Green Glue Question for Exterior Wall

I'm in the beginning stages of building a Workshop addition to my house, for woodworking. I've done a great deal of research about sound control construction methods, but I still have a couple of very specific questions regarding my situation.

I realize this forum is dedicated to HT construction, but almost every aspect of my workshop soundproofing efforts is just as applicable to Home Theater sound control. In fact, there are two reasons I would appreciate being able to ask for advice/opinons here: 1) I kept getting hits on this forum when Googling issues I still need to clarify, and 2) even though my specific questions haven't been addressed here yet, there is a lot of very helpful advice on this forum, and I'd really appreciate being able to tap into that knowledge.

I'm also more than willing to offer my two cents on any construction/soundproofing issue I can.

If the focus on HT is iron-clad and moderators believe workshop noise abatement techniques won't translate to Home Theater noise abatement, does anyone know of another site with forums targeted to my situation? Soundproofing is such a specialized area, I haven't had any luck finding one so far.

Thanks in advance! Great information on this site regardless.

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!

Also, one other thing: I'm using as my real-world example the workshop of a friend who is a retired professional cabinetmaker, which he built in his (much larger) back yard as a literal room within a room. He built a large exterior shell, inside of which is a 4' wide perimeter walkway completely surrounding the interior building. He uses the walkway for wood and tool storage, which adds to dampening effect. The inner structure also completely seals off with only one large doorway, on a different wall than the exterior door.

This is no exaggeration: with dust collector and a 20" Felder planer running under load, with the doors closed, you can stand 3 feet away outside the structure and hear absolutely nothing. Not a low hum, or faint whine, or anything. He built the shop 20 years ago, after a lot of research into the Sound Control methods of the time, so I'm hoping that with improved materials and knowledge, I can achieve something even close, in a much smaller footprint.

Is the reason you need to build an enclosure for the dust collector because the larger shop itself isn't soundproof enough to contain the sound?

I've been keeping my fingers crossed that my shop ends up being soundproof enough to contain the noise of the dust collector (the loudest machine in most home workshops). Otherwise, I'll be in the same boat as you, and looking forward to the answers to your question.

So far, suggestions I've received as sure-fire solutions for DC and air compressor (also loud beasts) are these: a high-end, high-dollar DC that generates less noise in the first place through fancy engineering. And a rotary screw air compressor, which are unbelievably quiet. And unbelievably expensive.

NICE !!

Could you run down the materials used in the muffler in your picture? (Great second picture for size reference )

As a friend pointed out very early in my planning: if my goal is to make the shop as air-tight as possible, I will most likely need some kind of air exchange, which I plan to do on the side that is attached to my house.

Thinking of doing it through floor, between joists and up through wall cavity, so as to isolate the intrusion as much as possible.

I've seen diagrams of that kind of ceiling/roof construction. Kind of like a staggered stud wall, but horizontal, and not connected by bottom and top plates.

My thinking for the totally separated ceiling and roof joists: I figure there are two reasons a double stud wall has higher STC rating than a staggered stud wall: 1) the DS wall doesn't have the shared top and sill plates to transmit sound like a SS wall, and 2) the DS wall has greater separation between one leaf and the studs of the opposite leaf.

The "Staggered Joist" ceiling you mentioned takes care of one of those problems (no shared support members), but doesn't separate the leaves and joists as much as a "Double joist" assembly would (where ceiling and roof joists don't overlap in the horizontal plane).

Hopefully someone more knowledgeable than me can give an opinion as to whether my proposed assembly would reduce sound transmission enough to be worthwhile? I'm operating under the principle that every little bit helps. There's no added cost or labor to build the separated roof/ceiling, but if it gains absolutely nothing, then headroom would be a consideration.

* * *

I realize there are already several issues in discussion, which I really appreciate (the more information the better). And that is what drew me to this forum in the first place. I think people dealing with HT and industrial noise are far more thorough, detailed and scientific than people dealing with workshop noise, because it is automatically an integral part of the HT design process.

But your post reminded me of an additional question I've always had, so I'll throw it in: Why aren't the studs staggered in a double stud wall? Why are they always shown in line with each other? Would it hurt the STC rating to stagger them? Or just have no effect?

Thanks again for all the replies so far. It's great to find a forum where the members have so much knowledge and experience with the issue.