blown cellulose insulation & air cavity between double w

I've done a couple of hours of net searching and haven't found anything definitive yet to guide me in this area... My drum practice room/studio has double wall construction. The inner walls are steel studs and the outer walls are wood studs. The room's walls will have two layers of 5/8" sheetrock with Green Glue. The floor is currently two layers of subfloor with GG. The ceiling will be one layer of 5/8" rock on RC. The exterior wall, under the floor and on top of the ceiling will have blown cellulose insulation. For one side of the room, the opposing double walls are only about an inch apart. Do I need to preserve this air gap or can the entire double wall structure be completely filled up with blown cellulose insulation. (It's the somewhat sticky type that shouldn't settle if it matters.) If the air gap needs to be preserved, is there a scheme for placing a set of batts where they won't fall out into the air gap? FWIW, the sub's price to blow the cellulose insulation in the whole house was cheaper than using fiberglass batts. Thanks! --David

Blown insukation is bad. IR-761 wikll help you on hta other questions. Warning though, over 350 tests and lots of tech data. http://irc.nrc-cnrc.gc.ca/fulltext/ir761/ir761.pdf Andre

AVare wrote:

Blown insukation is bad. IR-761 wikll help you on hta other questions. Warning though, over 350 tests and lots of tech data. http://irc.nrc-cnrc.gc.ca/fulltext/ir761/ir761.pdf Andre

IR-693 shows cellulose insulation being generally comparable with glass or mineral type fibers and I've come across a fair amount of information that either likes and dislikes blown cellulose from an acoustic standpoint...there are other considerations beyond acoustics for using the cellulose. My question is about preserving the air gap (whether using insulation batts or blown cellulose.) :) --David

My cursory look IR-761 also seems to show sprayed cellulose insulation to be comparable to the usual glass/mineral fiber batts. I've got the sprayed instead of blown stuff lined up which may not have been clear in my initial post when describing it as the sticky stuff. --David

The difference is small, but definite. As long as you are aware of it. The gap is considered inportant help to prevent transmission of vibration from one leaf to the other. Andre

The type of blown cellulose you seem to be referring to gets a bit stiff after installation, so will tend to "bridge" between frames, which will lessen the isolation somewhat; the heavier the bridge, the more the frames will act like single frames - so best case, no difference between blown cellulose and rigid wools - worst case, 8-10 dB difference (assuming a REALLY bad buildup stuck to both frames) Depending on your situation, you might be able to place some of that "ground cover mesh" used in gardening across your studs, staple it in place, then blow the cellulose in thru small holes you make in the mesh every few feet - you don't want to use NON-perforated material like Visquene (poly) film, because it will act as a membrane and create a third leaf effect (worse bass isolation) I've not done this, so can't be much more specific... Steve

What about the blown white fibreglass / type insulation? It looks like cotton. They shred up these big loafs of fibreglass and blow it in place. I had about 10 inches added in my attic last year and it made it much quieter in the house from outside sounds plus my energy efficiency went way up. I had thought about having it blown in to fill all the voids after I am finished building. I am still putting batts in the walls and ceilings, this would just go on top of that. Tom

That's a dry process and doesn't pack too tight, so should work fine... Steve

David, Tis a bad move ye be making here. If you re-check the data you looked at you'll notice that the walls using this material had only partial cavity fill - the material never touched both wall surfaces. If you do what you're proposing it will significantly lower your isolation values. Rod

knightfly wrote:

Depending on your situation, you might be able to place some of that "ground cover mesh" used in gardening across your studs, staple it in place, then blow the cellulose in thru small holes you make in the mesh every few feet - you don't want to use NON-perforated material like Visquene (poly) film, because it will act as a membrane and create a third leaf effect (worse bass isolation)

Using a mesh of some sort was our original thought and is what we intended to do on another set of double walls that were about 8" apart. I guess we can use a mesh on this wall as well as I'd like to preserve the air gap if possible. HOWEVER :) , we're not sure at what point a mesh will start acting like a membrane. There are mesh-like products specifically made to do what we need for spraying insulation, but I doubt anyone knows how they perform from an acoustic standpoint. My original plan was to spray insulation on the outer wall and then mount "chicken-wire" on the back side of the inner wall to hold fiberglass batts in place against the room's sheetrock. Do you think this would be optimum in this situation? Presumably the chicken-wire is not even close to being able to act as a membrane... --David

dmjung wrote:

My original plan was to spray insulation on the outer wall and then mount "chicken-wire" on the back side of the inner wall to hold fiberglass batts in place against the room's sheetrock. Do you think this would be optimum in this situation? Presumably the chicken-wire is not even close to being able to act as a membrane...

True... But in a consultation phone call I had with Steve (a.k.a. "knightfly") a couple of months ago, I asked him about the possibility of using chicken wire across the tops of my trusses to mitigate the possibility of a careleslly misplaced foot caving into an elaborately constructed outer leaf ceiling above my attic, and he recommended not using chickenwire for that purpose due to its tendency to vibrate at low frequencies and generate some really icky buzzing sounds. Honestly, I don't see why you want to preserve the "actual air" between the wall frame assemblies. Breatheable insulation counts as "air" in the mass-air-mass formula. Why not fill the cavity with Thermafiber Sound Atteuation Fire Blankets (SAFB) - Nominal Density (2.5 pcf) or Roxul Acoustical Fire Batt (AFB) - ≥2" (2.5 pcf)? This stuff will probably satisfy your local building department's requirements for a firebreak in the void between the walls without having to permanently couple them together with lumber. That's what I plan to do on my project. 8)

sharward wrote:

Breatheable insulation counts as "air" in the mass-air-mass formula. Why not fill the cavity with Thermafiber Sound Atteuation Fire Blankets (SAFB) - Nominal Density (2.5 pcf) or Roxul Acoustical Fire Batt (AFB) - ≥2" (2.5 pcf)? This stuff will probably satisfy your local building department's requirements for a firebreak in the void between the walls without having to permanently couple them together with lumber.

As an alternative to chicken-wire, there's also wide open meshes like the orange plastic construction fencing you see on job sites....but spraying the outer wall, placing some "open" insulation and then spraying again for the inner wall sounds like a winner. I don't need to worry about a firebreak in my house which probably gives me some more options with the "buried" air gap. :) --David

dmjung wrote:

As an alternative to chicken-wire, there's also wide open meshes like the orange plastic construction fencing you see on job sites....

Isn't that stuff pretty strechable and pliable, kind of like a plastic six pack holder? If so, I wonder if that stuff would sag (and, thus, separate insulation from the wall or cause the top of the fill line to sink over time)... :rol:

I don't need to worry about a firebreak in my house which probably gives me some more options with the "buried" air gap. :)

Really? Fires don't burn in Waco? I recall seeing some video footage in the '90s that would contradict that assessment! :lol: (Hope that didn't cut too close to the bone there...) Yes, I'm a bit paranoid, and I try to spread that disease whenever I have the opportunity. ;) Anyway, stick with "tried and tested" methods. When people experiment and do things they think will work, they often do not. That Thermafiber and Roxul stuff is tested, and the results are excellent.

The construction fencing is pretty stout...way thicker than your 6 pack plastic. OSHA approved. :) Having a fire break in just one wall section would be overkill. I was getting at that I could use glass or mineral fiber that wasn't fire rated...there's plenty of other combustible materials for any incoming incendiary devices. I've only got two walls where I have to do something special...the 1" air gap wall and the 8" air gap wall. What do people normally do when space is constrained like this? --David

What do people normally do when space is constrained like this? Gently fill it with loose fiberglass, unfaced - whatever you use, it should NOT be even close to rigid. If you're concerned with the stuff falling down after the walls are closed up, you can use either the netting you're talking about or you can put light lath across the outer studs, place the unfaced thin batts, and maybe do a loose stitch through the batts and around the lath every so often with something like a sail needle, or even 9/16" T50 staples (kind of like button-tufting on car upholstery) - By "constrained", am I correct in assuming your steel inner studs are already in place? If not, I'd have the OUTER frames blown with cellulose, trim back any that sticks out much, and THEN put up your steel frames and use standard fiberglass batts there. IF you use paper faced batts, put the paper next to the wallboard side so it doesn't create a third leaf... Steve BTW, only one layer of wallboard on the ceiling??!? For DRUMS??

knightfly wrote:

By "constrained", am I correct in assuming your steel inner studs are already in place?

Yes, all studs are in-place...by "constrained" I was referring to situations where the gap between double walls was very small. What we've ended up doing is using the blown cellulose on the outer wood stud walls and mineral fiber batts for the inner steel stud walls...the consensus at this point is we won't have a problem with the batts falling out. The inner drywall will be 2 layers of 5/8". The outer wall will be either a single 5/8" drywall and/or the actual outer wall of the house (hardi-plank). Green Glue is used between drywall sheets and on the studs...we obviously won't be able to tell if the GG made a difference, but the additional cost was minimal compared to the total project. (I'm taking pictures throughout the project and will post those if anyone is interested.)

knightfly wrote:

BTW, only one layer of wallboard on the ceiling??!? For DRUMS??

:) Yes, well, I don't play very loud....usually. This is a concern...currently we'll be using 5/8" drywall with hat channels. Above me is the roof and beyond that is my in-laws. Cellulose will be blown into the attic area. I'm kind of anticipating we'll be putting up another layer of rock on the ceiling. My goal is to get is pretty far along, set up some drums and see where we are soundwise. --David

dmjung wrote:

. . . I don't play very loud....usually. This is a concern...currently we'll be using 5/8" drywall with hat channels.

Hat channels? :shock: Yikes. :shock: Read this and be afraid... VERY afraid.

sharward wrote:

dmjung wrote:

. . . I don't play very loud....usually. This is a concern...currently we'll be using 5/8" drywall with hat channels.

Hat channels? :shock: Yikes. :shock: Read this and be afraid... VERY afraid.

Afraid of what? :) I know that the hat channel won't perform like "real" RC. It's curious that cheap hat channel hasn't been thoroughly tested for it's acoustic effects in various structures. It is definitely SOP for your average contractors/architects to want to use it for sound control issues...don't know if that's just ignorance, cost-cutting or they've historically found little real-world difference between hat channel and RC in most typical applications (eg flanking overwhelms any RC benefits.) --David

dmjung wrote:

It is definitely SOP for your average contractors/architects to want to use it for sound control issues...don't know if that's just ignorance, cost-cutting or they've historically found little real-world difference between hat channel and RC in most typical applications (eg flanking overwhelms any RC benefits.)

I'm going out on a limb here, but I think it's most likely ignorance (i.e., "they look the similar, so therefore they must be similar"). My garage has hat channel currently, but I think that was to reduce spacing to 16" o.c. on my 24" o.c. trusses. So, it might be SOP to use hat channel, but its purpose may be for something other than sound control. Bottom line, if isolation is your goal, even if only partially or mildly, I would avoid hat channel and use the real stuff.

dmjung wrote:

sharward wrote:

dmjung wrote:

. . . I don't play very loud....usually. This is a concern...currently we'll be using 5/8" drywall with hat channels.

Hat channels? :shock: Yikes. :shock: Read this and be afraid... VERY afraid.

Afraid of what? :) I know that the hat channel won't perform like "real" RC. It's curious that cheap hat channel hasn't been thoroughly tested for it's acoustic effects in various structures. It is definitely SOP for your average contractors/architects to want to use it for sound control issues...don't know if that's just ignorance, cost-cutting or they've historically found little real-world difference between hat channel and RC in most typical applications (eg flanking overwhelms any RC benefits.) --David

Being direct, read the the thread including this link: http://www.auralex.com/sound_isolation_rc8/RC_Page.PDF It explains how RC works, and the importance of the sides and holes in the sides. After reading that, you will understand why regular hat channel by itself is not of use in recording construction. Andre

dmjung wrote:

It's curious that cheap hat channel hasn't been thoroughly tested for it's acoustic effects in various structures. It is definitely SOP for your average contractors/architects to want to use it for sound control issues...don't know if that's just ignorance, cost-cutting or they've historically found little real-world difference between hat channel and RC in most typical applications (eg flanking overwhelms any RC benefits.)

David, I have no idea what your background is in the construction industry - but this is reality. 99% of contractors wouldn't know an isolated wall from a hole in the ground. They don't have a clue what the right products are or how to use them. That's why we have architects design buildings - and not contractors. It is NOT SOP for architects to use hat channel in place of RC for sound control. Any architect who does that should be shot and then have his liscense revoked (in that order). What in the world makes you claim that this is SOP? Perhaps you should check out "Architectural Graphics Standards" - which is published by the AIA (of which liscensed architects in America are members of). It clearly indicates resilient sections (not hat sections) in areas dealing with sound isolation - and refuses to get involved in even suggestions for designs relating to studios - they suggest that architects bring on qualified acousticians before getting involved in design of that nature. I would be very interested in where you got your info that this is SOP for architects, because it certainly isn't from the architects themselves. Sincerely, Rod

That's why you should be very afraid. :shock: ;-) Thanks for contributing, Rod. David, hopefully that clears things up for you. :-)

rod gervais wrote:

dmjung wrote:

I would be very interested in where you got your info that this is SOP for architects, because it certainly isn't from the architects themselves.

Rod, You are correct...I should have said "it seems to me" instead of "it is definitely SOP" and I initially had written it to refer to only contractors and added architects at the last moment while thinking about something else. My most recent experience with architects is with the one who had our new garage being built over a sewer line. :) --David

sharward wrote:

David, hopefully that clears things up for you. :-)

You guys are killing me... :) I had already read those materials and threads on RC and hat channel awhile back and wasn't planning to use RC -or- hat channel. However, we ended up with HC on a couple of the ceilings in the house including my "music" room. (It's a long story.) Rather than bother with taking it down we left it in place and I told them to use some of the extra Green Glue on them before putting up the ceiling rock. (I know, naughty, naughty, an untested structure scheme.) I think there's supposed to be about 18" of blown cellulose up there as well. My original question was about preserving the air gap in a couple of the double wall structures. The answer seems to be that the S in the MSM doesn't have to be "air" itself. True? Presumably anything less "springy" than air would perform worse and anything more "springy" should be better? (I couldn't figure out how to induce a vacuum between the walls. :)) In any event, we're preserving the air gap. My question at this point is where are the tests using HC as I'm curious about how inefficient it is relative to RC (or nothing at all.) I see tests using RC, but haven't seen anything on HC which is presumed to perform worse than RC (and I agree that it should.) OTOH, it would seem that anything (could be additional structure elements or even just a construction method) that reduces the total physical contact area between structures should reduce vibrations being transmitted between structures at the contact points...or more likely its a function of total contact -pressure- between the structures instead of area? In any event, the RC is additionally efficient due to its "springier" nature and the HC less efficient due to its maintaining a stiffer link between the structures. True? Untrue? Untested/unknown? It's magic--don't bother trying to think about it? I really suspect the HVAC will be where the isolation breaks down as I couldn't swing the idea of a completely separate system... :cry: --David

David, hat channel is designed to replace furring strips and is functionally at LEAST as stiff as a solid board. If you followed the link Sharward gave, I posted a horror story http://www.recording.org/modules.php?na ... ic&t=22469 Had Bob's contractor had a CLUE, the correct stuff would have been used and the problem solved. As it was, we left Bob trying to choose between suicide and homicide. That's enough "proof" for me, dunno about you... Steve