I was reading a thread over at Gear Slutz regarding "inside out" walls. Someone pointed out that they didn't provide the same quality of isolation as a conventional wall with drywall on the inside of the room. They felt this was due to not having the same size air space or the insulation in the cavity between leaves.
John's diagrams show insulation being glued to the outside face of the inside out wall. Assuming there's also good old pink stuff in the outside wall framing in addition to the 703 and for the sake of argument the same air space cavity between the two drywall leaves, is there any reason this wouldn't provide the same degree of isolation as a conventional two wall structure?
Inside Out Vs. Conventional Walls
Originally posted at johnlsayers.com, topic 14481.
Sorry to say, but that "someone" has no idea what they are talking about! There is no place in the equations dealing with MSM resonance for indicating which side of the mass you happen to have the studs on. It just does not come into the picture at all. It has no effect on isolation, one way or another. The isolation in an MSM wall is based on three basic parameters: The amount of mass in each leaf, the distance between the leaves, and the amount of insulation in the air gap. Period. Studs just aren't there. (Yeah, OK, air density, temperature, humidity, etc., also matter, but I'm talking about the construction of the wall itself: the things that you have control over.).Someone pointed out that they didn't provide the same quality of isolation as a conventional wall with drywall on the inside of the room.
Why would you not have the same size air space, or leave out the insulation? I've never seen John saying that you should not put insulation in the air gap! And in fact, all of his designs that I'm aware of seem to have insulation in the air gap. It seems like your "someone" is making assumptions about how to build an inside-out wall. Perhaps he should learn about how to ACTUALLY do it, before making such claims. And if you have a problem with needing a thinner airspace due to room dimensions or whatever, then you can compensate with more mass on the leaves. Simple. Until you get down to very small dimensions where things stop being so linear, if you halve the size of the air gap then you can double the mass on each leaf to compensate. The relationship isn't exactly that linear, but that's a rough rule of thumb).They felt this was due to not having the same size air space or the insulation in the cavity between leaves.
No reason at all! For the same size air gap, the same mass, and the same type/amount of isolation in the air gap, you will get the same isolation, regardless of how you support the mass. Period. In fact, for an inside out wall you should perhaps get slightly BETTER isolation than for a normal wall, since there are no studs in the air gap to interrupt the insulation. You can have 100% uninterrupted insulation across the entire surfaces, which can only help. The difference is minimal, of course, and perhaps not even measurable, but best case, inside out has that slight advantage (plus the real advantage of having the studs inside the room). One final point about the "someone": He might want to chat to some of John's very satisfied customers about how well their inside-out walls work... :) - Stuart -Assuming there's also good old pink stuff in the outside wall framing in addition to the 703 and for the sake of argument the same air space cavity between the two drywall leaves, is there any reason this wouldn't provide the same degree of isolation as a conventional two wall structure?
Thanks Stuart... this was precisely my thinking on the subject. Mass is mass, air space is air space and insulation is insulation...... I can't see how these factors would change regardless of inside out or conventional. Perhaps they were referring to a situation of two adjacent inside out walls with a very small air space between them leaving no room for insulation. This is not the case with the rooms I'm constructing. I've got existing rooms where we're removing the inner layer of drywall and building up the outside wall mass by inserting them into the wells between studs. A couple layers of that and some insulation and then we were going to frame inside out walls inside the existing structure. Thanks again for the info.Sorry to say, but that "someone" has no idea what they are talking about! There is no place in the equations dealing with MSM resonance for indicating which side of the mass you happen to have the studs on. It just does not come into the picture at all. It has no effect on isolation, one way or another. The isolation in an MSM wall is based on three basic parameters: The amount of mass in each leaf, the distance between the leaves, and the amount of insulation in the air gap. Period. Studs just aren't there. (Yeah, OK, air density, temperature, humidity, etc., also matter, but I'm talking about the construction of the wall itself: the things that you have control over.).Someone pointed out that they didn't provide the same quality of isolation as a conventional wall with drywall on the inside of the room.Why would you not have the same size air space, or leave out the insulation? I've never seen John saying that you should not put insulation in the air gap! And in fact, all of his designs that I'm aware of seem to have insulation in the air gap. It seems like your "someone" is making assumptions about how to build an inside-out wall. Perhaps he should learn about how to ACTUALLY do it, before making such claims. And if you have a problem with needing a thinner airspace due to room dimensions or whatever, then you can compensate with more mass on the leaves. Simple. Until you get down to very small dimensions where things stop being so linear, if you halve the size of the air gap then you can double the mass on each leaf to compensate. The relationship isn't exactly that linear, but that's a rough rule of thumb).They felt this was due to not having the same size air space or the insulation in the cavity between leaves.No reason at all! For the same size air gap, the same mass, and the same type/amount of isolation in the air gap, you will get the same isolation, regardless of how you support the mass. Period. In fact, for an inside out wall you should perhaps get slightly BETTER isolation than for a normal wall, since there are no studs in the air gap to interrupt the insulation. You can have 100% uninterrupted insulation across the entire surfaces, which can only help. The difference is minimal, of course, and perhaps not even measurable, but best case, inside out has that slight advantage (plus the real advantage of having the studs inside the room). One final point about the "someone": He might want to chat to some of John's very satisfied customers about how well their inside-out walls work... :) - Stuart -Assuming there's also good old pink stuff in the outside wall framing in addition to the 703 and for the sake of argument the same air space cavity between the two drywall leaves, is there any reason this wouldn't provide the same degree of isolation as a conventional two wall structure?
could some one point me to the Gearslutz forum on this.
I think this is it ...
http://www.gearslutz.com/board/studio-b ... kwool.html
Yup, that looks like the thread. It seems some folks over there are VERY confused about what an inside-out wall actually is, and the original poster isn't even building one, even though he called it that at first. What he is actually building seems to be a single-leaf wall with the studs on the inside.
But regardless of that, the questioning of John's concept is here:
"An inside-out wall is built exactly the same as any double wall construction with the exception that the layers of drywall are facing each other and the studs with insulation are facing the room interior. - One of John Sayers' designs. This is an excellent way to make a vocal booth...
Air space between the panels is always much smaller because there is no stud cavity to add air volume in the gap. There is maybe 2 to 4 inches of airspace between the drywall. Also, there is no sure fire way to secure any absorption in the cavity - so it is omitted, further reducing the transmission loss of the wall.
In my opinion, I would build such a wall on top of a normal double wall if the design needed such a thing. ie; Room A, gypsum board, stud with insulation, air space, stud with insulation, gypsum board ** then stud with insulation and fabric covering, Room B. This works great for a vocal booth where you need trapping and broadband absorption AND you get to have a great TL rating too."
(I underlined some of the comments, for emphasis)
(In the last paragraph, it even sounds like he might be advocating a three leaf wall, but then he goes on to describe an inside-out wall with THREE sets of studs... :shock: )
Like I said: Some folks are way confused over there...
- Stuart -
What it sounds like he is suggesting is a standard construction room, then putting another set of studs in to house the treatments.
His confusion sounds as if it is suggested that all rooms use inside-out construction, which could cause problems. As an example:
Room A-studs-gypsum-air(or space filled with insulation)-gypsum-studs-Room B.
While it can be done that way, it seems as if it would be extremely difficult. What I understood (and correct me if I am wrong) is that some rooms would be inside-out (in my case, I'm designing my control room as totally inside out), but all "mating" walls would be build as standard construction - thus guaranteeing me 4 1/2" (assuming 1" finished gap between the walls) air space for the spring.
If not, that poster seems to believe that 4 1/2" space is not enough for low frequency isolation, and that 8" air gap for the spring is what is necessary. Unfortunately, I haven't fallen back across the calculations for excited mass-spring-mass calculations (it's been a whole lot of years since college, and I've long since forgotten the formula) to determine the critical air space for myself (dependent, of course, on the mass) but it seems that the general consensus I have seen in my readings so far from you all with the experience is that 4 1/2" is enough to drop the resonating frequency below the 20Hz threshhold, so more air gap than that would be nice, but unnecessary. At a point where space is a premium (and it always is), it would just be wasting space.
Did I understand it correctly?
Pretty much. Just a couple of things to add: 1) You don't get isolation until about 1.4 times MSM resonance, and it only starts getting reasonable at about 2 times MSM resonance, so you want to aim for a frequency that puts you in that range. 2) If you need to halve the air gap then you can double the mass on each leaf to compensate. That's just a rule of thumb. The relationship isn't really linear, of course, and once you get below a certain point then it doesn't hold any more, but it should work reasonably down to at least 4 inches gap. In other words, if you would have gotten the amount of isolation that you need with an 8" air gap and two layers of 5/8 on each leaf, then you can get roughly the same isolation for a 4" air gap if you use 3 layers of 5/8 on each leaf, or two layers of something that is twice as dense as normal drywall. - Stuart -but it seems that the general consensus I have seen in my readings so far from you all with the experience is that 4 1/2" is enough to drop the resonating frequency below the 20Hz threshhold, so more air gap than that would be nice, but unnecessary. At a point where space is a premium (and it always is), it would just be wasting space. Did I understand it correctly?