Double walls filled with spray in insulation?

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Snarl 12/8
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Double walls filled with spray in insulation?

Post by Snarl 12/8 » Mon Aug 05, 2013 9:35 pm

I didn't want to hijack the "room within a room" thread, but I have a similar question, I think. I've been pondering forever the idea of doing a room in my basement. My wife the other day said, wouldn't the garage be easier? And she's so right. The room will have to be a little smaller, but that doesn't bother me incredibly much, but I have a question about spray in insulation.

I want to use spray in insulation because, to my understanding, you don't need a vapor barrier with it. The walls are uneven and the roof has the "ship lap" (I think) thing going on. Where it's not flat plywood against the roof beams, but slats of wood with gaps, so I can't press rigid insulation against it. I'd need to do a roof vent and leave a 1" gap, etc., etc.

So, if I built a second wall about 1" away from the current exterior wall and then I filled the resulting space with spray in insulation will my two walls still be decoupled? The spray foam would be continuous from the inside surface of the exterior wall of the garage to the exterior facing surface of the sheetrock walls of the recording space. I've never encountered the spray in insulation, I don't know how rigid it is, whether vibration travels through it, etc. If I can't use spray in insulation, I think I need to reroof the garage before I start.

Anybody know how this'll work?

Thanks,

PS has anyone here gotten rid of those 2x4's that cross the garage at the top of the wall to prevent the weight of the roof from pushing the walls out? What did you do? A truss of some sort?[/b]
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Nick Sevilla
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Post by Nick Sevilla » Mon Aug 05, 2013 9:47 pm

So, if I built a second wall about 1" away from the current exterior wall and then I filled the resulting space with spray in insulation will my two walls still be decoupled?
The answer is no.

Decoupling means "no touching" or in the case of studio walls, "barely touching".

If I were you I would use regular batting, it would not create a permament hard contact between the two walls. I would just sit there, especially if you don't stuff it in a lot.

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Post by llmonty » Tue Aug 06, 2013 9:08 am

I used spray insulation in the ceiling of my last space. I used 2" of it - sealed nail holes and other penetrations in an unvented roof design. I also needed it for R value requirements. I think back filled it with netted cellulose for $ savings and sound isolation.

This approach may work for you but it depends on a number of factors and a system design. Couple of comments:
-The gaps you are talking about I imagine could be fairly large in some areas -- 1/2" to an 1". These intuitively may be too large for spray foam. Foam wants to travel into every nook and cranny - I could see it potentially pushing out against your roofing materials. BTW, this isn't ship lap. Ship lap is like an open faced tongue and groove for lack of a better term.
-You would generally still have missing mass in those gaps. You could consider adhering plywood inbetween the ceiling rafters to add mass and to seal holes.
-You would need to decide on vented/unvented roof design and the ability to change it from whatever it is currently.
-Spray insulation is generally not good for sound isolation purposes and it acts as a "coupler" - exactly what you don't want. It is also VERY expensive relative to batts.

I am not expert, but those are my top of mind observations.
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jhbrandt
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Re: Double walls filled with spray in insulation?

Post by jhbrandt » Thu Aug 22, 2013 10:02 pm

Snarl 12/8 wrote:...I want to use spray in insulation because, to my understanding, you don't need a vapor barrier with it. The walls are uneven and the roof has the "ship lap" (I think) thing going on. Where it's not flat plywood against the roof beams, but slats of wood with gaps, so I can't press rigid insulation against it. I'd need to do a roof vent and leave a 1" gap, etc., etc.
Roof; you will need to install some molded air channel next to the roof sheeting to prevent condensation... this will duct air from the eaves or soffit vents to the ridge vent at the top of the roof.
You can then tack r13 or r19 (depends on ceiling joist size) with the frk facing down over this assembly.
Snarl 12/8 wrote:So, if I built a second wall about 1" away from the current exterior wall and then I filled the resulting space with spray in insulation will my two walls still be decoupled? The spray foam would be continuous from the inside surface of the exterior wall of the garage to the exterior facing surface of the sheetrock walls of the recording space. I've never encountered the spray in insulation, I don't know how rigid it is, whether vibration travels through it, etc. If I can't use spray in insulation, I think I need to reroof the garage before I start.
Spray foam is completely useless for air cavity absorption. It DOES work well for thermal. I would recommend against it's use unless you need the 'sealing' effect. If you DO use spray foam, apply ONLY a 1" layer on the EXTERIOR wall only- then add light-weight fiberglass insulation. - Interior partition; install only R13 for 2x4s and R19 for 2x6s. - FRK facing the room interior.
Snarl 12/8 wrote:PS has anyone here gotten rid of those 2x4's that cross the garage at the top of the wall to prevent the weight of the roof from pushing the walls out? What did you do? A truss of some sort?[/b]
Under most circumstances you DO NOT want to touch those cross members. Your building will eventually fall down. But if you are to remove those, you must replace them with something that will perform their function structurally. I.e.; Scissor-type structural support. I highly recommend that you consult with a structural engineer or at least get the advice from the city/council building inspector. They usually have recommendations and can be helpful - (if they're not nazis - LOL)

I have just posted a new paper on my site which you might find helpful:
http://jhbrandt.net/Sound-Proofing-The-Quest.pdf

Cheers,
John
John H. Brandt - Recording Studio, Performance Hall & Architectural Acoustics Consultants
http://www.jhbrandt.net

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