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:
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?
- 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?
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
- 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
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.