Well thanks for the welcome!
Ok, here's what the plan is so far (opinions/critiques/suggestions welcome): I'm currently doing the prep and grading for footings. The workshop will extend into my backyard, detached on 3 sides, attached to house on 4th side. 18' x 30' exterior dimensions, 16' x 28' interior. I plan to do the absolute most I can afford (doing all the labor myself, with assistance of friends) to "soundproof", since where I live houses have about six feet between them. The goal is to minimize shop noise so that even in daytime hours, the noise is never an aggravation to the neighbors. If it miraculously eliminated noise transmission enough to enable the occasional power tool at night, that would be gravy. But I'm staying realistic.
I'm going to build double stud walls (room within a room), with each wall on its own footing. Will pour footings with the same gap between them as walls (3")for maximum decoupling. 2x4 sill plates, studs and top plates. The inner "room" will have ceiling joists which are separated from the outer room's roof joists by the same air space as the walls. Yes, I will be sacrificing a couple feet of headroom, but 9' ceilings will be fine in exchange for really good sound control. Flat roof (to minimize the overall bulk of the building).
The inner walls will be: studs (24" OC) then 1 layer of 1/2" plywood (so I can hang shelving, etc. on walls without having to hit studs), then 2 layers of 5/8" type X drywall, with Green Glue or Quiet Glue (haven't pulled that trigger yet) in between (2 tubes per panel), staggered seams, sealed with acoustical caulk. All electrical on surface, so no intrusions into walls. Self contained dust collection (no ductwork penetrating walls). No glass windows, since there is no way to include windows that doesn't lower STC rating. (My idea for openable solid panels for daylight use is the subject of a future question.)
Ok, I guess that's enough background, although if anyone needs more information in order to offer advice, I'm more than happy to provide any details, from nail spacing to number and type of tools...
Here's my first question, which I'd think would be easy, but I've had no luck finding an answer:
In wall assembly diagrams which accompany STC test results, I see double stud construction for INTERIOR walls go: drywall/green glue/drywall/studs with bat insulation/air gap/studs with bats/drywall/green glue/drywall.
However, all my walls except one will be EXTERIOR walls. So working inside to outside, once I get to the outer layer of studs with insulation, what do I use for the two EXTERIOR layers of material with damping glue between? Has green glue been tested, or is there anecdotal evidence about its effectiveness between, say, two layers of Hardy board instead of two layers of drywall? If it DOES provide damping between two layers of exterior panels, will the final finish affect its damping properties? I'd planned on using stucco for mass, unless that would make the outer layer of the green glue sandwich too unequal to the inner layer, and reduce its effectiveness. (My understanding is that Green Glue has to be sandwiched between to identical layers to reach maximum effectiveness.)
Also, thanks a lot for the links to other Soundproofing-related forums. I will look through the links.
Thanks in advance for any and all comments!