HVAC for Seattle Studio Build

[Soundman2020]

am I right in assuming that you would simply spin the inner leaf 180-degrees so that the insulated portion is facing the interior of the room?

Correct. In other words, the studs face the room, and the drywall faces the cavity. You still need insulation in the cavity, though.

The advantage is that you now have the stud bays facing the room, and that depth can be used for most of the treatment. Basically, the face of the studs becomes the room surface, and is usually finished with some type of fabric, with the treatment behind it, or wood slats in front of it, as part of the treatment.

If so does the doubled-up drywall constitute one leaf, while the exterior sheathing and siding constitute a second leaf?

Correct. A leaf might consist of several layers of building materials, in fact. For example, you might have a layer of OSB on the studs, then a layer of drywall, a layer of Green Glue, then another layer of drywall. That "sandwich" all together makes up the leaf. On the other hand, the leaf might be just one single sheet of drywall. It all depends on what you are trying to accomplish.

How deep should the air gap be in this type of arrangement?

The air gap is the distance across the cavity inside the wall, between the face of the outer leaf "sandwich" and the face of the inner-leaf "sandwich". You need at least 4" there, and more is better. If you have less than 4" then the MSM resonant frequency is too high, and the wall isolates poorly in low frequencies. So assuming that your outer-leaf wall uses 2x6 studs, you already have 5 1/2" inches or air gap right there. You could have as little as 1/2" between that framing and the face of the "sandwich" of your inner leaf. That would give you 6" of air gap, which should be enough... but you'd still need to do the math to ensure that the MSM frequency is low enough.

The insulation inside the wall cavity is not counted here: since insulation is mostly air, it is considered to be part of the "air gap".

As I mentioned previously an inside-out wall assembly is not something I was even aware of, so thanks for the suggestion.

It isn't very common in normal construction, but it is in studio construction. As far as I know, John Sayers himself came up with this concept, and the forum has been called "the home of the inside-out wall"! So you sure are in the right place to learn about it...

So to be clear, the HRV/ERV sizing calcs that basically equate to room volume multiplied by air changes required per hour multiplied by 30-35% are a good way to go for studio purposes? If that's the case, it will save me quite a bit of cost.

Pretty much, yes. It might need some adjustment if you live in a very humid of very dry climate, or if you have very high or very low temperatures in some parts of the year, but that's the ball-park.


- Stuart -

[silky smoove]

Thanks Stuart, this is all in-line with what I was thinking and in the case of the ventilation saves me a bunch of cash. Fortunately Seattle is pretty moderate in both humidity and temperature extremes, which will make the HVAC much more agreeable in terms of how much system is required and how much it will end up costing me in the long run, haha.

Regarding the inside-out wall design: I will do some more digging on this, but what is the purpose of the insulation within the air gap? I've seen it referenced in John's plans (shown as glued onto the face of the drywall before it's tipped into place), but in my mind it seems counter intuitive to the whole "mass, air, mass" concept. Can you clarify that? In the meantime I'll do my own research to better understand the matter.

Thanks again!

[Soundman2020]

what is the purpose of the insulation within the air gap? I've seen it referenced in John's plans (shown as glued onto the face of the drywall before it's tipped into place), but in my mind it seems counter intuitive to the whole "mass, air, mass" concept. Can you clarify that?

Let me answer cryptically (mostly because I love answering cryptically! Why be simple and direct and clear, when there's a more circuitous route? )

So, your question is "What is the purpose of the insulation within the air gap? It seems counter intuitive to the whole "mass, air, mass" concept". My answer: "What is the purpose of the the shock absorber in your car's suspension? It seems to counter-intuitive to the whole "springy smooth ride" concept..."

Shock absorbers do for car-smooth-riding, exactly what insulation does for studio-wall-isolation.

"Shock absorber" is actually the wrong term, technically, because "absorbing shocks" is not what they do. It's the springs that absorb the shocks, not the shock absorbers. What they really do, is to damp resonance in the suspension system. They put a damper on the "bouncing up and down" thing that would happen i they were not there. Your car would never stop bouncing if it had no shock absorbers! Even tiny bumps in the road would set it off, bouncing away merrily.... and it would always bounce at the same rate, because your car suspension is a tuned system: it has the mass of the car on one side, the mass of the wheel/hub/axle on the other side, and a spring in the middle to connect them, so it is a resonant "Mass-Spring-Mass" system, just like your wall... MSM

MSM systems have this one notable feature: they resonate. They oscillate. They bounce. Hang a rock from a long rubber band, pull it down and let it go.... it bounces up and down forever, oscillating (resonating) at it's natrual resonant frequency. Just like your cars suspension would do if there was nothing to stop it, such as a shock absorber. And just as your wall would do if there was nothing to stop it, such as insulation.

Insulation is to wall resonance as shock absorber is to suspension resonance: it damps the motion. Technically, ti is a "damper", not a shock absorber.

The thing about resonant systems is that they don't want to stop resonating! Push a child on a swing, and they swing higher and higher. Even though you only give them a tiny little push on each cycle, the motion is huge. you push an inch or two, and they swing many feet high, and they keep on swinging after you stop pushing... because they "resonate".

Just like your wall.

It will resonate very loudly, and very long, at its natural resonant frequency, and it will not only pass that frequency through to the other side, it will also amplify it as it goes through, due to the resonance. And it will carry on resonating long after the frequency is gone.

Unless you damp it.

That's why you need porous absorption inside an MSM wall: because it damps all of the resonances going on inside the wall cavity, which would otherwise trash your isolation.

<CRYPTIC MODE = OFF>


- Stuart -

[silky smoove]

Thanks for that Stuart!

So I currently have R-21 fiberglass insulation in the walls with the faced (kraft paper) side of the insulation pointing towards the interior of the room as you would expect. As with most batts there's a slightly "bulge" to the face of the kraft paper extending into the room. Presumably the additional layer of insulation I put between the exterior wall studs (cavities filled with R-21) and the drywall on the inside-out wall will be touching the kraft paper face of the exterior insulation. It would make sense to me that the extra insulation, as well as the exterior wall insulation, will work best if it is not compressed, or at least compressed as little as possible. Rod touches on this in his book a few times. Should I take steps to make sure the insulation protrudes (bulges) beyond the stud bays as little as possible to avoid compressing either layer of insulation, or is that simply overkill? I've considered things like wooden furring strips, drywall hangers, masonry wire, etc. Thoughts?

[Soundman2020]

should I take steps to make sure the insulation protrudes (bulges) beyond the stud bays as little as possible to avoid compressing either layer of insulation, or is that simply overkill?

Overkill.

Correct that you should not compress it so much that it bridges the across the leaves, but you actually need a fair bit of compression to do that (something like 20%, if I recall correctly). Light compression is not a problem, and there's some evidence that it might even be beneficial, since it damps the actual surface of the leaf slightly.

As long as it's not crammed in tightly, you should be fine.

- Stuart -

[silky smoove]

Great news there! I just finished insulating the ceilings tonight and hope to have all of the walls done by the end of the weekend.

Thanks once again for all your help Stuart. It's been invaluable!

[silky smoove]

Are you SURE about that? Have you considered the size of the acoustic treatment that you'd have to hang on the walls if you do it your way, vs. the space you'd save by doing an inside-out wall for your inner-leaf?

Stuart, getting back to something we talked about earlier in this thread. I'm getting closed to finishing the installation of insulation along the exterior walls of the building (thermal envelope). The next phase of construction will be building the inside-out walls for the main room. The ceiling in this structure is vaulted with scissor trusses. Short of building the inside-out walls at less than full height and stacking a second set of trusses on them (and losing a lot of ceiling height in the process) I was considering using Whisper Clips and hat channel to decouple the drywall on the ceiling. Since I won't have trusses to attach to the top plates of the inside-out walls, how do people typically go about fastening said top plates of the inside-out walls to the structure when relying on clips/channel to decouple the ceiling? Attaching the bottom of the walls to the structure (sub-floor and floor joists) is certainly easy enough, but if I also attach the top plates of the inside-out wall to the existing building trusses won't that effectively eliminate any decoupling from the structure? Do people typically leave the top of the inside-out wall unattached to the rest of the structure if they're using clips/channel for the ceiling? That seems like it would help with decoupling, but I have fears about the stability of such construction. For instance, if some clumsy drummer trips and collides with the wall, the only attachment at the bottom would seem to let it get knocked over relatively easily. Looks like I need to pull Rod's book off the shelf again and see if I'm missing anything.

To reiterate my question for clarity: Since I won't have trusses to attach to the top plates of the inside-out walls, how do people typically go about fastening said top plates of the inside-out walls to the structure when relying on clips/channel to decouple the ceiling?

[Soundman2020]

Short of building the inside-out walls at less than full height and stacking a second set of trusses on them (and losing a lot of ceiling height in the process)

Why would you lose ceiling height like that?

If you build your ceiling inside-out (as I usually do), your acoustic ceiling height can be just an inch or so below the joists / trusses / beams / whatever-else-limits-the-height...

I was considering using Whisper Clips and hat channel to decouple the drywall on the ceiling.

You could do that, but you won't ge the same level of isolation as a fully decoupled independent ceiling will give you.

Since I won't have trusses to attach to the top plates of the inside-out walls, how do people typically go about fastening said top plates of the inside-out walls to the structure when relying on clips/channel to decouple the ceiling?

You would have to use acoustic sway braces on your wall tops, and you would have to deal with the resulting variable gap between the walls and ceiling. The gap will be changing all the time, as doors open and close, pressures change, the wind blows, the building moves and vibrates... You'll need to use a very flexible caulk to fill that gap, and the gap will need to be fairly large anyway, to accommodate the movement of the wall top. Not an easy task to accomplish...

Attaching the bottom of the walls to the structure (sub-floor and floor joists) is certainly easy enough, but if I also attach the top plates of the inside-out wall to the existing building trusses won't that effectively eliminate any decoupling from the structure?

Right! It would seriously degrade your isolation, since your inner-leaf walls would transmit sound to/from the outer-leaf, through that direct mechanical bridge. Not an option, if you need good isolation.

Do people typically leave the top of the inside-out wall unattached to the rest of the structure

Nope! You can't do that: the tops of the walls needs support. If you don't build a ceiling on top of it, then you need to support it resiliently against the outer-leaf, using acoustic sway braces or some other form of resilient mount. Of course, that also reduces isolation, but not as much as having a direct connection.

That seems like it would help with decoupling, but I have fears about the stability of such construction.

If you live in an earthquake area (I think Seattle is?), then you might also need seismic snubbers, to prevent the wall top from basing into the surrounding structure in an earthquake.

You'd need to get professional advice on the design, number, location, and characteristics of your sway braces and seismic snubbers: That's not something that you want to be calculating or figuring out yourself.

Those things also cost money!

That's why I pretty much always do full ceilings on top of the walls. They provide the structural integrity (assuming that your walls are also built correctly, with good sheer strength...)


- Stuart -

[silky smoove]

If you build your ceiling inside-out (as I usually do), your acoustic ceiling height can be just an inch or so below the joists / trusses / beams / whatever-else-limits-the-height...

I'm unclear as to how that construction would work. Since this is a vaulted ceiling (scissor trusses); Wouldn't I need a second set of trusses underneath the existing ones to support the weight of the isolation materials (planning on two layers of drywall with Green Glue between) and the acoustic treatments, as well as supporting the top of the inner leaf walls and keeping them from splaying? I've done some research looking for details, but have come up empty-handed. Can you cite some examples of this kind of installation?

Below are some pictures of the interior space that might better illustrate the structure in which I'm building. All wall framing is 2x6 spaced 16" on-center. Trusses are spaced 24" on-center.

Room interior looking north


Room interior looking south


Scissor truss at peak of vaulted ceiling


Scissor truss at exterior wall top plates

[silky smoove]

I found this thread which seems to shoot for a similar goal: http://www.johnlsayers.com/phpBB2/viewt ... &view=next

What I'm still unclear on is how the LVL beam (per Rod Gervais) is attached to the existing scissor trusses while still decoupling the whole system from the building structure. I'll be doing more research and pulling out my copy of Rod's book to find references, but if you can shine some light on that as well I'd really appreciate.

I know I've said it before, but it merits repeating: Thanks again for all your help on this Stuart. I really appreciate it.

[Soundman2020]

Ooops! Problem! Big one!

You cannot use that roof as your outer-leaf...

silky-DSC_3149-problem.jpg

It is a vented roof, and those are the inlet vents, under the eaves. From the sunlight pouring through the top, there's also clearly a ridge vent up there, to let the air out again.

silky-DSC_3149-sunlight.jpg

You cannot seal that up, so therefore that cannot be your outer leaf.

So, what you will have to do is to put drywall directly on the bottom of those trusses, sealed to the wall, and that will be your outer-leaf. Then you will have to build a proper "room-in-a-room" studio below that: four walls and a ceiling.

Yes, you will end up with a 3-leaf roof, but you don't have any options here. You'll need some extra mass on that middle-leaf, and a larger-than-normal gap between that and the actual inner-leaf, and you can still get good isolation like that.

I'm sure this is not what you wanted to hear, but fortunately there is a solution. I've done a few studios like that, and it works out just fine.


- Stuart -

[silky smoove]

Ooops! Problem! Big one!

There's been no shortage of those on this project, haha.

It is a vented roof, and those are the inlet vents, under the eaves. From the sunlight pouring through the top, there's also clearly a ridge vent up there, to let the air out again.

Exactly right. Airflow in through the eaves and then out through the ridge vent. This was the easiest way to meet code here in Seattle. So much so that in a pre-construction meeting with the city's building department, the inspector told me that he did not want to see gable vents on this structure. He was pretty clear about wanting eave and ridge vents, so that's what we went with. I also briefly explored the notion of a "warm roof" system where the vaulted portion of the structure is completely sealed up and insulated with closed-cell spray foam. The same inspector told me that if there wasn't adequate ventilation up there (despite lots of articles indicating that a fully sealed vaulted ceiling is truly the way to go) he would fail my permit inspections until it was changed. C'est la vie!

So, what you will have to do is to put drywall directly on the bottom of those trusses, sealed to the wall, and that will be your outer-leaf. Then you will have to build a proper "room-in-a-room" studio below that: four walls and a ceiling.

So from the top down I'd be looking at the following (please correct me if I'm wrong):
Asphalt shingles
Felt paper
Plywood sheathing
Scissor trusses
Drywall (single layer?)
Air gap
Drywall/Green Glue/Drywall sandwich
Inside-out ceiling framing

Is that correct?

Yes, you will end up with a 3-leaf roof, but you don't have any options here. You'll need some extra mass on that middle-leaf, and a larger-than-normal gap between that and the actual inner-leaf, and you can still get good isolation like that.

To be clear; The inner leaf is the drywall on the backside of the inside-out ceiling. The middle leaf is the drywall on the bottom of the existing trusses. The outer leaf is the roof sheathing, felt paper and asphalt shingles. Is that correct?

If that's the case, how much extra mass will I need on the middle leaf? Two layers of drywall?

How much larger of a gap are we talking here? I'm concerned about losing too much ceiling height.

I'm sure this is not what you wanted to hear, but fortunately there is a solution. I've done a few studios like that, and it works out just fine.

As I mentioned before, this project has been filled with things I didn't want to hear (mostly related to the site work outside the structure and permitting hangups), so I'm fairly numb to it at this point, haha.

Just thinking out loud here: Since the ceiling is becoming a larger complication than previously expected (by me at least), would I be better off avoiding the inside-out walls/ceiling and just going back to my original plan of using Whisper Clips and hat channel to decouple the interior from the structure? Would that still run into the same problems with requiring a third leaf on the ceiling?

[Soundman2020]

There's been no shortage of those on this project, haha.

OK, so it's a perfectly normal studio build, then...

The same inspector told me that if there wasn't adequate ventilation up there (despite lots of articles indicating that a fully sealed vaulted ceiling is truly the way to go) he would fail my permit inspections until it was changed.

Wow. Not so nice! And he doesn't even seem to know his own building code, because as far as I'm aware, across the USA that is an acceptable alternative... Oh well...

So from the top down I'd be looking at the following (please correct me if I'm wrong):
Asphalt shingles
Felt paper
Plywood sheathing
Scissor trusses
Drywall (single layer?)
Air gap
Drywall/Green Glue/Drywall sandwich
Inside-out ceiling framing

Is that correct?

Right. But I'd go with more mass on your "middle" leaf, and maybe on your inner-leaf as well, so we can get you a smaller air gap...

To be clear; The inner leaf is the drywall on the backside of the inside-out ceiling. The middle leaf is the drywall on the bottom of the existing trusses. The outer leaf is the roof sheathing, felt paper and asphalt shingles. Is that correct?

Right.

If that's the case, how much extra mass will I need on the middle leaf? Two layers of drywall?

Ahhh! Now that's the 64k$ question, now isn't it? Yep.

The answer is: "How much isolation do you need?" (measured in decibels). . .

How much larger of a gap are we talking here? I'm concerned about losing too much ceiling height.

Mass and air gap are interchangeable, to a certain extent. With more mass, you can go with less air gap. but first there's a question that needs answering: "How much isolation do you need?" (measured in decibels)

Since the ceiling is becoming a larger complication than previously expected (by me at least), would I be better off ...

No.

Would that still run into the same problems with requiring a third leaf on the ceiling?

Yep.

Your wall plan is fine: It's your ceiling that is the problem.


- Stuart -

[silky smoove]

I'll get back to you with specific decibel requirements for isolation. Currently on my phone and need to do that from my desktop.

In the meantime:

Would I be able to move back to a two leaf system if, after I pass inspection, I pull out the newly installed insulation, block and air seal all of the existing vents and then reinsulate using a "no air flow/warm roof" method? Would that even be achievable?

[silky smoove]

OK, so it's a perfectly normal studio build, then...

Precisely!

Wow. Not so nice! And he doesn't even seem to know his own building code, because as far as I'm aware, across the USA that is an acceptable alternative... Oh well...

I've had to make trips to the Seattle DPD and argue with them during the site development phase of the project. They often argue things because of how they feel rather than simply enforcing code. I overturned one such decision. I had a surveyor I contracted to do a property line delineation (to locate the studio building on my property) tell me "The Seattle DPD is a black hole where good projects go to die." No shortage of love loss there, haha.

The answer is: "How much isolation do you need?" (measured in decibels). . .

Let me set the scene for you: The loudest piece of equipment ever operating inside the space at any given time will live drums. Since there's only one room where sounds will be generated (mixing and tracking occur in the same space due to size limitations), I'm not worried about bleed in the rest of the studio. There's a storage vestibule, but that's inconsequential to this discussion. The building is located against an alley. There's a neighbor's house on the other side of the alley that is roughly 20' from the western wall of my studio building to the eastern wall of his house with a wooden fence in between. There are cars that go up and down that alley periodically, but it's not a major thoroughfare.

So with all of that in mind I would say that a drummer with a light touch will be in the 90-100db range, whereas a drummer that's an absolute basher might be well above that. 105db? 110db? 120db? I wouldn't be surprised if it were possible to get that high, but I suspect it will commonly peak out around 100-105db. The bass drum will likely be the sound that penetrates the isolation perimeter most readily simply due to its lower fundamental frequency than the rest of the kit.

If I could get the resultant sound outside the studio to be non-obnoxious to my nearest neighbor I'd consider the isolation a success. If I could get everything down to the 50-60db range I would be happy, although more than that would obviously be better. I'm using the db chart on the Sound Proofing Company's website to make relative comparisons to "real world" sounds and their apparent loudness (db). I have a good relationship with all of my neighbors, and don't plan on tracking loud instruments late at night, so the isolation concerns are somewhat mitigated socially. I'm definitely attempting to build such that I can maximize sound isolation while preserving as much internal working space as possible. A better end product is more important to me than cost savings.

In terms of isolating external sounds from getting into the studio is concerned: The loudest exterior sound will be a car driving down the alley. It will be around 5' from the exterior wall of the structure, but its a dead end alley with only two homes at the end, so the cars are typically traveling very slow (~5mph) and none of them are large trucks or anything noisy like a motorcycle. If I can keep that relatively quite noise from getting into the studio and ruining a perfectly good take I'll be very happy. SPC has an average street at 70db, so I'd estimate this to be in the 40-50db range.

Your wall plan is fine: It's your ceiling that is the problem.

Stupid ceiling! Haha

Reiterating my question from earlier: Would I be able to move back to a two leaf ceiling if, after I pass inspection and am done with the city, I pull out the newly installed insulation, block and air seal all of the existing vents and then reinsulate using a "no air flow/warm roof" method? I could physically block the vents with thin strips of lumber or metal flashing, caulking, and then shoot the whole thing with a closed-cell spray foam insulation. Would that be worth my time and money given my stated goals above? Or instead, in your estimation, will I be able to achieve my desired level of isolation or better despite the three leaf ceiling?

Thanks once again Stuart!