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HVAC flex in DD in joists

post #1 of 9
9/5/12 at 5:44am
Thread Starter 
Hi everyone. Fall is coming so I'm going to get back on my build. Next is the HVAC so it is in place and I will work around it then. I know this is one of the weakest links if not done right so I have a question. The HVAC will be for the theatre and the rest of the basement. Zone 3 of the house.

I don't have the room for a full soffit to run the flex in so I'm going to make mufflers up in the joists for the supplies and returns. About 4' will be run in soffit but the majority will be up in the joists. My question is the correct building on them.

My plan is:

1. line the top of the joists and the sides of the joists with DD.
2. what do i do with the bottom side? does the ceiling DD act as the fourth side of the muffler so I would bring the sides of the muffler down to line up with the channel on the ceiling? OR do I need my own DD bottom side to complete the 4 sides of the muffler? this second way seemed very sound but I didn't know if I would have some kind of triple leaf going on if there is a gap between the underside of the muffler and the finished DD ceiling on the channel?
3. Finally I'm going to use metal collars for the transition from the ceiling DD into the soffit where about 4' of soffit will continue. That will give me a tight transition. My question is the other end going back towards the main trunks where the flex will be just in the joists without muffler. where the muffler ends. can I just end my walls of muffler or do i need to bring DD down to pretty much make a complete box or by that point will the sound be controlled enough that the muffler DD stop and the flex continues back to the main trunk lines.

Thanks so much! Very sorry if this is wordy. I'm at work and didn't have time to do a drawing. PLus it wouldn't be good anyway.

jimi
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post #2 of 9
9/5/12 at 9:28am
Hi Jimi - #1 is correct. #2 isn't. You want to make a full box out of the joist space, so attach 2 layers of drywall to the bottom of the joist as the fourth side of the box. You may want to consult Ted White, but I did it this way (and have seen others using the same method) and I don't think there's any triple leaf concern present in that.

For #3, I think you're going to want to make a dead vent in the joist as well. I'm not 100% sure, but I remember Ted telling me that you want to have about 8 feet of dead vent to adequately attenuate the sound from the theater.

Hope this is helpful!
Ben
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post #3 of 9
9/5/12 at 10:41am
Thread Starter 
Hey Ben,

Thanks for that. I figured I needed that extra DD layer on the bottom. I guess to hold it in place I will just attach it to the bottom of the joists? If I do that won't it mess up my placement areas for my clips for the channel and the channel also? Meaning there will be drywall in the way in some areas?

I guess I can attach blocks to the insides of the joist bay to act as attachment points for the bottom layers of DD? I know they won't be decoupled but something has got to give eventually.

So 8' total length that's it? I thought it was more but if not that will be great. I can add DD around the flex as it exits the box I'll be making for the rest of it's journey back to the main trunks?

Thanks again,
jimi
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post #4 of 9
9/5/12 at 10:53am
You should give Ted a call. They have a great instruction manual on building a joist muffler that illustrates everything nicely, including the transition from the muffler to the room via a chimney.
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post #5 of 9
9/5/12 at 10:55am
Quote:
Originally Posted by J_P_A View Post

You should give Ted a call. They have a great instruction manual on building a joist muffler that illustrates everything nicely, including the transition from the muffler to the room via a chimney.

+1...yep, I intended to say this in my first post. Ted is the man for sure.
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post #6 of 9
9/5/12 at 11:58am
Thread Starter 
I've talked with him in the past. I didn't want ot bother him again. DO u guys have a link for that doc? I have the regular one about a dead vent and a soffit inside a room but not about up in the joists wit hthe transition into the room then. I was thinking again about that part on my way home. There is no way to have everything decoupled with that transition? If you guysd have a link to the joist bay muffler it would be great.

Thanks,
jim
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post #7 of 9
9/5/12 at 12:19pm
You have mail!
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post #8 of 9
9/5/12 at 5:14pm
Thread Starter 
Quote:
Originally Posted by Ted White View Post

You have mail!


lol thanks again!

jim
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post #9 of 9
9/7/12 at 3:15am
I'm sure Ted will get you the information you need, but if you check out my build, I had the exact same issue. I ended up putting GG, then 3/4" MDF, more GG and then 5/8" drywall on the bottom of the subfloor above and the sides of the joists. I alternated the layers by putting one layer on top, then one layer on the sides and repeated. I caulked the joints as well. After building the top and sides of the box, I installed the bottom two layers so that they were flush with the bottom of the joists (flex vent already installed). I used strips of wood temporarily screwed to the joists to hold the bottoms in place while I screwed and caulked them.

A couple of pictures:



I then built register boots from MDF and drywall and mounted them between the joists. They did not touch the joists so that they were isolated. I used isolation clips to mount them.



Boots installed (bottom not in place yet).



The boots are mounted so that the face is about 1/8" above the top of the DD on the ceiling. I then put a bead of caulk around the opening for the diffuser before installing the drywall so that it would be sealed, but not touching the drywall.

Hope this gives you some ideas.
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