Apologies in advance for the long wind...

Planning a ground-up residential outbuilding recording studio build. Zone 5, central Massachusetts. The name of the game in sound isolation is mass, decoupling, and air sealing. Two of these concepts will be very familiar to this crowd.

As we know, wherever air goes, moisture AND sound go as well, thus excellent air sealing everywhere is standard practice. For example, surface mount electric inside to prevent any holes in the walls, acoustic caulk at all perimeters of drywall, offset drywall seams, no windows at all, etc...

Double walls prevent thermal coupling AND vibrational coupling, so if sound shakes the inside wall, it won’t shake the outside wall as well if they’re completely mechanically separated. Even the fireblocking above the wall will be resiliently mounted to prevent coupling.

The third and final macro concept to sound control is mass and location of mass. When walls vibrate, sound is transmitted through them. The more massive something is (a wall in this case) the harder it is to make vibrate. Standard studio construction practice is multiple layers of 5/8” type-x heavy drywall, as far from each other as possible, maximizing air space.

The final concept also requires that you must construct with ONLY two ‘leaves’ (massive partitions), lots of mass on the outside face and lots of mass on the inside face. No significant mass layers in the middle....This relates to resonant frequencies of the cavity/air space, and this is where things get tricky.

Squaring the acoustic needs with green building takes some work. My expertise is in sound isolation, not building science, so I’m hoping someone will chime in with “please don’t do that or your walls will rot/fall down/make your life horrible, etc.

My plan is as follows.
Building is 32x42 with 12’ walls and 6/12 roof-

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ROOF

Zone 5 ‘Nature’ - Shingles - Mento 3k Connect - 23/32 Ply - 23/32 Ply - 3” ccSPF - 6” netted cellulose - **1 to 8’ of air space - 6” cellulose - 5/8” drywall - 5/8” drywall - Latex Acrylic - Studio Space

**Half of the building will employ scissor truss, with the finished fully-decoupled ceiling rafters supported by inside walls, built as close as possible to the roof for maximum ceiling height. The other half will have standard attic truss, with a finished ceiling below that (12’ above grade), so the distance between the roof and ceiling varies from around 1.5 at edges to 8’ at the peak of the roof.

The cellulose is in contact with the outside of the inside wall for damping of the structure.

Issues that concern me-
1. For ROOF condensation control, Lstiburek wants to see 40% air impermeable insulation directly on the sheathing, the remaining 60% to be air permeable. There isn’t usually so much uninsulated air within the assembly, is that going to cause issues? At the peak, we’re looking at an additional R-27 from air alone...?
2. I need at least R-49 for the roof. Is it OK to keep ~R-20/ 3” ccSPF and ~R-24/ 6” cellulose right under to the roof, and another ~R-24/ 6” cellulose on top of inside ceiling? I know this makes the sheathing real cold, but with the ccSPF fully covering the entire underside of roof, air and vapor can’t get to it from the inside.
3. To keep my mass layers heavy enough to stop sound, I have to double up on already thick sheathing, one layer of which will be taped for complete air sealing. Plywood has a not entirely favorable permeance already, so if we want the assembly to dry to the outside, are we hosed by the near 1.5” thick plywood?
4. I’ll have HVAC system installed in the air space formed by the attic truss. I’ll have issues if it’s also not totally air tight, right?

References-

https://www.buildingscience.com/documents/digests/bsd-149-unvented-roof-assemblies-for-all-climates

—————————————————————
WALLS

Nature - 1/2” T1-11 - 3/8” furring - Mento 1000 - 23/32 ply - 23/32 ply- 2x6 24OC w/cellulose - Intello Plus - 3” air space w/cellulose - 2x4 16OC w/cellulose- 5/8” drywall - 5/8” drywall - Latex Acrylic - Studio Space

For all double walls, the distance between walls will be consistent, without extra attic space like the roof. Can construct in a way to get 100% fill with cellulose.

Wall issues that concern me-
1. That 1.5” thick ply again on the exterior.
2. I see that in zone 5A, the test wall (from PDF below) with 12” dense cellulose isn’t working to control sheathing moisture, so they recommend an internal vapor barrier. It will be easiest to seal if it goes up right behind drywall, but I could also put it on the inside of the outside wall (also easier to seal) or the outside of the inside wall (not as good a seal...) is this vapor barrier a good idea and where does it go in the assembly?

References:
https://www.nrel.gov/docs/fy16osti/65147.pdf
https://www1.eere.energy.gov/buildings/publications/pdfs/building_america/monitoring-doublestud-wall-northeast.pdf

If you made it this far, I appreciates you! Go green!

Sounds like a lot of complexity and work. Why don’t you just do what the professionals do for new indoor theaters...use ICF? I suspect it will be a lower cost and higher performance solution.

Posted By sailawayrb on 24 May 2020 07:12 PM
Sounds like a lot of complexity and work. Why don’t you just do what the professionals do for new indoor theaters...use ICF? I suspect it will be a lower cost and higher performance solution.

I should have mentioned that I’m building this all by myself. Stick framing I can handle, ICF forms not so much. Materials alone will cost more than stick framing, factor in skilled labor and it blows up. My labor is remarkably cheap. Also, even an 8” core ICF with a resilient channel layer on the inside can’t compete at low frequencies with a traditional high mass double wall. If I framed an isolated 2x 5/8” drywall on the other side I’d be able to have rock concerts inside, but real $$$$$. Thanks for chiming in, though.

Confession of a insulation contractor. Started in business as an insulation contractor. The only way you will get a dense pack 12" of cellulose is to open the bundles of cellulose and install them like bricks. I can tell you it can happen as long as your check clears. We call it the 30,30 guarantee , 30 ft or 30 minutes whichever comes first. Come on it's just common sense.

Posted By smartwall on 25 May 2020 11:34 PM
The only way you will get a dense pack 12" of cellulose is to open the bundles of cellulose and install them like bricks.

Thanks smartwall! In your experience, what’s the maximum feasible density attainable with blown-in? Did you ever spray wet-applied? I’m also concerned with a 12’ column settling over time. Can you speak to that?

It will settle. You can wet spray with adhesive, but it would take multiple applications, with it drying between shots. BIBS would be the only way I would do what you want.

Posted By smartwall on 25 May 2020 11:34 PM
Confession of a insulation contractor. Started in business as an insulation contractor. The only way you will get a dense pack 12" of cellulose is to open the bundles of cellulose and install them like bricks. I can tell you it can happen as long as your check clears. We call it the 30,30 guarantee , 30 ft or 30 minutes whichever comes first. Come on it's just common sense.

I have been into these businesses a while. With the right pricing structure and a steady flow of client acquisition, a spray foam insulation business can be very lucrative, and you can generally cover your investments quickly if you do approach the process correctly. The spray foam jobs typically expect 50% in gross profits and about 25% in net profits. I went through a tough situation to select a brand name.

Spray foam is something I work with frequently and I can attest that this is 100% factual.