How many leafs in ceiling - considering hvac?

[rbarnes]

Hopefully I don't get flamed with this post, but I am building a home theater in my basement. The extremely informative posts that you studio designers have made has brought my plans to a whole new level. My questions revolves around the leaf concept - I think I understand that 2 is better than 3 in general, but my question adds an extra variable - the hvac system in my home. I have attached a drawing but my design from top to bottom (room in a room) is this:

hardwood floor (kitchen upstairs)
plywood subfloor
scissor trusses (2x4 with insulation and hvac trunk for main level)
---sheetrock here?---
air space
2x6 freestanding (room in room) ceiling with insulation
sheetrock in theater

So the brunt of my question is the 2 leaf design calls for NO sheetrock in the middle (as i understand it) but considering the hvac trunk for the main floor is in the middle there I am not so sure. Is my net result going to be quiter if i look at 2 leaves between theater and the venting at the risk of 3 leaves if viewed from theater and kitchen?

Not sure if my question is getting conveyed well.. to re-iterate what are your thoughts on using the sheetrock on the buttom of my trusses.

Thanks again.. keep up the good posting and I will keep up the lurking/learning!

-Rob

[knightfly]

Crap, I had hoped my answer got through before my broadband puked on me last night at about 4 am;

Bottom line here is that ducting WILL weaken your isolation, so all you can do is minimize the damage.

Your upper floor normally would be your first leaf, then the air gap and insulation between, and the ceiling of the theater (not including any acoustic treatment) should be the second leaf of mass.

When you have ducts in between, even thin metal will constitute a leaf even tho it's a wimpy one; the bad effects will still be there.

If you beef up the walls of the ducts with a couple layers of gypsum all around, you won't help your iso much in the LF range but you WILL cut down the sound entering the duct, by improving the m-a-m of the part consisting of one wall of the duct, whatever air/insulation is between that and your ceiling mass, and the ceiling mass itself - this will act the same as a m-a-m wall between two rooms, and can be calculated about the same - so two layers of gyp, a few inches of insulation and your ceiling is what determines how much sound gets into the duct.

Beyond that, if the duct is fairly long and has at least two 90 degree bends, it's about all you can do.

OTher than that, the larger the cross-section of the duct the quieter the air flow; noise caused by moving air is roughly equal to the 4th or 5th POWER of the velocity, so a larger duct makes a BIG diff in noise from that source.

I still haven't had time to chase down a DIY grille source that's quiet - if anyone out there knows of one, please post in the building materials forum... Steve

[rbarnes]

Thank you knightfly for your well considered response. Sorry you had to type it twice due to broadband outtages!

If only there where cut and dry solutions to all of our problems. I think I will follow your advise and forgo the sheetrock in the 'middle' that would have protected the vents and see how it goes!

[knightfly]

I'm not sure that's a good idea; do you have a sketch or pix as to where the ducts run to and from, etc? IN some of these cases, quad-leaf is the LEAST evil you can do; like you said, nothing cut-and-dried... Steve

[rbarnes]

Thanks for your reply, sorry for the delay in getting back to you. Here is one closeup that shows my situation. This shows the upstairs floor, the joists, the trunk line and the bottom of which where I could put the first layer of drywall. The room-in-room ceiling has not been erected yet as that would make all this very difficult!

Since this room is a subsection of the basement I think I will have to apply sheetrock to the bottoms of the joists (3 leafs) to contain the sound.. otherwise it will propogate through the joists to the rest of the basement. This would conclude my exterior room.

http://chewyoatmeal.com:81/gallery/Basement/aam

[knightfly]

Tried 3 times to access your site and kept getting a timeout; can you size any pix down to 750-800 pixels wide and post them here as jpg's?

Meantime, here's a diagram that may help... Steve

[rbarnes]

Thanks again for your replies and the effort spent crafting them. My hard drive failed in my server so it was down for a bit. It should be back up so you can look at some more pictures, but I am attaching the most pertinant one now. As you can see the duct goes perpendicular to the joists so there is really no way (well i am not going to expend the effort) to wrap the duct in sheetrock as it bumps agains the 'scissors' as it passes through.

Your drawing is very good, although I would note I was not planning on resiliant channel on the bottom of the joists, as that is the 'outside' room .. i will constructing a room withing that will have a 2x6 ceiling with more insulation in the joists and 5/8" sheetrock on the bottom (none on top).

Am I making too many comprimises here, or is this a decent balance of price/effor/performance? Can't have all 3

-Rob

[sharward]

. . . I was not planning on resiliant channel on the bottom of the joists, as that is the 'outside' room .. i will constructing a room withing that will have a 2x6 ceiling with more insulation in the joists and 5/8" sheetrock on the bottom (none on top).

So, if I read that correctly, you will have:

• Floor of living space above basement
  Outer ceiling panels attached to bottom of joists/trusses
  Inner ceiling panels attached to bottom of newly constructed 2x6 joists

I count three leaves, not counting the HVAC ducting. 3 leaves = bad.

When counting leaves, you count not only the stuff you're building, but also the stuff that's already there.

[rbarnes]

Sharward,

You are absolutely correct. Which comes full circle to my question in the first post. Generally 3 leaves are bad (or so you folks tell me!). So the choices seem to be:

3 leaves between theater and kitchen
vs..
2 leaves between theater and kitchen

The big question revolves around if adding the 3rd leaf may be better overall since from the ducting to the theater point of view thats 2 leaves.. keeping sound out of my ducts.. which may be better for the home as a whole. All theory here...

[sharward]

I'm sorry that I missed your introductory statement in this thread:

I think I understand that 2 is better than 3 in general, but my question adds an extra variable - the hvac system in my home.

Just goes to show what happens when one reviews only the most recent posts in the thread! Sorry!

[rbarnes]

I really hope I didn't come across as rude in my post. I know you where just trying to be helpful - which is a common trait here. Thanks for taking the time to share your thoughts.

[sharward]

I really hope I didn't come across as rude in my post. I know you where just trying to be helpful - which is a common trait here. Thanks for taking the time to share your thoughts.

You didn't, I'm glad, and you're welcome!

[knightfly]

OK, getting a better idea of things now; one more - do any of those ducts serve the theater area, or do they just pass by it on the way to other parts of the house? Steve

[rbarnes]

The square trunk runs over the theater area. Off of that large trunk parallel (but seperated by 5') to the round run in the picture will be two runs for the theater. The plan is to use flexible 6" ducting with the fiberglass/vapor barrier shell. That run will go about 7' in the space pictured, where it will do a 90 degree turn through the outer ceiling to drop down along the wall between the inner/outer rooms. When it hits the floor it will do another 90 and pass through the wall into the theater.

I found little information on ducting types and though the flexible would not transmit sound from the theater to the main trunk as readily as the sheetmetal ducts.. Comments on this welcome as well.

I will post a picture of the ducting thus far when i get home tonight.

[rbarnes]

Here is a picture of the duct run. The plan is to put insulation in the cavity now and sheetrock the whole ceiling in. That run will drop through the sheetrock and behind the wall of the room/in/room.