Is this two leaf or three?

[bcopenhagen]

Hi all-
My original post can be referenced here:
http://www.johnlsayers.com/phpBB2/viewt ... =2&t=18328

In short, it's a garage conversion to studio, 10' x 20', double-wall construction, with plywood stucco exterior, and 2x 5/8" sheetrock on the inner frame, with a combination of inside-out walls, and one "regular" wall. It's not ideal (parallel walls, small space, etc,) but it's also not 100% music studio. To "share" the space with my family, I will build a 10'x4' utility space into the front of the garage, outside the inner room, as well as a loft above that space.
The exterior has been done since may, and I've been using it as a workshop. Now comes the fun part of building the inner frame.

garage-studio-overhead.png

My simple question (I hope), is this: I will be insulate the roof rafters with open-cell spray-foam, which will be covered with the inner room ceiling. I am considering putting that insulation on the inner walls of the utility space. However, because I am required to cover that spray-foam with sheet-rock (for fire retardancy), I'm wondering if I end up with a three-leaf system.
It would be, from inside to outside:
2x4 frame with 2x 5/8" sheetrock outside (inside-out wall)
4' space
2x4 frame with 1x 5/8" sheetrock inside, spray foam between studs, plywood + siding exterior

THE QUESTION:
Is the 4' utility space big enough that it "nullifies" the effect of this being a three-leaf system? Or would I be better off to skip the spray-foam on that front wall, use a different kind of insulation, and only cover the exterior of the outside wall (plywood+siding)?

(FWIW, I am also considering building up that exterior wall with sheetrock in between studs. All other exterior walls are plywood + stucco)

Here's a diagram, if it helps: the front wall is pictured with no sheetrock, foam, or exterior coverings.
FYI, the loft will be free-standing, ie not attached to the inner room, by using hangers on the outer walls.

garage-studio-front_detail.png

Thanks a million,
Brian

[bcopenhagen]

Greetings-

I hope I didn't do something wrong for forum mojo. I'm just needing a little guidance as to the potential for this to be a 3-leaf system.

So, I guess this is a BUMP.

Thanks for any input.

Brian

[Soundman2020]

My simple question (I hope), is this: I will be insulate the roof rafters with open-cell spray-foam, which will be covered with the inner room ceiling. I am considering putting that insulation on the inner walls of the utility space. However, because I am required to cover that spray-foam with sheet-rock (for fire retardancy), I'm wondering if I end up with a three-leaf system.

Technically, yes that is a 3-leaf system, where two of the leaves are coupled. However, you did say:

4' space

Four feet is a pretty big air gap! The resonant frequency will be so low that it won't be a problem.

Is the 4' utility space big enough that it "nullifies" the effect of this being a three-leaf system?

Yes. Well, it doesn't really "nullify" it: it just drives the MSM resonance down so low that the 3-leaf thing won't be a problem.

- Stuart -