Exterior wall construction, structural sheathing

[muze]

Hi all,
I'm working on my room-in-a-room construction design, and I have a question about how to beef up my existing exterior walls while achieving a single exterior leaf (if possible) as well as maintain structural safety.

Brief overview:
I live in the Bay Area, in Northern California. I'm converting a detached 4-car garage (2-car space in front, and 2-car space behind), splitting it in half to a Control Room and Live Room. I am planning to build a room-in-a-room (MAM), and would like to achieve sound isolation for a rock band with drums. I will be playing soul, pop, R&B, jazz, and sometimes hard rock (but no heavy metal). I live in a residential neighborhood, so I'm hoping that the sound will be quiet enough that my neighbors either cannot hear within a few feet or that the sound will be quiet enough within a few feet. Unfortunately, I don't have precise dB numbers to reference. The detached garage is built on a single concrete slab, which is isolated on my side, and I'm pretty sure it's isolated on my neighbor's side as well (not connected to any of their concrete).

Exterior Wall question:
The detached garage was built in the 1950s, and the existing exterior wall is constructed using 2x4s and stucco with paper backing (half of the garage also as metal lath behind the stucco). The existing exterior wall, does NOT have structural sheathing on the outside.

Based on my research, and my own design brainstorming, I've come up with 3 possibilities for beefing up the exterior wall. Please refer to the diagrams below. I'm not sure which is the best approach.
A) Recommend approach of infilling each exterior stud bay with 2 layers of 5/8" sheetrock and Green Glue in between the sheetrock.
- Pros: Single exterior leaf
- Cons: Exterior walls still do not have structural sheathing. Will this be a structural issue? I will also be adding 2 layers of 5/8" sheetrock to the exterior ceiling.
B) Above recommended approach (A), while additionally installing 7/16" OSB to the inner face of the exterior wall.
- Pros: Adding structural sheathing to exterior wall
- Cons: Will exterior wall become 2 leaves? Or is it possible to infill the space between the OSB and the infilled sheetrock, to form a massive exterior leaf, that spans from the inner OSB to the outer stucco?
C) Pink fluffy insulation between exterior stud bays, 7/16" OSB on inner face of exterior wall, followed by 2 sheetrock layers + Green Glue sandwich on the inner side of the OSB
- Pros: Easier to install sheetrock on wall (rather than between stud bays)
- Cons: 2 leaves

Of the 3 exterior wall approaches, is there one that you would recommend (given some of my open questions), as part of a MAM design that is structurally safe and efficiently using mass for isolation? Or is there another approach that you would recommend?

Thank you kindly,
Alex

Exterior Walls.png

[Soundman2020]

I will be playing soul, pop, R&B, jazz, and sometimes hard rock (but no heavy metal). I live in a residential neighborhood, so I'm hoping that the sound will be quiet enough that my neighbors either cannot hear within a few feet or that the sound will be quiet enough within a few feet.

That's a tall order! All of those genres imply drums and bass. That's the hardest part to isolate. The loudest, at the lowest frequencies. It is possible, of course, but not easy and not cheap.

Unfortunately, I don't have precise dB numbers to reference.

You really should get a sound level meter (a good one, with both "A" and "C" scales, as well as both "fast" and "slow" integration), and do some measurements. You should also find out what your local municipal bylaws asn regulations say about how loud you can be legally...

The detached garage was built in the 1950s, and the existing exterior wall is constructed using 2x4s and stucco with paper backing

That's a good start, but you are going to need more than that.

The existing exterior wall, does NOT have structural sheathing on the outside.

You live in California. I would imagine that local code requires sheer strength in the walls, to deal with the stresses of earthquakes. How is that being handled currently? What is providing the sheer strength at present?

I've come up with 3 possibilities for beefing up the exterior wall.

Unless you are a structural engineer, you can't figure this out by yourself. You need to hire one. I would suspect that it might be possible to add gussets to the inner face of the studs, just in the corners, to provide some sheer strength. For example, he might tell you that a sheet of OSB or plywood spanning the first three studs on each end of each wall would be sufficient. You could then ask him where and how big to cut large holes in those sheets, to deal with the three-leaf issue without compromising the structural integrity. Hopefully, you can cut out large sections.

A) Recommend approach of infilling each exterior stud bay with 2 layers of 5/8" sheetrock and Green Glue in between the sheetrock.

That's excellent for adding mass, but does not do much for structural strength.

while additionally installing 7/16" OSB to the inner face of the exterior wall.

That's fine for structural (although I would go 5/8", rather than 7/16), but it screws up the acoustic isolation, since it adds another leaf.

Will exterior wall become 2 leaves?

Yes, and they will be coupled. Not a good situation, acoustically, but probably fine, structurally.

Or is it possible to infill the space between the OSB and the infilled sheetrock, to form a massive exterior leaf, that spans from the inner OSB to the outer stucco?

Infill with what?

C) Pink fluffy insulation between exterior stud bays, 7/16" OSB on inner face of exterior wall, followed by 2 sheetrock layers + Green Glue sandwich on the inner side of the OSB
- Pros: Easier to install sheetrock on wall (rather than between stud bays)
- Cons: 2 leaves

... but not much isolation, since the leaves are still coupled.

So far you have only provided details of what you plan for the outer leaf. We'd also need to see your plans for the inner-leaf, to help you determine the best overall plan. And we'd also need your numbers, regarding how much isolation you need, in decibels....


- Stuart -

[muze]

Hi Stuart,
Thanks for your reply. I've been reading your forum postings, as well as Rod Gervais' book, and they have been very educational.

I do have a dB meter, which I use to make sure my monitor levels aren't too loud. The inside volume will be a maximum of 110 dB, and I would like to reduce it to 30 dB within a few feet outside of the structure. I know 80 dB is a lot of reduction, so if 70 dB reduction is more realistic, then I think I would be okay with that.

The neighborhood I live in doesn't have a noise ordinance, but the city I live in does have a noise ordinance. However, it doesn't specify a max decibel level, only prohibits anything "unreasonably loud". If I can reduce the outside volume to 30-40 dB, then that should be a reasonable volume.
"5. The playing or operating of any radio, phonograph, orchestra or other musical device or instrument in a manner that is disturbing or unreasonably loud to a reasonable person outside the facility or unit from which the noise emanates"

After doing more research, I realized that my exterior walls use cut-in diagonal bracing, which met the code for resisting lateral load. Apparently, this was pretty common for houses built in the SF Bay Area in the 1960s (thread below). This garage withstood the 1989 Loma Prieta earthquake (6.9 earthquake), so the cut-in diagonal bracing did its job according to code.
https://www.reddit.com/r/HomeImprovemen ... behind_it/

I don't know how adding infill sheetrock to the exterior stud bays and sheetrock layers to the exterior ceiling, will affect the efficacy of the cut-in diagonal bracing. Maybe the additional mass to the walls and ceiling will provide some slight additional lateral resistance, or maybe they will contribute to additional lateral load. This is probably a question for a structural expert.

I will be working with a contractor who knows a structural engineer that we will bring on for consultation. I will rely on the structural engineer's final recommendation regarding safety and load, but I'm also interested in hearing how others have solved similar design problems, as a reference.

I will upload a more detailed plan soon, including inner walls.

Thanks so much!
- Alex

[Soundman2020]

inside volume will be a maximum of 110 dB, and I would like to reduce it to 30 dB within a few feet outside of the structure. I know 80 dB is a lot of reduction, so if 70 dB reduction is more realistic, then I think I would be okay with that

70 dB reduction is still pretty big! That's about the limit that you can expect from a project studio, provided that it has a very succulent budget. That's going to take some doing. A typical house wall will give you around 30 dB of isolation. a goal of 70 dB means that you have to stop ten thousand times from sound energy from getting through. Yes, literally 10,000 times more. Each time you go up the scale by 10 dB, you need to block ten times more energy. From 30 to 70 is four steps of 10 dB: 10 x 10 x 10 x 10 = 10,000. So aiming for a reduction of 70 dB is a really tall order. Most home studio builders are happy to get 50 dB (blocking 100 times as much energy as a normal wall). Some set a higher goal, and invest a lot more time and money in very careful design and construction, and are ecstatic if they hit 60 dB. But hitting 70 dB needs very deep pockets, and major attention to detail. That's pretty much the flanking limit for a concrete slab on grade.

I'm not trying to discourage you! Not at all. I'm just pointing out the realities of achieving very high levels of isolation. It can be done: right now I'm working on the design for a place where the design goal is 75 dB, and we should be able to achieve that. But in order to get there, there's a good budget in place with enough zeros on the end, because we will be floating each room individually on tuned springs.

I would suggest that you should probably set your sights a little lower. 55 to 60 dB is a realistic goal for a typical home studio on a reasonable budget.

How far away are your neighbors from the outside walsl? If you have the advantage of distance, you do get a roughly 4 to 5 dB decrease in intensity each time you double the distance. So if you have (for example) a level of 50 dBC at a distance of 5 feet away from your wall, that would be down to 45 dbC at 10 feet, 40 dB at 20 feet, and 35 dB at 40 feet. So if your neighbors are 40 feet away, then you'd be fine with 50 dBC outside your studio.

Since your goal is so high, you are going to need to design and build very, very carefully. Attention to detail in the calculations and the planning, will be key, followed be careful supervision of the actual construction. There's no room for error when you are aiming so high. Even a small crack under a wall that somebody forget to seal can trash your isolation. At this level, a crack just a few feet long and only 1/32" wide will cost you 10 dB of isolation, easily. Perhaps more. Designing the HVAC system for such high isolation is also going to need very careful attention. Silencer boxes with 60 dB insertion loss are not easy to do.


- Stuart -

[muze]

Thanks Stuart. I’ve lowered my isolation goals to 55dB to 60dB for my project studio, and also made some design compromises, because of practical budget reasons. I live in a residential neighborhood, so my garage is adjacent to the backyards of my neighbors, and unfortunately about 10 feet from one neighbor’s in-law unit.

I’ve decided to add plywood to the inner face of the exterior wall, because I want the additional lateral resistance. Unfortunately, this makes my exterior wall and inner wall into a 3-leaf assembly. I’m aware that I may need to compensate in other ways (more mass, and/or increased air gap between walls), to make up for some of the shortcomings of a 3-leaf assembly, but I’m not sure exactly what changes are worthwhile to make.

Below are my wall and ceiling descriptions, for both Live Room and Control Room.

Walls
- exterior wall: stucco, 2x4 frame infilled with insulation, 1/2" plywood, 5/8” drywall, Green Glue, 5/8” drywall
- interior wall: 2x6 frame infilled with insulation, 1/2” plywood, 5/8” drywall, Green Glue, 5/8” drywall
- 3” space, between exterior wall’s innermost drywall layer and interior wall’s frame
(3” + 5.5” = 8.5” air gap between exterior and interior walls)
- 6” air gap between Control Room’s inner wall frame and Live Room’s inner wall frame
(6” + 5.5” + 5.5” = 17” air gap between Control Room’s inner wall and Live Room’s inner wall)

Ceilings
- exterior ceiling: roof, plywood, 2x6 frame with no infill, 5/8” drywall, Green Glue, 5/8” drywall
- interior ceiling: 2x6 frame infilled with insulation, 5/8” drywall, Green Glue, 5/8” drywall
- large space between the exterior ceiling’s innermost drywall and the interior ceiling’s outermost drywall (exterior ceiling’s drywall will be installed along the slope of the roof, interior ceilings drywall will be installed flat)

I’m not sure what space I should leave between the exterior wall and the interior wall. I chose a 3” space, resulting in 8.5” air gap, because I remember reading somewhere that an 8” air gap is a good rule of thumb. Or should I make this air gap bigger to compensate for the 3 leaf assembly?

The large air gap between the Control Room’s inner wall and Live Room’s inner wall, is because I have a 4” wide load bearing beam in the center of the garage, so the Control Room’s inner wall and Live Room’s inner wall will be separated by the 4” beam plus 1” space on each side (6” total space, then adding in the 2x6 frame cavities = 17” air gap).

I realize my walls have more mass than my ceilings because of the 2 plywood layers, but since the ceiling has a larger air cavity, I thought perhaps the increased air cavity might make up (to a certain degree) for the lesser mass. I was concerned with overloading the ceiling with plywood in addition to the 2 layers of drywall, as well as losing some ceiling height, so I omitted the ceiling plywood. I'm going to end up with a low 7' 10" ceiling, which is not ideal, but I'm okay with it since a vaulted ceiling will be much more expensive.

Thanks!
- Alex