Some Questions - In Progress Garage Conversion

#1

Hi All - This is my first post! The detail and discussion I’ve found on this forum has been extremely helpful.

I have an in progress studio/practice room build going, and have a few outstanding questions I’d appreciate some advice on. This build is 1/2 garage conversion and has been going on for a few years now. I originally posted my design on the John Sayers forum to get feedback before I started.

Here are a few details to summarize what I’m working with currently:

One half of a detached two car garage (garage door opening walled off). The size of the room allows for a functional adjacent 1-car garage space.
Completely independent MSM construction, framed on the existing garage concrete slab, 2x4 frame walls, 2x6 frame ceiling. Structural engineer reviewed / approved the design and existing structure for the extra mass.
Exterior leaf is two layers 5/8” type X drywall, cut and fit in between the studs per the method outlined in the Rod Gervais book – on top of 1/2” exterior plywood sheathing.
Inner leaf is two layers of 5/8” type X drywall + Green Glue.
The space is a long and narrow rectangle, so I went with inside out walls on the sides to maximize the room width. Air gap between these walls is 6.5”. Ceiling is also inside out, however the two short walls are built standard as I have plenty of room length. Gap between those walls and also the ceiling leaves is ~10”. (initial MSM calculations were done with the 6.5” gap).
Air space is filled with the R-30 insulation
Minisplit / Heat pump for air conditioning installed w/ S shape line set penetrations.
I put in a long, narrow exterior window for light, with an independent frame on each leaf – (In progress)
Plan is to use Rod’s Super door design (In progress)
Going with ERV and silencer boxes for fresh air (in progress)

Final Room dimensions, leaf to leaf:

10’ 11 3/4” Width
19’ 10” Length
8’ 9” Height

Inner width, frame to frame is 10’ 4 5/8”

Here’s are snaps of my very initial model to get a sense of it:

Note - This rough original model has all inside-out walls vs. the changes I mentioned above, and the door opening has now been moved away from the corner.

My questions revolve around the three in-progress items:

Window Glass

My math for the window: I’m using two layers of 5/8” drywalls on each leaf – the inner leaf has green glue, the outer does not, but has the extra mass of the 1/2” plywood exterior sheathing.

Total MSM mass
2.2 psf x 4 (drywall) + 1.42 psf(plywood) = 10.22 psf

Now I’ve read that using green glue is similar to adding an additional 2 layers of drywall. If this is reasonable then the estimated
equivalent total mass = 14.62 psf

Two glass panes with a 1/4” difference in thickness:
1/2” glass 6.36 psf + 3/4” glass 9.1 psf = 15.42 psf

The glass I’m able to order in these thicknesses is laminate+tempered (aka safety glass), and in Rod’s book it mentions the glass thickness could be lowered further by an 1/8” each as the lamination provides additional isolation. However I’m leaning towards the thicker glass to keep things simple (overkill even), as there is a concern I get it wrong. Some questions -

How important is the recommended 1/4” thickness difference in the two glass panes to offset the coincidence effect. For example would 1/8” suffice if using laminate glass?
1. For example, if I go with 1/2"+5/8” glass, total mass would now be 14.62 psf - very close to the wall assembly. If the lamination provides additional isolation at this thickness, this should suffice. Going with 5/8” vs 3/4” pane is lower cost.
Does it matter which leaf (studio vs exterior) the thicker glass goes on?

The Door

For egress purposes, I would really prefer to have a single door which opens into the studio. Although an unlikely scenario, I’d feel better removing the remote possibility of someone accidentally blocking the door while I’m in there, preventing an exit if the extremely heavy door only swings out. Otherwise the super door seems perfect for this, but I have some questions.

For the door to swing into the room, it would need to be mounted on the inner leaf - is there any significant isolation issues if I do this? I was thinking there might be the potential transmission via the door jambs from the sound trapped in the air space? If it was on the outer leaf and the door opened out I wouldn’t be concerned.

Per Rod’s book 5/4” lumber is recommended for the jambs – I was planning on this and also lining the opening with neoprene sheet prior to installing.

In my design, any sound leaking from the door would be somewhat contained via the adjacent garage space before exiting the building.
Door alignment with the leaf – let’s assume it’s okay to install the door on the inner leaf – since this particular wall is ‘inside out’, aligning the door with the mass layer on the wall, would mean I could not fully swing the door open as it would hit the door frame when open. I will have a commercial door closer installed, so this might be a moot point?

Ideally I’d like to set the door back far enough for it to open as wide as possible for equipment and so forth. If I do this however, when closed, the door would not be in the same plane as the wall mass. I’ve obviously never done this before, so just trying to plan ahead.

My instinct is that having the mass align would be more straightforward, and the door opening only 90 degrees is likely not that big a deal.

I mocked up two options to visualize what I'm talking about:

1 – Door set back (allows for wider opening)

2 – Better aligned with Wall mass (opening is restricted)

Silencer boxes and ERV Sizing

It took me a long while to figure this out, but I feel like I’ve finally cracked the code on the basics at least. I’ve done all of my circulation and fresh air calculations (below for reference). I get how the silencer boxes work with cross sectional area to slow down the velocity – however what I can’t figure out is, what is the most straightforward way to calculate the added static pressure a fan sees by adding these boxes? In my search I’ve found various conflicting ‘equivalent length’ tables for the bends to attempt to figure this out myself, with no luck.

Let’s assume four total boxes using the Gregwor design – I count 9, 90 degree turns in each, that’s 36 elbows of equivalent length to consider plus the length of the supply and return ducts.

By getting the static pressure wrong, as I understand it I run the risk of burning out the motor on the ERV and/or not getting the necessary CFM. The ERVs all have a SP/ CFM chart to anticipate the CFM losses.

I don’t have space between the leaves for these boxes, however the exterior ones will be in the garage attic. I was also considering having only two larger boxes (example below), so that’s still an option.

For review here are my calculations, using the 6-8 air changes and 20/40% guidelines from this great post: https://digistar.cl/Forum/viewtopic.php?f=5&t=1136&p=5911

One other thing to mention is that 99% of the time, it’s just going to be me in this room. My thinking is I can get away with being on the lower end of the 20-40% fresh air range if it works out that way.

Room Size: 1,905.2 ft3
Circulation:

6 to 8 Air changes an hour = 190.52 CFM to 254.03 CFM
Mini Split spec says 306 CFM-542 CFM airflow - more than enough for circulation

Fresh Air - taking the required CFM for 6 to 8 air changes, and adding…

Fresh Air at 20% = 38.1 CFM to 50.8 CFM
Fresh Air at 40% = 76.2 CFM to 101.612 CFM

Velocity:
There are a number of ERVs with variable speed settings, so assuming if I can ball park the static pressure of the ducts + silencer boxes, I can work the duct design to target CFM ranges, and finalize the silencer box dimensions to get the velocity down between 200-300CFM.

Otherwise, with my current understanding I’m basically stuck on the static pressure variable at this point. Of course if I’ve somehow missed the mark on this, please let me know.

Here’s an rough model I made with just two larger boxes, just so I could see what sort of space logistics I’m dealing with. I have a 16”OC joist grid where I can drop the duct into the room (14.5” square area). I still need to model the four-box version, to see if hanging two inside the room is even feasible.

Thanks in advance for any feedback here and happy to provide more info if needed. I have a photos of the build as well and can share them in another post.

Best

Joe

#2

one thought - if you're doing decoupled walls and ceiling, why not decouple the door jambs? that hard connection between the frames will transmit a lot of sound. if you build the door on the (say the inner frame) and use a jamb extension on the outer frame leaving (say) 3/8" gap which you seal with backer rod and caulk to leave it soft, then you'll have it decoupled and the only single structural transfer is the single slab. decoupling the wall frames using something like Kinetics Noise wallmat etc can help there. note: the jamb + extension is for the window(s) as well.

#3

Hi Joe, and Welcome to the forum!

Absolutely agree with Glenn! Decouple completely for maximum isolation.

It's surprising just how much sound a hard connection can transmit. A really good illustration to convince yourself of this, to get an old-fashioned tuning fork. Tap it on a table then hold it out at arms length and listen... very quiet, because there is no hard connection: just air. Now tap it again, but this time hold the butt end down hard on the table surface (or a wall surface, if you prefer). Listen again. MUCH louder. The "hum" is very clearly audible (which is the entire purpose of a tuning fork!), and that's because there is now a hard connection from the vibrating tuning fork to the table surface. Think about how tiny that hard connection is: the butt of a tuning fork has a very small area, maybe about the size of a nail head.... yet it can transmit all that sound. Imagine how much sound can be transmitted by a hard connection a couple of inches wide and 20 feet long (roughly the perimeter of a large doorway).

I'll get back to you about the rest shortly...

- Stuart -

#4

OK, about your concern with the outer leaf door perhaps being blocked if it opens outwards: The simple solution is to use a sliding glass door there, if that is a concern, and maybe use Glenn's idea of an "aesthetic"barn-style cover over that door (not acoustic), such as he suggested in this thread: viewtopic.php?f=6&t=1142 The glass gives you visibility out through that door if you want it, and the "aesthetic" door can give you privacy and security if you need it.

I'd also suggest using glass sliders on the inner leaf, too. hinged doors take up a lot of valuable space in the room, because the swing through a large arc. You can't put gear, instruments, speakers, mics, people, acosutic treatment, or anything else in that arc, so it is "lost" space. A sliding glass door takes up no space at all. You can put whatever you want, anywhere you feel like, right up to the glass itself. A large hinged door can remove several square feet from your usable floor space, and also interfere with traffic flow, and sight lines. Sliding glass doors are a better option all round (except for price!).

Now I’ve read that using green glue is similar to adding an additional 2 layers of drywall. If this is reasonable then the estimated
equivalent total mass = 14.62 psf

Well, yes and no. Yes green glue does increase isolation substantially, but it doesn't add mass to do so. It adds "constrained layer damping" between the layers of drywall, which is exactly what an acoustic PVB interlayer does between layers of glass in a laminated pane. Ordinary PVB does that in glass too, but to a lesser extent. Acoustic PVB is thicker and better. PVB isn't the only interlayer material. There are others too, with varying degrees of acoustic performance (and price!). Check with your glass supplier to see what options he has for acoustic laminated glass.

There's no simple solid relationship between GreenGlue and drywall mass. There is a very rough approximation that does allow you to say that a wall built using GG between two layers of 5/8" drywall will give similar overall isolation to a different wall built with four layers of drywall without GreenGlue, but it doesn't hold for all cases and all frequencies: It's just a generalization, or a useful rule of thumb. Not sure if I'm making the point properly here: GreenGlue isn't the same as mass, it only produces effects that are somewhat similar to using more mass, but not the same.

Isolation isn't just about mass. It is about many factors, and one of those is "damping". A wall with reduced mass can still isolate decently if the leaves are "damped" very well, as compared to a higher mass wall that isn't damped at all. "Damping" refers to any material that can remove energy from a vibrating system by converting it into low-grade heat. That usually means a soft, resilient material, such as GreenGlue.... or acosutic PVB. When such a material is sandwiched between two rigid or semi-rigid layers (such as drywall, or glass), then it absorbs some of the vibration energy that would have been transmitted through otherwise. The mechanism that it uses to do that is actually surprising complex at the microscopic level, which is why there are various materials out there with various levels of performance: each is engineered for a specific purpose. GreenGlue is really good at what it does between layers of drywall, but would be a bad choice to put under your speaker, for example. Acoustic PVB is really good at what it does between layers of glass, but would be a lousy choice to put under your floor, or between layers of drywall.

So, that said, it would be better to first determine the transmission loss curve that you are expecting from your walls/floor/ceiling, then look at the alternatives that your glass supplier can offer and find the closest match.

...the glass thickness could be lowered further by an 1/8” each as the lamination provides additional isolation.

Perhaps... perhaps not! Here too it doesn't just depend on the mass, but rather on the type of interlayer. Here's a graph from one manufacturer that illustrates the point very well:

: laminated-glass-isolation-severl-types-graph.gif (14.18 KiB) Viewed 15847 times

: laminated-glass-isolation-severl-types-graph.gif (14.18 KiB) Viewed 15847 times

The blue curve is a single pane of glass 6mm thick, the black curve is two layers of 3mm glass laminated together using an ordinary PVB interlayer. The green curve is for the same 3mm+3mm glass but laminated using an acoustic PVB interlayer, and the red curve is for the same glass but laminated using this manufacturers own specialty interlayer. As you can see, you can't just say that "using laminated glass is the same as an extra 1/8" thickness". It doesn't work like that. Making that single pane 1/8" thicker (9mm, instead of 6mm) is NOT the same as using laminated glass. In fact, the coincidence dip for thicker glass will occur at a LOWER frequency, which REDUCES isolation at that frequency. Depending on the situation and the glass, that could be a big issue in some cases. Here's another image to illustrate that:

You can see that the thinner glass (3mm, green curve) has a high coincidence dip frequency, up around 4 kHz. The 6mm glass (double the mass, red curve) has better low frequency performance, but the coincidence dip has moved down to around 3 kHz. And the very thick glass(four times the mass, blue curve) has even better low frequency performance, but the coincidence dip is now firmly in the mid-range, where a lot of instruments put out significant sound power at their fundamental frequencies. In real terms: the 12mm glass would be better for isolating kick drums, snare, and toms, but worse for isolating crash, ride, high hat, electric guitars, keyboards..... as compared to the 6mm glass (green curve).

Yes, varying glass thickness for the two layers is a good idea, for the same reason as above; You will still have two coincidence dips, but they will be at different frequencies, which is a good thing. If you use the same glass in both leaves, then you will have a "weakness" at the same frequency on both sides. Moving one of those a bit by using a different thickness can help. However, if you are using glass that doesn't have much of a coincidence dip (because it uses a high performance acoustic interlayer, such as the red line on the top graph above), then it really doesn't matter! Since you don't really have much of a coincidence dip at all, there's no point reducing isolation by making one pane thinner! If that's the case, then it would be better to make them both as thick as needed to do the job (or as thick as you can afford!).

So it all comes down to talking to various glass suppliers in your area, seeing what they have to offer in terms of acoustic laminated glass, and choosing the best option for your situation.

- Stuart -

#5

Hi Glenn and Stuart - thanks for your replies!

On the topic of the door - decoupling the door frame in this way makes perfect sense and is in fact how I've built the window frame. I will incorporate this. Unfortunately the sliding door option is out, as the room is already built - I didn't even consider this as an option in the early design stage, and would have been the most ideal for the reasons you provided regarding space. However right now I'm looking at a standard rough door opening, sized wider to accommodate larger 5/4" jambs.

I will rethink my need for the door only opening inside, as it gives me more traditional options (dual doors, super door on the outside etc).

Regarding the window glass - I really appreciate this extra detail this give me some more to consider now. The glass supplier I'm working with currently has tempered+laminate glass available using EVA. Certainly not a special acoustic layer or any custom interlayer options so I will call around. After reading your reply I looked up EVA vs PVB and acoustic isolation and only found vague references vs real specs. I reached out to the supplier and they don't having any specifics for me either - I don't think their laminate glass is manufactured in-house unlike their other types. Of course to be fair they are not trying to sell it as specialty acoustic glass.

Lastly any pointers for me on figuring out added static pressure of the silencers? I'm working on an initial layout, so at a minimum I will have the total length of the flex duct runs to get started.

Thanks again!

Joe

#6

the glass laminations will add damping to the panel which is not necessarily accounted for using simple mass coincident charts... generally - match the mass of the wall you're put it on - go slightly heavier than lighter when not an exact match. i've used 3/4" (3x 1/4") laminated glass on both side without issues.

for a 3 panel duct attenuator (commonly seen on these forums), figure it's the equivalent of 100ft of duct as a good starting point. in reality, given the system should be running 100% when occupied, the flow inefficiencies (along with lowest system settings etc) should not be an issue. it really only becomes a challenge if you're trying to integrate with an existing home system which would not be running continuously and retrofitting to multiple zones a problem. then you're going to get sporadic temperature shifts, moisture / odor "waves", and CO2 pockets etc between cycling.

#7

Thanks Glenn!

I’ve worked out an initial design for some silencers – I’m looking at a small Fantech ERV that is rated for 70cfm at .4 in wg (FIT70E). It’s got 4” round duct connectors, I ‘m estimating about 25’ total length between the ERV/Outside/Silencers. 400’ of equivalent silencer duct (~100’ per silencer estimate) at increased duct size of 7.5”. My assumption is that I calculate the total friction loss and then compare to the ERV performance chart.

Using friction loss calculator online, I get 0.23476 total in. wg at 100CFM and 0.11921 in. wg at 70CFM. This seems way lower than expected, did I do this right? Even if I double these numbers I’m still well in the operating range of the ERV.

ERV Ducts: 25' | 4" round / Friction Loss in wg @ 100cfm: 0.16352 / @ 70cfm: 0.08303
Silencers: 400' | 7.5" square / Friction Loss in wg @ 100cfm:0.07124 / @ 70cfm: 0.03618
Total Friction Loss: @ 100cfm: 0.23476 / @ 70cfm: 0.11921

For velocity, I calculated going from 4” to 7.5” (70cfm) changes from 802fpm to 191fpm.

I updated my model with the correct door and joist placement and mocked up a rough layout to get an idea of how this is all going to fit – I have plenty of space in the attic. I can run flex duct from the ERV in the back of the garage (not modeled), to the metal duct and run that through the insulated ceiling joist. On the inside of the room, I think can build the silencers in place, attached to the cieling and wall framing. The one in the back of the room will send the supply air in front of my mini split, mounted on the back wall.

I imagine there are a few schools of thought on how to connect the silencers between the leaves? I have a straight drop available through the joist grid. If the silencers have the same mass as the leaf, can I just use insulated flex duct?

Joe

#8

Careful there! Your diagram shows you using ordinary duct going through the two leaves to join the two silencer boxes. There's a section of bare duct in both cases, between the leaf itself and the silencer box. There's very little mass in a duct wall, as compared to the silencer walls, so you'll lose some isolation there. Sound can potentially get into / out of the duct, completely bypassing the silencer.

What I normally do is to make a square wood "sleeve" of the same thickness as the silencer wall, and cut a square hole in the leaf, the same size as the "sleeve". The sleeve then protrudes through the leaf, into the cavity, ending a short way past the leaf (or even flush with the leaf). Then you can use flex-duct to link the ends of the two sleeves, in the cavity between the leaves.

That way you maintain continuity of the mass throughout: leaf and silencer box are connected by the sleeve, which should have about the same mass as the leaf. Just make sure to seal the penetration really well, with plenty of caulk. Cut the hole in the leaf a little larger than the sleeve, and caulk the hell out of that gap!

- Stuart -

#9

the key objective of the silencers is to plug the hole in the mass layers. the inter-mass gap is what it is. so a simple flex duct is ok there. what you don't want is to hard connect the two mass boundaries. one way i do that when i know the inter-mass gap will be "noisy" is to build a set of inter-connected sleeves with duct board and drywall with a rubber seal - this protects the flex duct and the two sides have no hard connection -- limiting the potential for noise in or out. generally this happens in "shared" spaces with (e.g. a condo) where they have shared air spaces across units for pipes etc and there is no way to fully close those. otherwise if the silencers are mass equivalent and properly damped (along with reduced air speeds etc), then the exposed flex duct in the inter-mass gap is ok.

as a note, it's a good idea that the in-room silencer component is expanding the volume to further reduce the air velocity and preferably into a lined plenum for further expansion. the slower you can move the required volume of air, the less likely you are to have noise due to air movement (you likely cannot hear it but your mics can)...

#10

Soundman2020 wrote:Careful there! Your diagram shows you using ordinary duct going through the two leaves to join the two silencer boxes.

Whoops! Modelling error

Cutting some corners in sketchup for the post, but thanks for pointing that out. I'll add in an extension / sleeve to my design to make sure it's all going to fit properly in advance.

Soundman2020 wrote:Then you can use flex-duct to link the ends of the two sleeves, in the cavity between the leaves.

gullfo wrote:Then you can use flex-duct to link the ends of the two sleeves, in the cavity between the leaves.

This is exactly what I was curious about initially, and as usual, really helpful detail in your responses! I have a few additional things to consider.

Thank you both!

Joe

#11

#12

Hi Guys - question on my electrical setup. I basically have two options - one 20amp circuit for the entire room, or two circuits (maybe a 15amp) just for lights - my thought process was to keep the dimmers on a separate circuit to reduce noise. Practically however a single circuit for a few LED lights for one room is overkill and takes up space in my subpanel... (although I may redo the entire garage as 1) 'outlets' 2) Lights, to make it more practical).

If the objective is reducing noise on the main circuit from the LED dimmers, does having separate circuits even help? Just questioning some assumptions I've had. I've read up a little on variac dimmers, and Lamp Debuzzing Coils etc. Just trying to keep it simple - also if the noise is mostly radiated (vs conductive) RFI, I assume separate circuits won't mitigate that.

Thanks

Joe

#13

it depends on the panel - if you have 240v into the panel as split 120v, then lights (and "dirty outlets") on one side and clean outlets on the other will help as the noise can only transfer through the common neutral or ground (which should basically be at or near zero).

#14

Interesting - in this case it's a 100amp sub-panel, which ... might be a separate concern with noise coming down from the main panel to begin with? It is however 240v split, so I could arrange the circuits on each side as clean and dirty as you suggest.

#15

yes, you'll have to factor into the equation the noise from the primary panel - so finding the leg which has the least amount of noise would be your "clean" side. so things like induction motors, old dimmers, old electronics, etc can all generate noise, voltage fluctuations, spikes, and pathways to neutral and grounds which should be at zero (part of this depends on your home grounding methods - stakes vs cold pipes etc - and in doubt should check with a properly licensed electrician as messing with grounds on live circuits can result in serious injury and death).

Soundman2020 - Studio Design Forum