffle is properly build and installed on the opposite side of the original wall, new wall is constructed 1.5 inches away from original wall, how do you connect the return (hole in the original wall) to the vent (hole in the new wall) without coupling the 2 walls?

If I add drywall I've connected the 2 walls. If I run a solid piece of ducting I've also connected the walls since it doesn't 'float' and will need to be attached to both walls.

Maybe this pic will help ... how do I build/construct the checkered red part ... I can't think of a way to get the air flow from the rehearsal space to the baffle without connecting (coupling) the walls.


Sebg 2nd March 2012 11:03 PM

Why wouldn't you just use the flex duct that you already have? Is it really flexible (soft) or is it semi-rigid?

LegGodt 2nd March 2012 11:13 PM

I'm thinking about using the flex I have. I just want to make sure that is the best option.

Also, cutting out a rectangle and properly sealing it is going to be easier than a circle for the flex duct I have so I wanted to see if anyone had another option. Is there a flexible duct available that is rectangular or something different I can use?


John White 2nd March 2012 11:14 PM

Yes, just continue it into the room and run it down the wall. Box the whole thing out as if it were a soffit duct. One bend may not cut it though. If you're concerned put two bends in venting it back at the top.

LegGodt 2nd March 2012 11:36 PM

@John ... I wasn't planning on running the flex duct in the wall. I'm building a system similar to Figure 7.6 in Rod's book (Isometric view of a split air-conditioning/heating system). I will have a DIY baffle in the Utility Room so I don't have to worry about bends as your described them. They're already covered in the baffle.

I just need to know how continue the returns through the isolated wall assemble. Figure 7.6 only shows the return connecting to the outside wall (Mechanical Room ... my Utility Room). How do get the return to continue to the inside wall (Sound Room ... my Rehearsal Room) without connecting the 2 walls and decoupling the system at that point?

John White 3rd March 2012 03:44 AM

I must have misunderstood. Then I'm afraid I still don't completely understand your question. So is the DIY baffle not connected to the outside wall? It's just isolated?

Sorry if I'm a bit thick. Can you post a photo?

LegGodt 3rd March 2012 04:00 AM

I did ... post #3

gullfo 3rd March 2012 03:22 PM

If you didn't put the baffle inside the room or directly attach it to the inner isolation layer, then you likely need to create a heavy "duct" into the isolated room and leave the baffle side decoupled - thus avoid bridging the 2 walls... Attach to the isolation wall your duct wrapped with drywall and leave it just short of the baffle box. Seal up with backer rod and caulk. Then line the duct from the room to the baffle box with duct liner insulation (smooth air flow over the gaps). Or as suggested, use flex duct in lieu of the duct liner.

Rod Gervais 3rd March 2012 04:06 PM

No reason to over complicate this -

It's really pretty simple - you need to get air into the room - so you need to penetrate the skin of the assembly on both sides.

You want to decouple the walls - and flex duct is a good means of achieving that result.

They make duct connections you can utilize to make it from your round flex to square /rectangular

Look here for ideas......


gullfo 3rd March 2012 05:45 PM

1 Attachment(s)
i was thinking more along these lines...

Sebg 4th March 2012 10:13 AM

I think Glenn's point is that there needs to be a baffle box on each side of the double-leaf wall in order to maintain the acoustic integrity of the wall. In addition, flex duct is required in order to prevent bridging of the double studs. In general, I agree this is the best approach.