Name- Eddie 

Climate Zone 5 Massachusetts

I’m a Grammy-award winning recording studio engineer + producer designing a residential recording studio in Massachusetts. I’m learning building science, but what I know is sound isolation, so thats where we’ll start. 

Heres the crash course- The most practical way to keep sound in and out of a recording studio room is with mass, air space and then more mass. In general, more mass or more airspace = better performance. Ideally the mass portions of the assembly are as far away as possible. Very specifically, we’re going to build an air-tight sufficiently massive 2×6 wall facing nature, an air space in between for fluffy stuff, and another air-tight sufficiently massive 2×4 wall facing the music. The roof will be held up by the outer walls, the ceiling by the inner walls. There will be no direct mechanical (or thermal) coupling between outer and inner walls or ceiling/roof. So far so good, we’re building a double-wall…

We’ll have 3 layers of 5/8 drywall on the inside (with visco-elastic sound deadening compound in between) and 2 layers of taped 5/8 CDX ply sheathing (not ZIP) the outside. For sound, air-sealing is incredibly important- where air goes, sound goes. For this reason, gaskets and caulk goes under sills, sheathing and drywall seams are staggered and each layer except finish is acoustic caulked at all seams. Besides a handful of windows and a couple doors, there won’t be any other holes in the drywall shell. We’re pretty serious about this. All electric will be surface-mounted, no can lights, no recessed boxes. Electric will get in to rooms through the floor. This also means we’re 100% destined for an unventilated roof, as we can’t go poking a bunch of ventilation holes in our sufficiently massive walls. Anyone with a blower door fetish is welcome to come hang out while they try to push some air through this assembly. So…incredibly well air-sealed double-wall construction, no problem, right?

The drywall shell will be damped with 8″ R-25 Unfaced Ecotouch on the ceiling, with 3.5″ R-13 Unfaced Ecotouch in the 2×4 wall bays. This insulation must be pressed up against the drywall, and will damp the walls to keep them from resonating. If you hit this wall with a hammer, it should give a short dull thud, not a long resonant boooom…at least if we don’t want to hear that exact same resonant boooom on the other side of the wall. Which we don’t. Sure, its thermal insulation, but it’s also doing the work of converting vibrational energy into heat. 

To meet code, we need an extra R-24  for the roof and + R-7 or so in the walls. I was thinking of flash & batting w/ 4″ spray of ccSPF (+R-24) under the roof deck + This helps mechanically stiffen everything up + adds some additional mass, even better air-sealing and will prevent any hot moisture-laden air from ever hitting the back of the sheathing. Then I can come back with more unfaced batts to get to code.

Questions:

1. Per BSI-100, air-permeable insulation (unfaced FB Batts) should be placed directly underneath air-impermeable insulation (ccSPF). If I’m planning to maintain my air-gap between walls, I’ll have 4″ ccSPF under the roof deck with a 3.5″ unfaced batt under that, then a small air gap, then an 8″ unfaced batt under that. As we move towards the peak of the roof, there will be even more of an air-space between insulative layers. Is that OK? I’m also into batts because I can install them myself. https://www.buildingscience.com/documents/building-science-insights-newsletters/bsi-100-hybrid-assemblies

2. Per BSI-100 in Zone 5, I’m looking for 40% air-impermeable insulation on the roof and 30% on the walls. I’d love to be able to run up the ccSPF as thick as I want but $$$ and I don’t want to be hermetically sealed-in with any more off-gassing foam than I have to be, and neither does the planet. I’m therefore limited to how much insulation I can use by how much ccSPF I’m willing to spray to stay within ratio. Here’s my plan- 

3. For ‘sufficient mass…’ I’ll be using two layers of 5/8 CDX to sheathe the wall facing nature. More mass = stops more sound. By using CDX on the outside instead of ZIP & some Mento 3k  we should be able to dry to the outside should anything outside the ccSPF get wet, right? On the inside, despite all the layers of drywall, we’ll only have vinyl paint on it and no other vapor retarder, so come spring, we can dry inward from the ccSPF?

4. I’d love to avoid a roof rain-screen for acoustic reasons. I haven’t gone into detail on this, but adding another relatively well-sealed layer of mass (roofing nailbase) close but not touching (air gap) to another layer of mass (the roof deck) will raise the resonant frequency of the wall/roof system, reducing its ability to stop sound at lower frequencies….and that’s bad. I’m in Zone 5, so we’re just below where Lstiburek (Per BSI-046) considers a vented roof rainscreen necessary to prevent high snow pack from insulating the roof deck enough to cause ice dams. If we do a great job on insulated eaves, I can omit the over-roof rain-screen without worrying about it every winter? Is this going to hurt my drying potential when something invariably gets wet?

https://www.buildingscience.com/documents/insights/bsi-046-dam-ice-dam

5. Was planning on going ‘chainsaw retrofit’ style with the eaves. Cut rafter tails flush with the wall framing and run the ply all the way to the roof for real easy and effective air sealing against the roof deck. Then we’d build the eaves off ledgers to the same insulative spec as the roof assembly so any heat plumes coming off the wall are directed out and away instead of warming the roof and melting the snow. No problem, right? Worth it? Anyone know a better way? 

Thx to  anyone that made it this far!