A question about existing Joists- Garage conversion

Hello The usual applies to me, long time lurker, first time poster. Thanks in advance for reading my post, I would be appreciative of any and all advice. I'm thinking about putting a build diary up but I thought I'd ask a question that's been on my mind for a while, first. My garage conversion measures 19.5' by 12'. It will have at most 8'8" ceilings, but that depends on how I go with the ceiling (this question). I think it will likely end up around 7'10-8'. It is a detached building, although we share one wall with out neighbor's garage, which is a mirror to ours. But neither garage is attached to our houses, and it sits behind the house with neighbors not too far away. Consequently, I'd like about 50db of reduction so that I can track some loudly amp-ed guitar or mix at a comfortable level. I will not have clients or bands there, and I don't think I can fit a drum kit anyway. My garage has the main floor area (currently gutted with a new slab and DPC) as the dimensions above, with a loft area above this with sharply sloping eaves, which is only 6'2" high at its apex. It is also the same dimensions otherwise, albeit with the sloping roof, not as much is usable. This loft will be my machine room/soldering area. The main floor area will be open plan, no booth or subdividing. I've been reading Rod Gervais' book, and info on this site as well, but I can't seem to find a clear answer to my specific problem. My garage roof is designed to be upheld by the 6" thick joists that run the entire long way of the garage. Because these rest on the existing exterior walls (14'' thick stone), and are attached to the far edges of the roof, they stick out past the exterior wall into the outside world. Like a triangular mushroom shape. So these joists are a major issue in preventing sound escaping via flanking. It seems clear to me that any new ceiling (and walls) I construct has to be decoupled from these joists, or they will be a flanking path to the outside. The question I have is that beyond the new 2-layer ceiling that will sit decoupled, should I put 2 layers of drywall on the bottom of the joists themselves? I know the latter 2 layers will form one mass with the joists, but will I gain any sound proofing by doing: existing joist---2 layers drywall attached to joist---air gap---new 2 layers of drywall vs: existing joist-----airgap---new 2 layers of drywall Mainly I'm trying to decide on using resilient channel for the new ceiling to maintain some ceiling height. So should I have 2 layers on the joists, then resilient channel, then 2 layers of new ceiling hung on the channel, or just skip the first 2 layers? I am not particularly concerned about sound going up (due to not having anyone above), more sideways. Thank you very much for any help and I hope my wordy description makes sense.

Hi Carl, and welcome! :) It's good to see someone who did their homework, and took the time to follow all the forum rules! Photos and diagrams would help to better understand what it is you are dealing with, bit if I understand correctly, it seems to me that this might be one of those occasions where you have no real choice but to build a 3-leaf ceiling. If you can't seal the area above the joists, then sound will, indeed, escape, and you won't have a proper MSM system up there. So it seems like your best bet might just be to put mass on those existing rafters (drywall, OBS, plywood, etc), and seal off the space underneath for building your actual inner-leaf. So that "ceiling" mass will become your middle leaf. Then, to get a high level of isolation (50 dB is pretty high) you'll probably need to build a separate stud and joist frame inside that space, and put a couple of layers of drywall on one side of that, for your inner leaf. To maximize your final ceiling height, you could use Rod's concept of "interleaving" the new joists with the existing joists, assuming that you put the "middle-leaf" mass above the existing joists, or between them. - Stuart -

Carl Hungus wrote:

I've been reading Rod Gervais' book, and info on this site as well, but I can't seem to find a clear answer to my specific problem. My garage roof is designed to be upheld by the 6" thick joists that run the entire long way of the garage. Because these rest on the existing exterior walls (14'' thick stone), and are attached to the far edges of the roof, they stick out past the exterior wall into the outside world. Like a triangular mushroom shape. So these joists are a major issue in preventing sound escaping via flanking.

Air borne sound is the issue in this respect, the path of least resistance, where air goes so does sound, etc. There is nothing uncommon about this issue and it is typical of any structure available. The air path required to extract heat from a roof has a joist/rafter system that gives access to convection to work cooler air from the ground to move up and in and hotter air to move up and out. Contrary to popular belief adding mass to this (the rafter) will only add cost to the job but will not help you in any way. Far from it. Encapsulating the air path with a hard boundary reduces/removes the ability of air to move freely in this cavity and adds an area of uncertainty as to how much heat is being removed and how much is retained and this is never a good thing to the health of the structure subject to the most extreme of natures natural heating cycles.

Carl Hungus wrote:

It seems clear to me that any new ceiling (and walls) I construct has to be decoupled from these joists, or they will be a flanking path to the outside. The question I have is that beyond the new 2-layer ceiling that will sit decoupled, should I put 2 layers of drywall on the bottom of the joists themselves?

The first part is accurate, a flanking path. The second part is no.

Thanks very much for the replies guys. I've started a build thread that has some pics that might make my issue even more clear: viewtopic.php?f=2&t=17903 Please let me know if I need to post more pics and what your thoughts are. Cheers!

Brien and Soundman Assuming you had a moment to look at some of my build thread pics (happy to attach here if you prefer) how would you address the issue then of the air gaps. I have attached a crude sketch to illustrate. I normally would prefer to not have the 3 leaf system, not waste money as Brien suggests, but I'm not sure how best to keep the air from flowing over the bricks between the joists. many thanks

Hi Carl, Here is the solution I would use. On the inside of your brick wall, seal the cavity between the the loft floor joists, the loft floor and the brick wall. Make sure to use the same amount or more mass as your existing brick shell. Insulate these areas as they become part of your wall system. The standard is board foam, sealed around the edges, or spray foam. Now add a second layer of board to to the top of your loft floor for extra mass, I believe there are examples of this in rod's book. Make sure the structure can support any extra mass being added. As a note, if you have an stairs up to the loft, close this off with a wall, door etc. as part of your first leaf. This should give you a sealed first leaf. Build your second leaf inside of it. Steve Ps I looked at the other thread, wanted to double check that you have a vent at the top of your roof and that you left a clear airspace under the insulation so that the roof can breathe properly.

Hi Cold Thanks for your reply. I do actually have roof vents on both sides of the roof for that reason. The building inspector asked for them and air holes to pass through the roofing batons, so that should be ok. I was actually thinking about another option which is to use RC on the joists to decouple 2 layers of drywall from them as the outer leaf. And then from the same joists, but spaced around the RC, use the isolated hangers in Rod's book to float another double layer 100-150mm under that first outer layer. That way the sound leaving the room will still hit 2 leafs before it hits the joists. Obviously its not perfect as the channel and hangers aren't a desirable as a truly removed ceiling but I just can't afford to lose 12-14" from a 8'8" ceiling. I'm hoping that the channel and hangers are decoupling enough to constitute a 2 leaf system with some effective dB reduction.

You mentioned your ceiling joists are 2x6", and are running the long length of the room ... that's 19 ft ??? If so ... I don't think you'll be able to hang any drywall off of those joist. There are other options is this is indeed the case. just an observation.

" I do actually have roof vents on both sides of the roof for that reason." You have a gable roof with vents on either end?