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 333 times

: laminated-glass-isolation-severl-types-graph.gif (14.18 KiB) Viewed 333 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.

Soundman2020 - Studio Design Forum