1/2 Garage Conversion – Design review / advice

Hi All! This is my first post – I’ve been searching this forum for months after reading the Rod Gervais book collecting as much info as possible to get my design together. Thanks to all who contribute and maintain this forum! The amount of quality information here is outstanding, and I’ve already found many answers along the way. I would be very grateful for any feedback and guidance as I work to finalize the design. Overview Goal of the project is to convert 1/2 of my 2-car detached garage to an isolated practice and recording room. I’m currently focused on an MSM plan for a single open space. I’ll be using the room for DAW work, practice, recording and the occasional jam session. Isolation Requirements The garage is located on the edge of my property, 5’ from the property line, from there another 12’ to my neighbor’s house. Using a Galaxy Audio CM130 (slow / C setting), a friend and I took measurements both inside (~5’ from the instrument) and at the property line nearest to my neighbor. Instrument | Inside | Outside ‘silence’ | 47.5 | 56 Trombone | 93 | 58 Tuba | 92 | 65 Guitar Amp | 102 | 74.5 4pc Drums ‘medium’ | 108 | 78 4pc Drums ‘loud’ | 110 | 82 I’m going with 41hz as the target lowest dominant frequency – this is low E on the Tuba as well as bass guitar. I’ll be regularly playing the above instruments in the room – excluding drums. Since I’d like to host a drummer occasionally, we measured that as well as to baseline, however I realize total drum isolation is likely not possible given the current MSM design I've come up with. Inner room space is the trade off. There is also the issue of outside sound (aka, ‘the parade of weekend lawnmowers’), so keeping noise out is obviously a concern too, although I have some flexibility on when I use the room for recording. A running lawnmower on the neighboring property near the garage reads around 90dBs outside. My current room design has inside out walls and ceiling with a 6 3/8” space between the leaves. The MSM Calculator gives me the following (using the inner/outer leaf details outlined in the below plan): TL of Leaf 1: 34.89 TL of Leaf 2: 32.93 F0 (insulated cavity): 19.99 F1: 161.90 The chart is showing 73.82 TL across the spectrum, then dropping to 55.46 TL between 160hz-20hz. Assuming I can get close to this, I feel like this is a reasonable amount of isolation based on what I measured, and for my usage requirements, but I would like to know if I’m interpreting this correctly or not. The Garage The garage bay door in question has already been closed up with similar foundation and wall construction, and inside 1-car bay has been gutted. I framed in space for a narrow window for natural light and have quotes on various sizes of laminate glass, up to 5/8”. Otherwise, the garage walls are built on top of a 8x8x16 concrete block foundation (two rows, partially recessed. About 10” tall), with a poured concrete floor in the center. The slab is slightly sloped towards the front, not 100% level. Framing is 2x4 walls, 2x10 ceiling joists, 1/2” plywood attic floor, 2x8 roof rafters. The attic is completely unfinished, and has a ridge vent. There is a 120v supply but no heat or AC. Wall construction is: Cedar shingles, 1/2” plywood sheathing, R13 insulation (pink stuff), 1/2” drywall. The ceiling between the garage and the attic has white R30 insulation. Drywall and insulation is still intact in the other garage half. Framing - Outside view: Overhead (roof framing hidden): The working space for the room, is approximately: 11’ 9”w x 20’ 8”d x 9’h. The Design Plan I’ve been spending time modeling the framing details in Sketchup to plan ahead as much as possible, and to help visualize things. My focus has been primarily on the construction plan and dealing with the existing structure. The resulting room is simply a large rectangle – I’ve decided to keep the room to a single open space, and haven’t yet considered any additional layout features for treatment. The Outer Leaf

• Seal and beef up other existing walls between studs: Acoustic Caulk, Backer Rod and 2x 5/8” drywall + GreenGlue.
• I need to frame a 4th dividing wall, and add mass: 2x 5/8” + GreenGlue, plus a layer of 1/2” plywood, to match the existing, exterior walls):

(view from back to front)

• Add similar mass to the attic ceiling to complete outer leaf (Questions about this below)

Inner Room/Leaf

• Inside out 2x4” walls with 6 3/8” space between the leaves:

• Inside out ceiling construction, 1/2” below and parallel to existing joists. 2x8” joists

HVAC I have a few specs to ask about in the questions below. Basic summary:

• Wall mounted mini-split for AC & Heat
• Fantech or similar (ERV) for fresh air / stale air exhaust.
• Four silencer boxes for the fresh air system connections

Electric

• Single entry point for the 120v lines, and surface raceway inside

Final inner room dimensions accounting for the current MSM design: Height 8’ 3/4” Width 10’ 3 5/16” Depth 19’ 11 5/8” Questions: 1. Block Foundation My biggest concern currently is the foundation blocks that the existing walls are built on. The block is obviously thicker than the 2x4 wall, so the MSM space is reduced significantly at the bottom. My initial design has the inner room framed on the slab, with just a nominal space (1/8”) away from the block so that the wall framing is not touching along this ~10”h area of block (from slab to top of block foundation). Also there is the concern of mass. I’m fairly confident these are 8”x8”x16” concrete blocks with the two holes inside, and not 100% solid blocks. Although these are often filled with mortar as they go in. Should I be concerned about the effect this pinch point will have on the isolation and the overall design? Any practical suggestions for this type of situation? Here are the choices I came up with, Option #1 being the current design. The inner room is tall and very long, but width is at a premium. Option #2 gives me additional MSM space, and lowers the resonant frequency slightly. Option #3 provides for a slightly larger room with the same MSM space as Option #1, but raises some more questions about the sound transfer. 2. ‘Beefing up’ the outer leaf ceiling Since I have access to the top of the ceiling / attic floor – can I consider adding mass on top of the existing floor vs the ‘sheet rock between the studs/joists’ (under the floor) method? This is how I currently have it modeled. There would still be some challenge getting the panels up there, and working in a tight space. However, the room is long with 9 joist bays around 24” long, so I was considering other options. 3. Third Leaf section For the North and South wall-ceiling connections, there is about 1’ or so of sloped ceiling which will create a third leaf section with the roof sheathing. The roof cannot be used as the outer leaf to allow for ventilation. This is the solution I came up with: I can tighten up the space along the roof significantly if needed (yellow line), only a small gap is needed to fit a prop vent, but I wasn’t sure which direction to go in here (more / less space, more mass on these panels, etc). Should I be approaching this differently? I’ve seen cathedral ceiling examples here where the roof runs along the entire ‘outer leaf’, but not when it appears in a smaller section like this. Likewise, should I also be taking care to slope the mass of the inner leaf ceiling accordingly to maintain an equal space? I wasn’t sure how critical this is. I would likely end up leaving the frame as-is for strength, and devise a way to slope the mass within the ceiling module corner if needed. 4. HVAC I’m not done with this design yet, but here is where my research has led me so far: I’ve read here that a good rule of thumb is 6 air changes per hour for a studio. The room will be approximately 1633 cubic feet with the current design. If I overestimate for 6 people in the room max, then the average cfm needed is: (1663 * 6)/60minutes = 163.3 cfm. If I understand it correctly, and go with a wall hung mini-split, it would need to support an average cfm of 163.3 for the purpose of circulation only. Heating and cooling requirements to be calculated separately for the same unit. Then I need fresh air. At 15cfm per person, again let’s say 6 people max = 75cfm (minimum) to be added / subtracted to the room. I haven’t figured out the duct size / static pressure / air velocity equation yet to determine the silencer design. In the meantime I have been looking at specs of Fantech ERVs for fresh air. It can get very humid here in the summer, so I’m not sure an HRV will manage. Am I on the right track here? Once I figure out the static pressure and air flow calculations I will design the silencer boxes / duct system. 5. Mini-Split conduit Lastly I’ve read an example here how to penetrate the leaves for an electrical conduit, and for the silencer boxes, and will add those into my design soon. However what about the conduit for the mini-split line set? These are the flexible tubes that carry the refrigerant and would require a somewhat larger diameter conduit vs. the one for 120v. Would this be handled similarly to the electrical conduit? The example I’m referring to is a S-shaped conduit penetrating the leaves at different spots, cut in the center, and rejoined by neoprene. So that's the plan and process so far - I’m looking forward to any feedback on this! It’s been quite a learning experience. Cheers and thank you! Joe

Welcome Joe! Excellent post. You seem to have a great grasp on things! I'm short on time here so I can't get into much detail.

(view from back to front)

Add noggins on either side of your door! Like this: Put 3 in the first stud bay, then 2 in the next, then one in the next.

Should I be concerned about the effect this pinch point will have on the isolation and the overall design?

I'd go with option 1. The inside out design opens up endless possibilities for treatment all while maintaining your room space acoustically.

Since I have access to the top of the ceiling / attic floor – can I consider adding mass on top of the existing floor vs the ‘sheet rock between the studs/joists’ (under the floor) method?

Working with small pieces of sheathing makes for easy handling but lots of caulking. I assure you that adding mass on top of the sheathing inside of your attic will be the easiest method. Beefing up from below is a ton of work.

For the North and South wall-ceiling connections, there is about 1’ or so of sloped ceiling which will create a third leaf section with the roof sheathing. The roof cannot be used as the outer leaf to allow for ventilation. This is the solution I came up with:

This looks good.

Likewise, should I also be taking care to slope the mass of the inner leaf ceiling accordingly to maintain an equal space? I wasn’t sure how critical this is

This is acting as a third leaf and ultimately, the further away the leaves are from one another the better. Also, your middle leaf should have a lot of mass, so if you can have more on that, do it.

Am I on the right track here?

Yes! Be sure to calculate your sensible and latent loads to properly size your heat pump.

Lastly I’ve read an example here how to penetrate the leaves for an electrical conduit, and for the silencer boxes, and will add those into my design soon. However what about the conduit for the mini-split line set? These are the flexible tubes that carry the refrigerant and would require a somewhat larger diameter conduit vs. the one for 120v. Would this be handled similarly to the electrical conduit? The example I’m referring to is a S-shaped conduit penetrating the leaves at different spots, cut in the center, and rejoined by neoprene.

S with neoprene for conduit is the way to go. For the line set you don't really need conduit but still use the S method and a ton of caulk! Greg

Hi Greg - A tremendous help so far, Thank you!! I'm going to focus on working out the HVAC details / silencer box design next. Cheers! Joe