HVAC Soffit construction?

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

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

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

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

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

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

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

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

John, Here's one more drawing. doug

Anyone?

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

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

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

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

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

"I am mostly concerned with sound escaping through the ductwork. It is heating and air return. " - "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." - Yeah, people in Hell want ice water too... :cry: Seriously, I'm not making fun of you, it's just that that much isolation requires a "total package" approach - if your ductwork is plain old house construction, you may need to replace it with ducting that is designed specifically for sound attenuation. High isolation requires multiple bends and absorption INSIDE the ducts, as well as a much larger cross section. This is because the noise that is generated by moving air is proportional to the 4th or 5th POWER of the air velocity. So a slight reduction in air velocity (larger diameter duct) results in a large decrease in sound generated. Also, sound WILL "swim upstream" almost as easily as "downstream" - think about it - typical air velocities for ductwork are around 300 feet per MINUTE - sound travels in air at approximately 1130 feet per SECOND. So upstream sound may only travel at 1125 feet per second, instead of 1130... One possible "fix" for your ducts - if your ducts run parallel to the ceiling joists, it may be possible to get your "mass-air-mass" construction ALL between the duct and the studio - I would need to see a drawing showing the duct cross-section as related to the ceiling framing to know if this is possible. Your floating floor will have only a couple inches of air gap, so won't attenuate possible flanking noise as well as one with 4-5 inches of gap. The insulation in the floor isn't there for heat, it's there to break up sound waves and lessen the coupling between the floated floor and the concrete. Once sound gets into the concrete, you're screwed - it will conduct all over the foundation and framing, and re-radiate multiple places in the house. This is why successful floated floors are at least heavy wood (like 2" or more just for sheathing) or concrete. Where you're floating two separate floors, first I would make them thicker and heavier and with a larger air gap if possible - second, I would make each floor different in mass/materials than the other. This is because any sound that flanks through one floor into the concrete will be less likely to share sympathetic resonance with the other floor if they have a different resonant frequency. Remember, the ultimate goal of sound isolation is to get all surfaces resonant at sub-audible frequencies - this can only be done with mass and air space. The closer you get to this goal, the less coupling between rooms. If walls are floated on a floated floor, there is no need to float the wall on the floor - the function of "floating" is to isolate inner leaves from structure in order to minimise flanking noise. That would be accomplished by the floor suspension, as long as the walls aren't allowed to have hard physical contact with structure. There are sway braces available that handle this last part. Also, light gauge steel studs or Resilient Channel will further isolate the inner wallboard leaf from structure. Doors and windows are another source of disappointment in sound proofing - doors need to be hermetically sealed when closed, and double doors with a sound lock (think foyer) are necessary for anything more than STC 45-50. Overly makes one door that's rated at 55 dB if I remember correctly - it only sells as a complete frame and door and seal package, and it costs just over $5,000. To get close to that, you need tandem doors, each with a good drop threshold and well-made seals. "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?" - Hey, that last question was two questions - :wink: First, you could do that - note, however, that I drew the RC's pretty close together - if it works out that the normal RC position is relatively close to the soffit, I would just add an extra RC right where the cleat needs to be. The extra weight of the soffit won't be any more than if you continued horizontally with 2 feet of double drywall. In addition, the hat channel isn't as resilient as the single legged RC (be careful here though - at least one company (Dietrich) has two different grades of RC, only one of which is rated for ceiling use) Screwing the cleat direct to steel stud - Yes, you can do that - I'm hoping you're using the light 25 gauge studs, otherwise RC will be needed. Just remember to make sure you caulk everything thoroughly with REAL acoustic rated caulk - even over the net, the large tubes of that aren't much more expensive per unit than the small, WRONG ones at Home Depot. As I said previously, if you really want to isolate ceiling from wall, you could insert a strip of RC where the outer corner cleat is drawn, then caulk thoroughly. This would break the hard path between the vertical soffit wall and the horizontal soffit bottom. Be sure and download the USG handbook while you're at it, lots of valuable info in that - see the "stickies" section. Don't get "gunshy" about asking questions, I've been doing this long enough to know you're nowhere NEAR through asking questions :wink: ... Steve

Steve, I just completed rereading every single post on the construction forum. I have access to the internet in the control rrom at work. We are doing multiple runs of a very slow low tech show. My brain hurts. I answered some of my own questions but a few more popped up. I have the space to build a typical foyer type sound lock. Roughly a 6'x6' room with a door to the control room and a door to the live room. 1. Should I isolate the floor of the foyer from the live room and control room (each will have its own floor)? 2. Should I build my double walls between the foyer and LR CR and then build another double wall at the entrance to the foyer? This would seem violate the mass-space-mass rule by having another mass or m-s-m so close, in effect setting up a triple or quadruple leaf situation? I am trying to build with new condition salvage materials from several local re-use sources. i have found screaming deals on T&G ply and T&G OSB at $4/sheet and new 2" solid doors surplused from a hospital for $10 ea. i also located about 20 4'x8'sheets of "2 compressed fiberglass for a few bucks each. I don't know enough about this product to know if it is usable. It is foil backed and has no brand name or info anywhere on it. Can I use this beneath my floor instead of rockwool. I'm sure I CAN use it, but is it better or worse than rockwool? Okay, enough questions. As usual..thanks. Take your time with any answers. I'll keep reading and researching. BTW, like you said, those people in hell want icewater, but they probably would make do with a cup of hot chocolate as well. I'm going for an ice water studio, but I'll dig the heck outta a hot chocolate one. Doug

(1) Yes, definitely. You do NOT want the floors to flank noise between rooms. (2) The sound lock should be NON-parallel walled if possible, the doors should NOT be parallel to each other (preferably at right angles to each other), and the lock should be heavily absorbed. One way to do it without getting into a multi-leaf situation is to put triple layers of wallboard on all walls that border the lock, then heavy absorption in frames and then either perforated metal and/or cloth over, so that the entire interior of the lock is absorbed but that you still have two leaves between any two areas when the lock is closed. One thing that doesn't seem to be clear yet, probably because I've not stressed it since it doesn't apply in the majority of cases - If you look at the attached graphic, you'll note that TWO identical walls (STC40 example) are NOT WORSE than one, just not as efficient use of materials as the STC63 example, which uses the exact same amount of materials and almost the exact same amount of wall thickness (+ two layers of wallboard) - So multi-leaf walls are not total disaster, just not as efficient (in most cases) as using the same materials in different designs where there are only two leaves. I'm still working on a graphic explanation of this, but basically in my mind building double leaf walls on each side of a sound lock actually makes MORE sense. I'm not quite ready to clarify that, but will as soon as I get time to finish the graphic examples and text layout. Stay tuned... Steve

Steve, Thanks for the latest input. I have been hunting/gathering materials and have completed the tear-out of the sixties era basement treatment. I hope to begin building soon. Questions: 1. I was looking at the pix of the Blue Bear Sound project and noticed the HVAC ductwork is very much like that in my basement. They seemed to have wraped it with something before framing. Could you take a look and tell me what it is? Sheet block? I will spring for the extra $$ if it will help any. Do you recomend a layer? 2. As I was searching for a local source of acoustic sealant I ended up on the phone with an acoustic consultant and he said that they use 20 year non-hardening silicone. You've cautioned against Home Depot/Lowes.etc cheap caulking products but (one more time?) can I use this? As an aside, I am 48, so if I get 20 good years out of the silicone (and consequently the studio) I'll be happy. 15 yrs in a road band insured my ears will give out by the time I'm 68, but that's another issue. Thanks, Doug

I'm not able to tell from the pix what Bruce used to wrap his ducts - maybe he'll check in soon and you can ask him - or you could PM him, and that might speed things up. This is one case where the expensive sheet blok would be warranted - if fastened to the metal ductwork with contact cement it would damp the vibrations of the metal and cut the noise level both directions, probably by quite a bit. The phrase "20 year non-hardening silicone) could also be used to describe actual acoustic rated caulk, so it's kind of ambiguous as to whether your acoustic guy meant the HD kind of stuff or actual acoustic caulk. The real acoustic stuff is "floppier" than the regular stuff, in addition to NOT ever hardening. Even buying it on the net isn't any more expensive than getting the NON acoustic stuff at HD - the tubes are more expensive, but they're quart size instead of 10 ounces. Here's one link if you can't find it locally - local sources would be possibly commercial drywall contractors - http://www.acousticalsolutions.com/prod ... /caulk.asp You may be able to contact Titebond directly and see if they have local distributors you can deal with - reports of local purchase typically run around $4 to $4.50 per 29 oz tube - even though HD doesnt' carry the caulk, you CAN get the large caulk GUN at HD, they run about $14. Personally, I'm 58 and I STILL wouldn't use the wrong stuff. All it takes to ruin a sound wall is a 1/64" crack a foot long, caused by a marginal caulk job shrinking and pulling away from a surface. Not worth the chance unless you want to build stuff for the rest of your life instead of USING it... Steve

Hi again...I don't know if it is proper to just keep appending my origional post with new questions or if I should start an entirely new post for each subject. I am getting ready to lay my sleepers (2 x 4 laid on their flat side), fill with rock wool, and cover with T & G plywood to build my floor. I am having a real delima over putting down a vapor barrier or not. I read the basement science info and they basically said don't build anything in any basement. Well, my basement is the only space available, so i'm going to put my studio there. The floor and walls are bone dry, even after weeks of rain. I can't bring myself to just lay the insullation on the bare concrete. A number of RSD forum's readers have sucessfully built basement studios. My question is: Should I lay down a vapor barrier or not? And reasons. Thanks in advance for your wise help. Doug

Continuing this thread is fine - few threads stay focussed all the way through anyway. Generally, you want things that MAY get wet to be ALSO able to DRY. Rockwool shouldn't need a vapor barrier, but your floor sheathing probably should have one. Any wood in contact with concrete should be pressure treated, so if you're laying your 2x4's flat on the concrete they should be pressure treated. If they will be floated on neoprene or EPDM, it's not as vital. Either way, you can put the rockwool between them touching the concrete, then lay Tyvek house wrap over the whole thing and put down your floor sheathing. The tyvek is able to breathe somewhat, but will still protect the underside of your wood floor material from excess moisture. From your description, it sounds like the outside of your basement was done in a good enough way to keep excess moisture from being a problem. Lucky you... Steve