Room in a room build

Mastiff

Senior Member
I'm going "all in" on a room in a room practice room build. I'm tearing my space down to nothing and trying to do everything right. Hopefully this is the correct spot to document the project. I know I would have liked to have had more examples to check out before we started.

Here's the starting point, at least the first I really took pictures of:





The pics show the two opposite corners of the room, which is at the back of a garage space. The room is about 10x11. The black walls are to the exterior of the building and you are seeing the weatherproofing material on the back of the sheeting. The other two walls are to the interior of the garage.

The first step, now complete, was to seal up every single crack or seam in the room with caulk. Next we beef up the outer walls with additional drywall. For one of the walls facing the garage, this was done by putting a sheet of 5/8" drywall on top of the existing drywall with Green Glue in between (then caulking all seams). For the other walls, that can't be treated from the outside, we will be putting additional drywall in between the studs. Two layers of 5/8" for the exterior walls (which otherwise only have sheeting) and one additional layer of 5/8" drywall for the interior facing wall. The ceiling also got an additional layer of 5/8" with Green Glue.

Here are pics as it stands now, with everything caulked (note some white, some clear) and cutting of drywall strips on process.







If people are interested, I will keep posting stuff as progress continues.
 

Mongrel

Silver Member
Yes-please keep posting your progress.

To be honest, from the description, I was expecting to see a “free standing” isolated\insulated box within the space-totally separated from any attachment (vibration transfer points) to the walls of the existing “room” (in this case the garage) and only having the cement floor in common. Space is your friend....

Definitely i lnterested in how your method works out. Keep us posted...
 

GetAgrippa

Platinum Member
So is the room within a room where the internal room is completely isolated from exterior room? Will you build a raised floor to isolate from concrete or is that a concern? Will it be temp controlled? Sorry got lots of questions and yes keep posting your progress!
 

Mastiff

Senior Member
I didn't describe the whole plan. Yes, in the end there will be another room inside the room, with two doors. I'm going to risk it and try to have windows as well, since people say it can be done with proper glass (which I have on order). I've been researching a ton recently, along with a contractor I'm working with. The main goal is to have two (and only two) extremely heavy walls with the largest air gap possible in between and no air leaks anywhere. The inside room will only touch at the floor, which is concrete slab and most people think shouldn't be an issue. We'll be using an isolation strip between walls and slab as well, though there are mixed opinions about whether this adds much value. I only need to keep sound from escaping the building (other spaces on the same slab don't matter).

Minimal HVAC is planned and will entail drawing air from an adjacent room that is conditioned and pushing it back. All breaks in the inner or outer walls will have a muffler/silencer, so there will be four of these total.

So right now we're at the stage of beefing up the outer walls. The inner room will come next, possibly after addressing some of the HVAC penetrations.
 

Mongrel

Silver Member
I didn't describe the whole plan. Yes, in the end there will be another room inside the room, with two doors. I'm going to risk it and try to have windows as well, since people say it can be done with proper glass (which I have on order). I've been researching a ton recently, along with a contractor I'm working with. The main goal is to have two (and only two) extremely heavy walls with the largest air gap possible in between and no air leaks anywhere. The inside room will only touch at the floor, which is concrete slab and most people think shouldn't be an issue. We'll be using an isolation strip between walls and slab as well, though there are mixed opinions about whether this adds much value. I only need to keep sound from escaping the building (other spaces on the same slab don't matter).

Minimal HVAC is planned and will entail drawing air from an adjacent room that is conditioned and pushing it back. All breaks in the inner or outer walls will have a muffler/silencer, so there will be four of these total.

So right now we're at the stage of beefing up the outer walls. The inner room will come next, possibly after addressing some of the HVAC penetrations.
“Ahhhhh....!” So THAT’s the plan! Thanks for the clarifications.

Nice!

definitely keeping an eye on this thread....
 

Mastiff

Senior Member
The beefing up of the outer layer is now complete. We'll make one more pass over to make sure every nook and cranny and nail hole is totally sealed. We were going through caulk so quickly, the contractor decided to start using rubberized roof coating, so that is now slathered over all seams. All four walls and ceiling now have two full layers of drywall, with the new layer being heavy 5/16". The ceiling and one wall, the ones with full sheets instead of strips in between studs, have green glue in between the layers. Green glue is expensive stuff, so for the strips between studs we used carpet glue (Roberts 3095) which some people think is a good substitute and much cheaper. Apparently, the company is in the process of getting their glue tested for STC so they can legitimately market it for this purpose.

The bottom picture is a foam mock up of the baffle/silencer boxes that will be used to get air into the room without compromising isolation. The real items will be made from 1" MDF.





 

Mastiff

Senior Member
Here are pics of the inside room in process. Walls done, ready for ceiling. There's an inch gap all around (from studs; actual air gap will be 7" or more between actual walls). 2x4 construction for walls, ceiling will be 2x6. 24" o.c. stud spacing is preferred acoustically. Ceiling will be 16" for strength to hold the weight of two layers.



 

Mongrel

Silver Member
Looks great, thanks for the updates and pics.

Questions-

Is there a recommendation for batt type insulation between the studs on those partitions?

Also-curious if the doubled up/glued rock is better than two normal framing/rock partitions (rock/insulation/rock) separated by “space” for a sound barrier?

thanks!
 

Mastiff

Senior Member
Rock Wool is generally a favored insulation type for acoustic stuff, which is what I'm going to use, but I've also read that there is not much difference between that and plain old fiberglass. I've seen several expert build ups online that went with fiberglass. So, I guess I don't have a definitive answer. My understanding is that it's mostly to stop echo in the cavity, not so much to "stop" sound per se. It requires mass to do that.

One thing I learned early on in my research was that there is a thing called the "triple leaf" problem. The idea is that you want two heavy walls with a big air gap in between, and if you put multiple layers (walls) instead (like a third wall in the middle), you can make the overall isolation much worse because of how the layers resonate. Big gaps push the resonant frequency down, and low frequencies are the hardest to stop.

My understanding of the acoustics is a little shallow, but the triple leaf concept has come up over and over, and best practice is definitely to have two walls with as much mass in them as possible and as large a gap as possible. The next level consideration has to do with the wall materials themselves and how they resonate. Green glue is in this category, and stud spacing.
 

Mongrel

Silver Member
Rock Wool is generally a favored insulation type for acoustic stuff, which is what I'm going to use, but I've also read that there is not much difference between that and plain old fiberglass. I've seen several expert build ups online that went with fiberglass. So, I guess I don't have a definitive answer. My understanding is that it's mostly to stop echo in the cavity, not so much to "stop" sound per se. It requires mass to do that.

One thing I learned early on in my research was that there is a thing called the "triple leaf" problem. The idea is that you want two heavy walls with a big air gap in between, and if you put multiple layers (walls) instead (like a third wall in the middle), you can make the overall isolation much worse because of how the layers resonate. Big gaps push the resonant frequency down, and low frequencies are the hardest to stop.

My understanding of the acoustics is a little shallow, but the triple leaf concept has come up over and over, and best practice is definitely to have two walls with as much mass in them as possible and as large a gap as possible. The next level consideration has to do with the wall materials themselves and how they resonate. Green glue is in this category, and stud spacing.
Thanks! Great info-appreciated!
 

Mastiff

Senior Member
Worked on HVAC today. Below are pictures of the muffler boxes, made from 3/4" MDF and lined with acoustic duct liner. Two of the boxes go up in the attic and will connect the outer layer of the drum room to the adjacent room, one for in and one for out. One leg will have an inline fan to pull air in. The adjacent room has a mini-split AC/heat unit. The next picture shows the boxes in place in the attic, and you can see the openings into the ceiling of the air conditioned room. The 4th pic shows the penetration point into the outer layer of the drum room. There are two like that.

Two more boxes will be used and mounted to the inside layer.









This shows how the electrical boxes look when treated with putty pads. These fully wrap the back of the box to keep sound from escaping. One the sheeting is up, the gaps will be caulked as well.



Finally, here is one metric crap-ton of insulation waiting to be installed on both the inner and outer walls.

 
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Mastiff

Senior Member
Significant progress today. Installed insulation in both inner and outer walls, and installed the muffler boxes inside the room. The inside room will have a layer of 3/4" MDF and then a layer of 5/8" drywall on top of that. Some of the MDF is installed.

In this pic you can see the inner and outer layers of insulation. There's a 1" gap between them.



A pic of inner MDF partially installed:



Muffler box inside the room. It's upper outler will be connected by flexible duct to the penetration in the outer layer ceiling.

 

dboomer

Senior Member
Muffler box is cool. But you can achieve almost the same thing by using flex duct and putting at least three 90° bends in it
 

Mastiff

Senior Member
Muffler box is cool. But you can achieve almost the same thing by using flex duct and putting at least three 90° bends in it
But flex duct itself has little or no isolation compared to two heavy sheets of drywall with green glue in between. Without the box, you have a 4-6" hole in each layer with only flimsy ductwork blocking the sound. FWIW, I didn't invent this, but learned about it on the acoustic design forums.
 
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