Sealing the top of the air gap of a double wall? - AVS Forum
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post #1 of 7 Old 12-13-2013, 10:19 PM - Thread Starter
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My upcoming theater is in an outcropping of a one story house, which means that three of my walls are exterior walls and the space above me is the attic. The exterior walls are concrete block. I've already completely ripped out the insulation and furring on those walls, so it's currently bare.

I'm hoping to strike the ideal balance between insulation and acoustics and so I'm planning on creating a decoupled second 2x4 wall (making it a double wall) with floating joists on top. The 2x4 wall will have rigid foam insulation on one side; fiberglass in the stud bays, and two sheets of 5/8" drywall. In short, this:



In text:
7 1/2" concrete block
1" air gap
1 1/2" rigid foam insulation
2x4 wall with fiberglass in stud bays
5/8" drywall
5/8" drywall

(Maybe Green Glue in between the drywall layers? Not sure if that's necessary on the exterior walls.)

The floating joists are in the attic and so they are going to be covered with R-50 (ultimately) of blown cellulose.

But what about that air gap? I can see two problems with it, as-is. One is that the blown cellulose is bound to filter down inside of it. Second (and more important) is that any number of critters and bugs could potentially get into that cavity and get trapped there.

What I'd like to do, then, is seal off the top of that gap in some way. How can I do that without creating a notable link between the two walls and possibly negating the whole point of decoupling the walls in the first place?

Also, I'm not creating a triple leaf design, am I? I'm going on the assumption that the rigid foam is considered "air space" and not a "leaf" in this context. If I'm wrong, tell me now!


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post #2 of 7 Old 12-16-2013, 09:35 AM
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Quote:
Originally Posted by granroth View Post

In text:
7 1/2" concrete block
1" air gap
1 1/2" rigid foam insulation
2x4 wall with fiberglass in stud bays
5/8" drywall
5/8" drywall

(Maybe Green Glue in between the drywall layers? Not sure if that's necessary on the exterior walls.)

Or this:

Concrete block
1 1/2 foam with adhesive attached to block, edges and seams sealed
1" air gap
2x4 walls (drawing doesn't show top plate assuming their is one) with fiberglass
5/8" OSB (shear for freestanding wall, and easy attachment of room finishes)
GG
5/8" DW

If your concerned about inspections/ fire stops you may also want to add a compressed piece of Roxul AFB between the old top plate and the new top plate. This would also alleviate your concerns with the blown insulation filtering down between the walls .

You didn't mention your 4th connecting wall, if you also build a separate free standing wall and rest your joists on the new wall you'll have a true Room in a Room which is the best for decoupling.

Personally I would splurge for the Green Glue. Although you mention exterior walls, there is still a fair amount of ambient outside noise from neighbors dogs, traffic, planes etc. you wanna keep out.... and the GG dampening would be the icing on the cake to really benefit from your other efforts.

Happy building...
Brad


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post #3 of 7 Old 12-16-2013, 08:18 PM - Thread Starter
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Quote:
Originally Posted by KNKKNK View Post

Or this:

Concrete block
1 1/2 foam with adhesive attached to block, edges and seams sealed
1" air gap
2x4 walls (drawing doesn't show top plate assuming their is one) with fiberglass
5/8" OSB (shear for freestanding wall, and easy attachment of room finishes)
GG
5/8" DW

If your concerned about inspections/ fire stops you may also want to add a compressed piece of Roxul AFB between the old top plate and the new top plate. This would also alleviate your concerns with the blown insulation filtering down between the walls .

You didn't mention your 4th connecting wall, if you also build a separate free standing wall and rest your joists on the new wall you'll have a true Room in a Room which is the best for decoupling.

Personally I would splurge for the Green Glue. Although you mention exterior walls, there is still a fair amount of ambient outside noise from neighbors dogs, traffic, planes etc. you wanna keep out.... and the GG dampening would be the icing on the cake to really benefit from your other efforts.

Happy building...
Brad

Is there a particular reason to attach the foam to the block wall rather than the stud wall? My thinking on attaching it to the stud wall was that I'd get a thermal break from the studs while obtaining some level of air sealing.

Using OSB instead of drywall is interesting. It costs more (roughly twice?) than drywall and doesn't have the same mass... but does allow for attaching things pretty much anywhere. Good point that it would be much better support for shear forces than plain drywall, although I wonder if two layers of drywall would be more than enough.

I am, indeed, going to have a separate free standing wall on the fourth side so that I can have true floating studs! The fourth wall will be:

1/2" drywall (existing)
2/4 stud wall (existing) with fiberglass insulation (new)
1" air space
2/4 stud wall with fiberglass insulation
5/8" drywall
GG
5/8" drywall

And maybe even a third layer of drywall? I'm going to go with a dual door, too, for the airlock effect. I really want that wall to be as soundproof as is reasonable, since it is shared with the rest of the house.

As far as using GG on the exterior walls... man, I keep going back and forth on that. I think it'll come down to cost. The reason I think I may not need it is because 7 1/2" of slump block wall (heavier per sq ft than normal concrete block) is supposedly over 46 STC all by itself. Perhaps having a decoupled stud wall with double layers of drywall and no GG would still result in a total wall with STC in the 60s? Would GG maybe be unnecessary?


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post #4 of 7 Old 12-16-2013, 09:56 PM
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How loud is you environment? I think this is bordering on overkill ( sometimes a good thing).

I'm not sure OSBis needed for shear wall - this is not load bearing, the block wall is.

I'm not sure the air gap is better than filling the space with insulation panels - sound waves can build in an air gap.

There may be more potential for ceiling sound transfer than the walls in your case.

Sorry, not a lot of good advice from me - but I'm scratching my head a bit on this.
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post #5 of 7 Old 12-16-2013, 10:07 PM - Thread Starter
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Originally Posted by lespurgeon View Post

How loud is you environment? I think this is bordering on overkill ( sometimes a good thing).

I'm not sure OSBis needed for shear wall - this is not load bearing, the block wall is.

I'm not sure the air gap is better than filling the space with insulation panels - sound waves can build in an air gap.

There may be more potential for ceiling sound transfer than the walls in your case.

Sorry, not a lot of good advice from me - but I'm scratching my head a bit on this.

I don't mind a bit of overkill every now and then tongue.gif In all seriousness, there isn't a significant amount of outside noise -- my neighbor occasionally does some kind of metalwork in his driveway (20 feet away) but that's already not very loud even with no special sound control measures. It's more that I want to already have a double wall on the exterior for insulation and it won't cost me much more to really bring the sound control to the next level. On the interior, I really want to be able to blast my theater as loud as I want and for it to not be heard at all on the other side of the house. At ALL!

The inside wall is sort of load bearing in the sense that that is where the joists will be, that will be holding up the (relatively heavy) ceiling and insulation and like. I'm going to treat it mostly as as load bearing, as a result.

I'm FAR from an expert on sound (far from even a beginner, actually), but some of the stuff I've read recently claim that an air gap is pretty essential. In particular, the Soundproofing Company articles and this article specifically: http://www.soundproofingcompany.com/soundproofing101/triple-leaf-effect/

As far as my ceiling goes, I'm going to have the floating joists with two layers of 5/8" drywall with Green Glue in between and then R-50 blown cellulose on top of it (it is the attic, after all). I'm led to believe that it doesn't get much better than that for ceilings.


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post #6 of 7 Old 12-18-2013, 11:45 AM
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Quote:
Originally Posted by lespurgeon View Post

How loud is you environment? I think this is bordering on overkill ( sometimes a good thing).

I'm not sure OSBis needed for shear wall - this is not load bearing, the block wall is.

I'm not sure the air gap is better than filling the space with insulation panels - sound waves can build in an air gap.

I use the term "Shear" rather loosely in this context, no one expects earthquakes in AZ or 100Mph lateral force winds to pop up in the theater (unless you have a hellava sub). Although drywall would be sufficient, I prefer OSB to keep the walls from racking in a freestanding room, along with the additional benefit of being able to attach room finishes anywhere. The differences in mass between OSB and drywall is a commonly acceptable trade off.

The 1" air space is just the distance between the new wall and the existing wall/foam, when you put appropriately sized insulation in the stud space it typically expands to fill the entire space. The entire space does not need to be filled to reduce cavity resonance, You just don't want to over compress the insulation. So R13 is fine.
Quote:
Originally Posted by granroth View Post

Is there a particular reason to attach the foam to the block wall rather than the stud wall? My thinking on attaching it to the stud wall was that I'd get a thermal break from the studs while obtaining some level of air sealing.

Air gap between the block wall and foam and foam and interior wall divides your air cavity into two smaller ones.. in essence a triple leaf=BAD
you still get the thermal break between conditioned and unconditioned space by attaching to the block wall.
Quote:
Originally Posted by granroth View Post

As far as using GG on the exterior walls... man, I keep going back and forth on that. I think it'll come down to cost. The reason I think I may not need it is because 7 1/2" of slump block wall (heavier per sq ft than normal concrete block) is supposedly over 46 STC all by itself. Perhaps having a decoupled stud wall with double layers of drywall and no GG would still result in a total wall with STC in the 60s? Would GG maybe be unnecessary?
Quote:
Originally Posted by granroth View Post

I don't mind a bit of overkill every now and then tongue.gif

Welcome to the "Land of Overkill"!! They should post a sign somewhere.. wink.gif

A quick look from the precipice of the sound proofing "Rabbit Hole". -

To understand the benefits of Green Glue its important to understand the impact of resonance on soundproofing.

(As a side note its more meaningful to think in terms of TL -transmission loss than STC in room construction)

The mass of the assembly and the depth of the airspace are factors that determine the "resonant frequency" of the assembly. Think of resonance as mass on a spring and in this case, the spring is the airspace in the wall. This mass-spring will oscillate when forces are applied. The frequency at which these oscillations are at their maximum is the "resonant frequency" of the wall assembly, and at these frequencies even a small amount of force (sound pressure) can greatly increase the amplitude of these oscillations. Increase either the mass (increased mass requires more energy to move) or increase the airspace (reduces the stiffness of the spring) and the resonant frequency moves lower. Ideally you'd like to move the resonant frequency below the audible range, however 20 layers of drywall or 36" deep air-cavities are impractical as examples.

Decoupling improves TL in the mid and high frequencies but decoupled walls can actually perform worse at lower frequencies. The effective limit of decoupling is ~ 1.4 times the resonant frequency of the wall. For the sake discussion, say the assembly you propose has a natural resonant frequency somewhere between 60 and 80 HZ, (maybe higher maybe lower) the TL of the decoupled wall starts falling off around 80-110 HZ..... clearly where the subs start kicking in, and the TL can decrease by as much as 20-30 db around resonant frequency and below.

Resonating walls can also introduce noise back into the room, say a wall has a resonant frequency of 80 hz, any impact around the resonant frequency can cause the wall to "ring" at 80 hz

Dampening (Green Glue) helps reduce this resonance and improves the effectiveness of your decoupling efforts to lower frequencies. Dampening reduces the amplitude and time of the oscillations, robbing the system of energy. In essence dampening is critical to TL in the lower frequencies we are looking at, particularly in combination with decoupled systems.

My .02, Hope that makes some sense, maybe others will comment.
Brad

p.s. I'm in Gilbert also.
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post #7 of 7 Old 12-18-2013, 05:55 PM - Thread Starter
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Quote:
Originally Posted by KNKKNK View Post

Air gap between the block wall and foam and foam and interior wall divides your air cavity into two smaller ones.. in essence a triple leaf=BAD
you still get the thermal break between conditioned and unconditioned space by attaching to the block wall.

AH! That was the last question on my original post and one that I was really hoping to get an answer to! That is, is rigid foam considered a "leaf" or an "air gap". The former makes sense since it is rigid, but the latter makes sense because it does a terrible job of absorbing or blocking sound. I am now realizing that I don't need an answer to the question: if I put the foam against the block wall, then it doesn't matter if it's a leaf or a gap.
Quote:
Originally Posted by KNKKNK View Post

To understand the benefits of Green Glue its important to understand the impact of resonance on soundproofing.
(snip interesting stuff)

That does make sense. It's definitely an argument for using Green Glue, even if it will increase the cost a bit. We'll see if it passes the WAF.
Quote:
Originally Posted by KNKKNK View Post

p.s. I'm in Gilbert also.

With a basement?! How did you pull that off? eek.gif


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