HVAC Soffit construction?

Eggman · Post by **Eggman** » Sat Jan 24, 2004 10:01 pm

Newbie (to this forum) here. First off....GREAT website. One of the most useful and informative I have ever found. I've learned more about studio design/constructionin the last few weeks than I have in the last 25 years.

Now my question. I have a 16'X20' future studio space in my basement. I want to bisect it into two 16' x 10' spaces, one a live space, the othera control room. I have read and reread info from this forum and feel I have a workable plan. The room is already divided by a beam and four posts. I plan to build a wall in each side of this beam. No problem. BUT on each side of the beam is a 20"x8" sheetmetal duct. I want to soffit around these ducts, one in the CR and one in the live room. How do I frame this soffit without tying it into both the wall AND the ceiling of each room thus loosing the wall/ceiling 1/4" gap and framing isolation. I have worked as a carpenter through the years, so I can handle almost any method. I just can't find the detailed info.
Question 2. A quick one! The floor willbe 3/4 T&G plywood over Tough deck turned sideways on 1/2 neoprene as sleepers. Do I HAVE TO put in insolation between the sleepers. How can I really keep it off the floor?

Thanks ahead of time for your reply. Can't wait to get to work.

Doug

Post by **John Sayers** » Sun Jan 25, 2004 12:16 am

Welcome Doug - I'm not sure by your description what your problem is. I assume you are saying you want to add a second ceiling but what do you do with the HVAC ducts. The only way I know is to add cross support beams from wall to wall just under the duct height and build UP from there to your ceiling.

See drawing:

Yes you must put insulation between your floor joists. You could in fact lay out the insulation (in rolls) first and build your floor on it.

cheers
john

Eggman · Post by **Eggman** » Sun Jan 25, 2004 4:10 am

John,
Thanks for your quick answers.

I will try to clarify my question. Yes, I want to add a second ceiling. The diagram you drew is correct, except I am going to add a double wall directly beneath the beam to divide my 16 x 20 space into two 16 x 10 areas.The hvac is running parallel to the beam on each side. One duct run will be in one room, the other run in the other room.

I feel I have a pretty good grasp of the two leaf principle, etc. I have viewed many of your drawings and will use them as a guide when constructing my inner and outer walls as well as the ceilings.

My understanding is that the ceiling and walls should not touch each other. How do I frame in a soffit without fastening it to BOTH the wall and the ceiling? Won't this violate the rules if I tie the ceiling to the wall?(via the soffit).

Okay, please give me your thoughts when you have a minute.

Thanks again,

Doug

Post by **John Sayers** » Sun Jan 25, 2004 10:26 am

You have two options IMO

Either turn it around 90 degrees and have two support beams in both planes or do what I've been trying ( unsucessfully so far) to get people to do..... remove two poles by adding a couple of steel beams. It's not major surgery!!

Then revert to my orignal drawing.

cheers
john

Eggman · Post by **Eggman** » Sun Jan 25, 2004 5:53 pm

Hi John,

Once again, thanks for your time. Please have patience with me.

I still don't think I have stated my question clearly enough. I think the two room info was clouding the issue. Maybe I can just ask a more theoretical question. Here goes.

In a room withm a horizontal HVAC duct run up in the ceiling/wall corner (the whole length of the room) how do I frame the soffit/ceiling intersection and the soffit/wall intersection?

How do I maintain the ISOLATION between the ceiling and wall if the soffit framing touches both(the ceiling and the wall framing)?

Do I hang the (vertical) face of the soffit off the ceiling framing (the floor joists)? Or do I make a hard framing connection above and somehow float the (horizontal) bottom of the soffit off the wall?

Hope you'll take one more shot at this. More thanks!

Doug

Post by **John Sayers** » Sun Jan 25, 2004 7:14 pm

Doug - I wish you could draw your layout - but here goes.

YOu are going to build an inner wall (room within room) OK? YOu are also going to have a new ceiling just under the original but not touching. The new ceiling will attach to the new walls. So we now havce a room within room.

Now your problem is that you have to get around the ducts that are attached to the original ceiling? So you must provide a support below the ducts, but not touching the ducts, that spans from new wall to new wall and acts as a support for the new ceiling - which is what I drew for you.

That's as I see it - you'll have to draw it for me if it's different.

:)

cheers
john

Eggman · Post by **Eggman** » Sun Jan 25, 2004 8:19 pm

Okay..we MIGHT have made a breakthrough in this...

First....I am attaching my ceiling to the bottom of the floor joists using the two leaf system described many times in this forum ( layers of different thicknesses of sheetrock airtight against the bottom of the upstairs floor, insulation then RC hung from the joists) So my ceiling is ISOLATED from the walls, NOT resting on them. My walls will be sitting on top of my floated floor( ala this forum's methods) and isolated from the joists and ceiling using brackets and neoprene as discribed in the wall construction sticky.

So I have a HVAC duct along the corner in each room. In many many photos of studios on this website there are soffits just like the one I am trying to build. It looks to be a garden-vartiety soffit, just like you would find covering any ductwork in any regular room found in any home. But I want to keep the wall and ceiling from being tied together in any way, so I can't just frame in a soffit like it was run of the mill framing, can I?

I understand you diagram, but it still does not exactly address my problem. The soffits I see in all the studio photos don't have this style of support.

How about if I had just asked, " Hey John, how do I frame a soffit and have a high degree of sound isolation with the ceiling, wall, and floor being discrete leafs (leaves?) in the system.

I have an ancient computer (a model T) and I am fairly incompetent and inept at using it. I will try to see if I can draft some sort of diagram within the next few days to describe what I am talking about.

I can't tell you how much I appreciate your time and effort.

I'll be back. Doug

Eggman · Post by **Eggman** » Fri Jan 30, 2004 3:01 am

Hi John,

I am going to try to attach a crude drawing that might help. Please take a look. I am interested in the soffit to wall connection and the soffit to ceiling connection. Framing-wise that is.

As usual, thanks for your time.

Doug

Eggman · Post by **Eggman** » Fri Jan 30, 2004 3:17 am

John,
Here's one more drawing.

doug

Eggman · Post by **Eggman** » Sun Feb 01, 2004 7:55 pm

Anyone?

Eggman · Post by **Eggman** » Tue Feb 10, 2004 6:46 pm

MR KNIGHTFLY...Steve? Maybe you could help me with this.

I have spent every available minute reading, reading and rereading all the posts on this forum. I think I have a suitible design and understand the 2-leaf principle. I found most of my materials. I will post some info on a great and inexpensive Seattle source for neoprene.

That said, I still need to know how to from around the ductwork and still maintain isolarion between the wall and ceiling. If I attach the framing to both the wall and ceiling won't I be setting up a "flanking" situation, transmitting the sound from one to the other. Or should I be only concerned with the sheetrock surfaces coming into contact with each other. Maybe I am confused. I think I frustrated everyone by asking the same question over and over again but I still need to find a solution so I can begin to build.

I first knew I wanted to make records when zI saw the beatles on Ed

If you or anyone else has a minute to give me your $.02 worth it would be greatly appreciated. Thanks

Doug

knightfly · Post by **knightfly** » Tue Feb 10, 2004 6:59 pm

Doug, it's not as critical in this case as you might think - if your wallboard and your ceiling wallboard are both resiliently hung, then flanking isn't a problem if you were to build a soffit around your duct and hard mount its framing to the ceiling and floor wallboard. You mainly don't want your duct to contact the wallboard anywhere, nor do you want your inner leaf's layers of wallboard to hard contact the framing. A lot of studios have intersecting walls whose wallboard has firm contact with the other wall's inner layers - this seems to make a couple of dB difference in some cases, but it's not the end of the world.

I'm starting 12-hour nights soon, and if I get lucky I'll modify one of the drawings I posted in the wall sticky, to show you a couple of ways you can go with your soffit. I'm out of time right now, and I'll have to think about it a bit before I draw, so hopefully I'll have some detailed ideas for you in a day or two... Steve

Eggman · Post by **Eggman** » Tue Feb 10, 2004 7:27 pm

Thank you Steve. That is the info I needed.

A couple of quick questions that you can answer at your leisure.

I will hanf my ceiling layers on RC but was planning on useing metal studs for the wall. My understanding is that i won't need RC if I use light guage metal. Sooo, should my soffit be wood w/ RC or metal. or could I frame w/ plywood (no 2x framing at all) and cover with wallboard layers hung on RC. In other words, just box in with plywd and cover w sheetrock/RC?

Won't there just be one leaf between the room and the ductwork inside the soffit? Will this carry the sound throughout the house via this ductwork, thus defeating all the other effort? Do you know what I mean?

Sorry for the incomplete sentence in my previous post. My dog jumped onto my mouse and clicked on the submit. What are the odds?

Thanks for your time. The is a GREAT forum.

Doug

knightfly · Post by **knightfly** » Fri Feb 13, 2004 6:30 pm

Hey Doug, finally got to this -

Part of the answer would depend on how loud your ducts already are when the AC or heat is on. Generally though, two layers of wallboard will kill what escapes the ducts on the outside. Figuring out how to keep a mass-spring-mass condition otherwise can get really tricky. One thing you might do is to glue homosote or celotex to the duct on the outside and THEN box it in with drywall which does NOT touch the duct. That would dampen the duct and lower the amount of noise to be blocked.

Do NOT, repeat NOT, put up plywood/RC/wallboard around the duct - this constitutes a mass-spring-mass condition all by itself, and will worsen the isolation for you.

Here's a quick markup of one of the other drawings I've posted, showing one way to do this - you could also put RC on the outer corner cleat and caulk thoroughly if you want to keep the wall/ceiling from being hard-coupled at all... Steve

Eggman · Post by **Eggman** » Fri Feb 13, 2004 9:44 pm

Thank you Steve. I am mostly concerned with sound escaping through the ductwork. It is heating and air return. No AC as there is not much call for it in Seattle.

Since the ducts are located inside the "spring" of my two leaf system, won't they act as a conduit and broadcast studio sounds throughout the rest of the house? Or will the layer of celotex take care of this? Will a layer of sheetblock-like material be better. I am interested in getting the highest practical STC level possible. I'll spend a few more bucks to get there. I would like to be able to record drums or high volume guitar at midnight and have my wife and dog be able to sleep upstairs.

I ramble.

One more question while I've got your attention. Someone else asked this but I couldn't find the answer. Do I screw through my bottom wall plate, through the sill sealer and into the floated floor. Won't the fasteners couple the wall to the floor, thus defeating the whole purpose of the sill seal layer to float the wall?

....okay, two more questions...I am floating my 3/4 T&G plywood on 5/4 Tough Deck sleepers (availible at lowes) laid flat on top of 1/2 neoprene. This gives me 1 3/4" clearance between the concrete floor and bottom of the plywood. How can I keep the rock wool from touching the concrete. I read about how to weave a net to suspent the insulation, but I don't see that really working in this case and not sagging over time. If I am floating a floor over concrete in a basement, do I really need the insulation? I am doing two rooms side by side with two seperate floated floors.

Last question...really....Back to the soffits. Can/should I substitute a "hat channel" for the RC that is supporting the cleat/vertical face of the soffit?
Also, I am using metal stud walls, so can I skip the RC onthe wall/soffit intersection and screw my soffit cleat to the metal stud?

Whew...done with the questions. Take your time in getting back to me, if you need it. I am a theater tech and have been at it 12/7 for the last few weeks and will be for the next few, so I can't start building until next month. It will give me time to fix things in the design stage rather than the construction phase.

This could be the greatest website of all time. I can't express a big enough thanks for all the info I have gleaned and everyone's patience.

Doug