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post #541 of 1650 Old 11-09-2012, 01:45 PM - Thread Starter
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Ted, yet another question. For sound attenuation purposes, I would assume that the total insulation thickness is what we're after. If that's true, then the R8 duct has 4" of insulation. 2" all the way around. So a sound wave would have to pass through 4" of insulation (at least) before impacting another rigid boundary. That's still more than the ~3-1/2" you get with R13. To put it differently, an insulated duct is like having a 2" layer of insulation on each wall of the joist muffler, correct?

Am I thinking about this wrong?

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post #542 of 1650 Old 11-09-2012, 05:38 PM
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Not thinking wring, but we're normally looking at insulated flex + R13. wrapped around, or as close to that as possible in the space provided.

Less insulation can be offset with longer runs

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post #543 of 1650 Old 11-10-2012, 04:40 AM - Thread Starter
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Makes sense. Once I get everything together, I'll see just how much space I've got left. After moving the duct around and looking a little closer, I "think" I'll be able to get one layer of R13 on the bottom. Fortunately, it's a 19' run of flex.

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post #544 of 1650 Old 11-10-2012, 05:22 AM
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That sounds great. Please keep taking the great pics!

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post #545 of 1650 Old 11-10-2012, 02:35 PM
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I am following this HVAC and soundproofing talk closely.

JPA I am sure it is somewhere in your thread but why did you go with a joist muffler instead of a soffit muffler? You mentioned that you are very concerned with keeping sound in and I am sure after talking with Ted he told you the same thing that he told me about the superiority of the soffit muffler for this purpose. With a joist muffler I am really worried about puncturing the sound envelope before the muffler has done its job and the sound going directly up into the floor above just as you stated.

Soundproofing concerns aside I am looking into doing the joist mufflers like you because of HVAC concerns. I just don't think I will have enough space to put my ducts. Is that why you are going this way as well?

One strategy I have given thought to is simply lining the entire joist with dw just as you have done and then lining it with 1-2" of OC 703 and maybe not on all sides. This would provide quite a bit of rectangular cross section for good cfm. And if you have 19' of length I would think that the sound would have enough distance to bounce into the fibers several times as long as you are making a few 90 degree turns along the way.

What do you think?

btw I might have been someone who was talking about keeping sound out for a low noise floor but in the end I know that a theater that I cannot use while the children are asleep is going to be a very frustrating experience. So I am concerned with both.
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post #546 of 1650 Old 11-10-2012, 03:54 PM
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I wouldn't use unfaced 703, as the fibers may be dispersed with the airflow. FRK may be an idea worth exploring.
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post #547 of 1650 Old 11-10-2012, 05:36 PM - Thread Starter
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Jedi, you are correct. My soffit height is only 7", which is not really enough to run my duct in it and have space for insulation. So, after agonizing over it A LOT, I finally decided to go with a joist muffler. Using the joist cavity as a duct does sound like a good idea, and Mr. Tim brought up a good point I would not have considered. I'm not familiar with FRK, but I'll be looking it up now smile.gif

Your post about the noise floor is the one I was referring to. What's interesting is the timing. When I read your post I was thinking +1, and "that's right!" and "amen, brotha!". But that night I had to give up on the video games because I had to keep the volume so low I couldn't hear the dialogue (despite the effects being loud enough to wake the kids). I was just frustrated, and I hope that I'll be able to enjoy both the low noise floor and watch movies loud enough to understand what's going on when I'm done with the theater smile.gif I didn't mean for my post to sound like a jab at you, and I hope it didn't come across that way. I was just venting.

Mr. Tim, as I mentioned above, good points! I'll have to look into the FRK

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post #548 of 1650 Old 11-10-2012, 05:51 PM
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No problem at all. And I really do care about preventing sound getting out.

Regarding FRK. So I think that is the kraft paper faced insulation. This is commonly used as DIY sound treatments where one would desire reflection such as between the surround speakers. I plan to do this myself.

The problem I could envision is the high frequencies just bouncing around the muffler without being absorbed. I don't know exactly what frequencies pass through the kraft paper and which are reflected.

Anybody have experience with this?

Is FRK the same as Kraft paper?
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post #549 of 1650 Old 11-10-2012, 06:29 PM - Thread Starter
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Looks like FRK might be referring to faced OC703. I wonder about just using duct liner. It would probably depend on the thickness, and I have no idea what the stuff costs.

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post #550 of 1650 Old 11-10-2012, 07:10 PM
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I wonder if one could use JM Linacoustic? I will be treating my front wall with it anyway.

This page says it is treated with a resin that should prevent the fibers from becoming airborne and is supposed to be used as duct liner in the first place. As long as the absorption characteristics are favorable it should work.

http://www.jm.com/insulation/performance_materials/products/AHS-329_linacoustic_rc.pdf

-or what about using the denim based cotton insulation?
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post #551 of 1650 Old 11-11-2012, 03:55 AM
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Linacoustic was designed for lining the interior of ducts, so absolutely it could be used. However, if I am understanding the absorption coefficients correctly, 1" of 703 FRK is a better choice.

Maybe a combo of some sorts would work, but that is well beyond my understanding.. if it's a good idea at all.

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post #552 of 1650 Old 11-11-2012, 06:08 AM
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How close is this in-joist duct that you are constructing to the HVAC equipment it is connecting to? And how far is the output of the supply from the exit of the room?

I think the in-joist duct silencer may not be necessary at all and in fact can be a bit detrimental, as Ted mentioned. If you have an in-joist silencer constructed of nothing but hard surfaces directly attached to the structure, you are essentially building a drywall "drum" that has the disadvantage of sound getting transferred (and possibly amplified) into the floor above . . . and all done in the name of capturing sound trying to escape the room through the HVAC.

If you are not too far along in the process, I would not construct the in-joist silencer and just run the flex line as normal, especially since you have double drywall above and a shell with clips and double 5/8" with Green Glue yet to come.

If you think of how difficult it would be for sound to enter through your HVAC supply and navigate all of the twists and turns contained within 19' of flex line to get out of the room, you can see where building a "drum" of drywall behind the shell may not be the optimal tradeoff.

Interested to hear your thoughts.
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post #553 of 1650 Old 11-11-2012, 06:46 AM
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The Joist Muffler does not have decoupled surfaces, which makes this the least desirable option amongst soffit mufflers or Dead Vents. There are a lot of times when running a muffler through a joist is all you can do, however. Certainly no amplification, as the system has significant added damped mass and absorption. They work well.

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post #554 of 1650 Old 11-11-2012, 07:11 AM
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That's exactly what I am talking about - it is the least desirable option. My point is there exists a tradeoff between trying to capture sound escaping the room by using an in-joist muffler and the sound leaving the room through normal sound waves.

Given the size of the supply opening, the number of twists and turns in the supply path and the overall length of the supply I really don't think there is that much cause for concern that an in-joist muffler needs to be constructed to capture sound leaving through the supply at the expense of constructing a hollow, uninsulated cubic volume that lacks decoupling immediately above the ceiling.

I would think the insulation of the flex duct would completely fill the space within the joist providing for at least two layers of insulation within the joist and its length, position and pathway essentially eliminate most sound from leaving in the first place. It just seems like a lot of effort for something that may not have any payoff at best and provide a less optimal solution at worst.
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post #555 of 1650 Old 11-11-2012, 07:23 AM - Thread Starter
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My supply line is a straight run from the opening in my theater to the trunk line in my mechanical room. I'll try to post a model later. Thats two 90's. One leaving the theater and one dropping into the trunk line.

So, is the concern sound traveling down the duct into adjacent spaces? Or, are we concerned about sound getting into the space above the theater ceiling (between the ceiling and the floor above)?

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post #556 of 1650 Old 11-11-2012, 07:28 AM
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"...the opening in the theater." That's my concern. That initial opening(s) for supply and return. I always ask when a sound wave exits those openings, what framing structure will the vibration enter?

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post #557 of 1650 Old 11-11-2012, 07:44 AM
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Quote:
Originally Posted by Ted White View Post

"...the opening in the theater." That's my concern. That initial opening(s) for supply and return. I always ask when a sound wave exits those openings, what framing structure will the vibration enter?

So in this case it is entering into a hollow drywall volume (lined with ductliner) that is directly coupled to the structure of the room above (albeit one side of this volume has two layers of drywall with Green Glue). The other case has a volume filled with a thick flex duct and the same two layers of drywall with Green Glue.
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post #558 of 1650 Old 11-11-2012, 01:25 PM - Thread Starter
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First off, I appreciate everyone taking the time to look at my build thread and post comments on my build.

Here is the existing plan that just shows the ducts in the joists. Both runs are just straight shots in the joist bay.

TheaterDucts.jpg

Now, considering the recent discussion, I've taken another look at my layout, and I "THINK" I may be able to run my ducts into an unfinished closet adjacent to my mechanical room. I could build a ceiling mounted dead vent for both lines in there. Here's the layout showing my HVAC lines (and crossover) in relation to the theater/mechanical room/closet.

HVACLInes.jpg

One drawback to this option is I will need to colocate my HVAC supply and the crossover from the adjacent room. So I would have a plenum with both lines in it. That also means I'll have a vent on the face of my screen wall. Something like an 8"x8" return register. It would be painted black of course, but I wonder if it would be objectionable?

Supply.jpg

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post #559 of 1650 Old 11-11-2012, 05:10 PM
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Aside from supplying at the front (or front-ish) of the room, you should be returning at the rear of the room. Without going through all the pages of this thread again, if I recall correctly it was not possible to return from the rear - is this correct? Having the return and the one supply so close will create its own little "microclimate" as the conditioned air from that supply will be drawn straight to the return without the opportunity to mix with other air in its room.

Regarding the short soundproofing discussion from this morning - were you intending to put the in-joist dead vent where the 90 degree elbows are before the supplies go into the room? I can see using a dead vent for your shorter supply run in the adjacent room, but I don't think you would get much value for your time and effort if you tried to put one in for your longer supply run IMHO.

One word of advice - the Illuminati of this forum recommend rigid sleeves for any shell penetration and flex duct for the rest. My room is very similar to yours....I simply cut the inner flex short and then left enough of the insulation cover so that once I attached the inner line to the elbow I had enough of the insulation sleeve from the flex line to cover the entire length of the elbow and straight rigid piece. You will have to cut the length of the insulated sleeve so ONLY the rigid passes through the shell, leaving only enough gap to run a healthy supply of acoustic caulk around the rigid penetration.
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post #560 of 1650 Old 11-11-2012, 06:26 PM - Thread Starter
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Good tips as always.

I do have a return at the rear that utilizes a 400 cfm inline fan. The second line at the front is for providing "fresh" air from the adjacent rooms. Ideally this would come from an ERV, but I can add that later if needed. I just wanted to add the duct now while I have a chance.

I was intending to run joist mufflers above the room and stop them in the adjacent mechanical room. I don't think sound traveling down the duct will be an issue for me because of the 90's in the mechanical room. I suppose at this point, the question is whether it would be better to run a dead vent with both supply lines, and enter behind the screen wall. The downside is two large holes next to each other (aside from the other redesigns this will cause wink.gif)

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post #561 of 1650 Old 11-11-2012, 07:39 PM
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Well, if you have seen my comments in other threads in this forum you will know I am not a fan of dead vents when a fully-optimized forced-air solution is available to be designed and installed. The exception would be if the position of any duct is very short to the outside of the room or the HVAC system - then I am fully on board with using one as a muffler to isolate the sound either escaping the room from the audio system or entering the room from the HVAC system.

Having the ERV is an interesting option...but wouldn't that simply connect directly to your HVAC system's main return air supply line to introduce the fresh air?
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post #562 of 1650 Old 11-11-2012, 09:52 PM
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Can you box out a small area of the bedroom or game room and put a small dedicated AHU in there? It seems like your design is getting complicated.

 

 

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post #563 of 1650 Old 11-12-2012, 08:02 AM - Thread Starter
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I think you may be right that I'm making this too complicated. Unfortunately, our HVAC budget was spent long ago (I'm not convinced that 2-stage compressor was worth the money, but that's another story).

However, I think I may eliminate the 7" crossover. Running just the 8", I don't think I will get quite 4 air exchanges per hour, but I wouldn't have exactly been meeting the intent of that anyway. I need around 300 cfm to get 4 exchanges, and while I have a 400 cfm inline fan, I feel like there will be significant line losses in my setup. The duct model calculated around 180" cfm from that 8" line. Once I add the exhaust fan, I should squeeze a few more cfm out of it. So, I think that will be sufficient.

With that in mind, here's yet another iteration of this thing. Again, I think I can get a dead vent in that unused closet. The trick will be mounting it to the ceiling rather than a wall. I'd have to go over the details of that with Ted. The advantage here is a single 8" penetration, and I get to use a dead vent. There are some other aspects that I'm not thrilled about with regard to layout changes, but compromises will have to be made.

NewHVACLayout.jpg

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post #564 of 1650 Old 11-12-2012, 09:18 AM
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Is the duct on the left side of the picture in post 558 the return?

I feel your pain.. I planned and planned and planned.. and when I went to fit the pieces together.. well.. it did not go as planned. biggrin.gif

As long as you can get a good dead vent in that closet, I think you have a workable solution. I would use a short piece of flex to connect the diffuser to the pipe coming through the wall.. but I'm sure you already knew that.

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Why would you cap the first 8" line instead of just running it all the way across to the other side of the room, especially since you are CFM deficient? I think that is an easy choice to keep for the room's cooling.

I also think you are spot-on for using the dead vent, but I think it should be shuffled to immediately inside the room or immediately outside the room instead of having several feet of duct before it reaches the dead vent.
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post #566 of 1650 Old 11-12-2012, 09:56 AM - Thread Starter
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Quote:
Originally Posted by Mr.Tim View Post

Is the duct on the left side of the picture in post 558 the return?
I feel your pain.. I planned and planned and planned.. and when I went to fit the pieces together.. well.. it did not go as planned. biggrin.gif
................

My return is at the rear. Here's another model that shows it as well as the location of the fan. It will be located in my equipment closet and dump into the mechanical room.

HVACExhaust.jpg

Just when I thought I had all this worked out........ CRASH! I suppose it's better to work out the kinks now than later, though smile.gif

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post #567 of 1650 Old 11-12-2012, 10:06 AM - Thread Starter
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Why would you cap the first 8" line instead of just running it all the way across to the other side of the room, especially since you are CFM deficient? I think that is an easy choice to keep for the room's cooling.
I also think you are spot-on for using the dead vent, but I think it should be shuffled to immediately inside the room or immediately outside the room instead of having several feet of duct before it reaches the dead vent.

I don't think I'm following you on this. I can either run the 8" across the top of my ceiling, or run the 8" behind my screen. There isn't enough volume from the trunk line to support two 8" lines into the theater.

As far as the dead vent location. If I run the 8" across the top of the room, there isn't a good spot for a dead vent. The mechanical room ceiling is too crowded, and there isn't enough space in the joists. That was why I was originally planning for a joist muffler. Running the duct in front of my screen gives me room for the dead vent in the closet ceiling.

Either case should give me the same flow (within a few cfm I would think).

EDIT: I also meant to mention that my soffit is only 7", so I don't have room for a dead vent of soffit muffler inside the room either.... 9' ceilings seemed so high to begin with smile.gif

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post #568 of 1650 Old 11-13-2012, 08:13 AM
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Quote:
Originally Posted by J_P_A View Post

I don't think I'm following you on this. I can either run the 8" across the top of my ceiling, or run the 8" behind my screen. There isn't enough volume from the trunk line to support two 8" lines into the theater.
As far as the dead vent location. If I run the 8" across the top of the room, there isn't a good spot for a dead vent. The mechanical room ceiling is too crowded, and there isn't enough space in the joists. That was why I was originally planning for a joist muffler. Running the duct in front of my screen gives me room for the dead vent in the closet ceiling.
Either case should give me the same flow (within a few cfm I would think).
EDIT: I also meant to mention that my soffit is only 7", so I don't have room for a dead vent of soffit muffler inside the room either.... 9' ceilings seemed so high to begin with smile.gif

I didn't realize your are CFM deficient on the system to the point of not being able to support the two 8" ducts you would typically need for a room that size loaded with people.

My original point was if you run the 8" across the room in between the joists that a joist muffler would not be necessary because of the elbows, the length of the duct and the seclusion of the duct behind a thick, acoustically soundproofed ceiling. My opinion was that it is better to have a full 8" insulated flex duct fill the void of the joist cavity. This way you don't have to mess around with a joist muffler and you have plenty of fiberglass insulation to not only isolate your duct (and deliver conditioned air, of course), but the insulation will also absorb sound. The drywall box of the joist muffler you were constructing seems to have no use or advantage in this situation.

Running the first 8" supply across the room also obviates the need for a separate dead vent in the mechanical room ceiling.

I think this solves all of your issues with the supply, with the exception of the ERV (which I don't where this is connected to your system. Hopefully this is clearer than in my last post.
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post #569 of 1650 Old 11-13-2012, 08:33 AM - Thread Starter
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Ah.. Much clearer; I understand now. The caveat to the above is 8" supply will cool the room (with some margin to spare), but does not provide the 300 cfm necessary for 4 air exchanges per hour. 300 cfm of cooled air would be WAY too much to maintain humidity levels in the room. I don't currently have an ERV, but it's one of those items that I may consider down the road depending on air quality in the house. I mentioned it because the 4-6 air exchange recommendation is based on exchanging indoor air with "fresh" air. Not necessarily just exchanging air between rooms.

From a sound standpoint, my main concern is that I don't have any data points to draw on with regard to sound leaking from the room. It would seem that an 8" hole in the ceiling will allow a substantial amount of sound intrusion into the cavity between the theater ceiling and the floor above. While I have a layer of damped drywall on a large portion of the subfloor, there are still areas that don't, and this is not decoupled. So it seems that sound entering this cavity will propagate to the floor above easily. This is why the joist muffler seems necessary.

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post #570 of 1650 Old 11-13-2012, 09:15 AM
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Wouldn't the 8" hole in your ceiling be dropping into a soffit? Normally the soffits have their own DD+GG+insulation which would help shield room noise from getting into the soffit and up through the 8" hole.

Regarding ERV... I read surprise/complaints about the Trane FreshEffects ERV that it's basically a stand alone system that doesn't talk to the main HVAC blower. The 2 Broans I have are the same way. So you can hook it into your main return if you want, but then you either need to be running your main blower 24x7 (which I don't like because it picks up water off the coils) -or- accept that effective ventilation will be hit & miss. Another option is to connect ERV as a separate loop to ventilate 1 room and let the asynchronously running HVAC system redistribute that air through normal operation. There are other variations. It has a blower in the unit, so you could probably also use it as a booster fan like you have planned for the Panny.

 

 

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