Detached Garage/Studio Space Sacramento CA

[Funcastle]

Project Overview:

I’m looking to convert my detached two-car garage in Sacramento, CA into a multi-use space where half of the building will be used as a motorcycle garage/workshop/storage space and the remaining square footage will be used to construct a project studio/practice room. I will summarize the details of the building in this overview as needed, but I have also included a fairly comprehensive spec sheet, drawings, and photos below.

My initial plan is to build a wall that bisects the current building east to west, and another that intersects the new wall and the south gable end of the building. (See attached floor plan) The existing square footage is ~548.5’ (24’X24’ to the outside of the framing, 23’5”X23’5” to the inside of the 2X4, 16”OC, wall framing). My budget, framing expertise, and existing roof framing (more on this later) preclude me from building a room inside of a room. I know that I will not be able to achieve pro-level sound isolation, but this is a compromise that I can live with.

Ideally, I would like to be able to practice with my four-piece band (drums, bass, guitar, vox) at any hour and not disturb my nearest neighbor. We currently practice now until 10pm, at which point we have to shut it down. The building, as it sits, is basically an empty shell with exposed studs, four soffit vents and a box vent on the roof. There is a 16’x8’ aluminum roll up door (with insulated bays) on the north end, and a metal hollow-core door and 5’x3’ window on the south end; basically it’s like practicing outdoors sound-wise. I took SPL reading last practice and were the following:

Two-minute average of in-room ambience:
34dbA - 53dbC

Four-piece band playing lively (drums, guitar, bass, vox):
In-room: 100dbA – 112dbC
Outside nearest neighbors house (approx. 20’): 75dbA – 86dbC

I’m not quite sure what my neighbor is hearing inside of his house, but for this exercise let’s assume that his external walls have a 30 STC rating.

At the very least, I would like to be able to track guitars and bass (inside an iso box if need be) and monitor around 80dbC - 85dbC, preferably with the sub on, late at night and into the early morning.

Below are some sketched of the existing floor plan and what I plan to do with the space. The room’s main purpose will be to serve as a control room where I can mix, overdub guitars and bass, and also track drums (with in-ear monitors) for my own project demos and less-professional recordings for friends. Also, as noted above, I will use the space to jam and practice. For the latter however, acoustics are much less of a priority; all we need are four walls, a roof, and electricity.

FLOORPLAN.jpg

ELEVATION.jpg

Existing building details:

Location: Sacramento, CA

Type: Detached two-car garage 24 x 24’ (to the outside edges of 2x4s) 576 s.f.
Empty shell (No drywall,OSB, plywood, etc) Garage door opens to an alley and the building is ~85’ behind the house. The nearest neighbor is 20’ away from the garage to the west.

Foundation: Concrete slab

Walls: 2x4 16” o.c. bolted to slab with double top plate measuring 97” in height. Exterior is 7/16” sheathing with 1x2 trim on the plywood seams.

Roof:
Style: Gable
Pitch: 4:12
Cover: 1 layer composition shingles over paper and 7/16” plywood sheathing.
Venting: 4 soffit vents and 1 central box vent 1’ off the ridgeline.
Framing: 2x8 non-continuous ridge board and 2x6 rafters, 24” o.c. 2x6 ceiling joists on every other rafter (48” o.c.) that sit on the top plates and bolt to rafters. The rafters in between have 1x6 collar ties nailed to the rafters 18” below the ridge board. The rafters with the bolted 2x6 joists also have a vertical 1x4 member that is nailed to one side of the rafter, where it meets the ridge board, and the 2x6 joist. Additionally there are 2 2x4 kickers at each gable end of the building that are nailed to the top plate and ridge board; they are oriented at a 45 degree angle. Also, there is a flat 2x4 bridging member that runs the length of the building, gable end to gable end, that are nailed 48” o.c. to the top of the joists. (See sketches)

Openings:
Aluminum roll-up door 16’ W x 8’ H on the north gable end.
Hollow-core metal door 38” W x 82 ¾ H r.o. on south gable end.
Single pane aluminum framed sliding window 60” W x 36” H r.o. on south end.

Electrical:
Single-phase 50A subpanel. Wire is run underground in schedule 80 PVC from the main panel approx. 85’ away, and is stubbed up through the slab and bottom plate. Sub panel is recessed in wall and is grounded through the slab.

Existing SPL Readings (Slow response):

Two-minute average of ambient in-room measurement:
34dbA - 53dbC

Four-piece band playing lively (drums, guitar, bass, vox):
In-room: 100dbA – 112dbC
Outside nearest neighbors house (approx. 20’): 75dbA – 86dbC

Budget: $3000-$5000


Starting my Project:
Here is how I plan to start and the order in which I plan to do things:

Beef up roof framing for a Coffin-Style Ceiling:
1. Add new bolted 2x6 top plate joists to the rafters that lacked them (in the studio portion of the building only). (Existing Rafter 1 img)

Roof framing page 1.jpg

2. Remove the existing 1x6 'C' struts from the rafters I put the joists on in step 1.
3. Add a new 2x6 struts to the same rafters and then add the vertical brace fastened to the rafter, where it meets the ridge, and the newly added 2x6 strut.
4. Remove the pre-existing vertical "B's that were fastened to the rafter and pre-existing top plate joists. (Existing Rafter 2 img)

Roof framing page 2.jpg

5. Repeat step three for the remaining rafters.

Roof framing page 3.jpg

***The drawings say that I will use 2 layers of 5/8 gyp. That is not the case, the drawings were made before I decided on RC1 and one layer of 5/8 gyp.

Eliminating Roof Venting in Studio Room and Seal Roof Shell:
For the rafter bays, I plan to build an unvented roof system in the studio portion of the building. To do this I will first seal the box vent hole by cutting, gluing, and caulking 7/16” plywood cut in the shape of the opening. Then I will nail 1x2 (3/4” x 1 ½” actual) furring strips to the inside of each rafter bay, flush with the underside of the 7/16” plywood sheathing, oriented on edge. I will then glue 1 ½” polystyrene insulation board to the underside of the sheathing, it will sit flush with the 1x2 furring pieces. I will then nail/screw and additional layer of 7/16” osb to the furring strips, leaving 1/4” gaps between the adjacent pieces of plywood and inside rafter edges. Then install 3/8” backer rod and caulk with OSI 175. For the two soffit vents, I will toenail 2x4s, cut to size, and caulk the heck out of it.

Existing Walls:
I plan to seal all cracks, gaps, and holes with OSI 175 and backer rod (where applicable). Remove the widow on the south gable end and cover hole with 7/16” plywood, leaving the ¼” gaps and backer rod and caulk.



A Few Questions Before I Start

Room Modes
I used the amroc room mode calculator to find dimensions that land in the “Bolt-area”, in my case 18’ x 12.5’ x 9’. If you look at the elevation drawing, you will see that there is three feet on the west side of the studio that has a sloped ceiling from 9’ to 8’. Is this going to screw everything up? Or will I still be in the ballpark?

FYI, my listening position will be a few feet off of that west wall, and I’m assuming, although it has yet to be designed, that I will probably have a lot of treatment and trapping along the wall and in the corners. I don’t know if that will have any bearing.

Sound Isolation of Wall and Ceiling Assemblies
From the reading I have done, it looks like the wall and ceiling assemblies I have chosen will yield roughly a STC45. (See attached reference from the NRC-CNRC publication)

WALL ASSEMBLY.png

The beefed up external walls and ceiling will have an outer leaf surface mass density close to that of 5/8” drywall and the walls that divide the studio and garage, and the studio and airlock are identical. Am I correct in this approximation?

What does that mean in practical terms, with the SPL levels I gave earlier and the proximity of my neighbors?

Doors
The door from the studio to the airlock will be solid core, with additional mass (5/8” drywall and maybe some absorbent material such as rock wool), GM trunk rubber (per the bible), thermal lock threshold, and drop seal. Can I skimp on the door from the airlock to the backyard or the door from the airlock to the garage?

Airlock/Mechanical Room HVAC
I have seen threads on here (and a section in Rod Gervais’s book) discussing a HVAC solution where one utilizes a through-wall heating and cooling unit in fresh air mode in a “mechanical room”, and then uses a system involving an air exchange ventilator (I’m looking at the Fantech AEV1000) and silencer boxes to circulate fresh, conditioned air into and out of the studio space. In my case, would cutting a hole for the wall unit in my airlock make sense, sound isolation-wise? (see floorplan)

Would I be better served with a mini-split ductless system mounted in-studio and a AEV/silencer box system fed from the garage portion of the building? Any input or rough design help would be much appreciated.

RC-1
I know all RC is not created equally. Does anyone have experience with RC1 from Lowes? Rod’s book says that RSIC-1 clips with hat is superior, but it’s not within my budget. I am however, down to spend a few extra hundred bucks for another 5db of mean isolation. Would that be a reasonable result?

Dummy Check
I tried to be as thorough as possibly with my description and attachments. I’m sure more questions will come up as I move along with my build. Is there anything that is glaring that I need to address before starting? There has to be at least one, “can’t see the forest for the trees scenario in there.”

Thanks for the awesome forum,
Zac

[Funcastle]

After working on sealing up the outer shell of the building and talking with a friend who has built a few rooms, I have decided to change my plan a little bit. I will be editing the initial post to reflect those changes in a few days once I am organized.

Now back to caulking...