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I am hanging a double layer of 5/8" type-x drywall with Green Glue, a soundproofing compound, sandwiched in the middle. If I hang the first layer with the face facing outward like normal and then stagger the second layer over it, i'll have about a 5" pocket where the two joined beveled edges of the first layer would recess away from the first layer. My understanding with Green Glue is that it needs to be between two rigid layers, so this 5"x8'-wide pocket would present an area where the sheets are not touching.

My idea is to hang the first layer with the backing facing outward. When I hang the second sheet, the two layers will be back to back with no pockets in the middle.

It may not seem like there could possibly be any issues with this, but my experiences in home improvement have taught me to ask even the most loaded questions since it's better to risk looking like a fool up front than be in a full body cast down the road.

Edit: I am also aware that perhaps a pocket of dead space would probably only server to make the structure slightly more soundproofed. However, I think its best to have both flat sides facing one another to aid in the spreading out of the glue.

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You seem to be very focused on making this room completely sound proof, you're not building a "murder room" are you? –  Tester101 Nov 5 '12 at 19:49
    
HA!! That's what one of my Facebook friends said a while back. Exactly that: murder room. –  oscilatingcretin Nov 5 '12 at 19:52
    
Have you taken this to AVS? I presume that's where much of your initial design started... but surely people there have already built such rooms. –  Matthew Nov 5 '12 at 23:25
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3 Answers

up vote 2 down vote accepted

Green Glue does stipulate to be used between rigid layers. However, even the beveled edge of the drywall meets this definition.

The edge of the drywall is beveled so that the mud and tape when finished doesn't create a "hump" in the wall at the seam. If you don't mount the drywall with the beveling in the right direction (towards the room) then you will notice all the seams.

Let me preface by saying that I think the green glue installation will be just fine even against the beveled edge of the layer beneath but if you still want to make sure the surfaces are smoothly mated then you should install the inner layer backwards and the front-layer correctly.

That is, the stud-attached layer has the beveled side facing the studs. This leaves the rough edge (without bevel) facing the room. Then the finish layer adhered its rough side to the wall... thus the finished, beveled side faces the room.

On a side-note, are you also using low-density soundbard and insulating the walls between the framing members?

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I am using regular insulation and type-x drywall (73 lbs per 4x8 sheet). However, this wall is being completely suspended by being mounted upon five rows of metal furring channels which are snapped into RSI clips which are fastened to the studs. The STC this wall will yield will be such that I won't need special insulation like Safe 'n Sound. –  oscilatingcretin Nov 5 '12 at 19:31
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My only concern is the holding ability of the backing paper against the bugle screw heads. I've never installed drywall backwards, so I have no idea if it's a problem. Installed conventionally, the whole system works just right. The gypsum compresses a bit, the paper deforms without breaking, yielding a reasonably strong connection. Overdrive the screw just a bit, the paper breaks, the gypsum cracks, the screw holds nothing. It doesn't take much to mess up the system, I've no idea what will happen backwards.

I honestly don't think the bevels will effect the STC at all, but if it makes you feel better, simply fill the bevels with mud. You don't even need to tape it, so what if it cracks. You only need a single pass, it doesn't have to look pretty. I think it's worth the little bit of extra labor to use the system as it was intended.

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Your answer prompted me to try this Google search: google.com/… . None of the sources from that link indicate this is an issue. I am hoping it's not because I already hung the first layer =\. –  oscilatingcretin Nov 5 '12 at 23:39
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If the heads pop on the first layer it won't matter anyway. The 2nd layer will hold the first layer. –  The Evil Greebo Nov 5 '12 at 23:54
    
I screwed these in right this time using a special drywall bit that keeps the head from breaking the paper. I have 25 screws per 73-lb 4x8 sheet and none of the heads have broken the paper. I am personally confident in it, but I am but a novice and yield to the expertise of you guys on here. So, Greebo, you think installing backwards is okay so long as it's not a sloppy job of punching the screws through the backing paper? –  oscilatingcretin Nov 6 '12 at 0:03
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@oscilatingcretin Don't break the paper, and you should be fine. –  Tester101 Nov 6 '12 at 12:44
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I would not apply the first sheets of dry wall with the back side out. The problem is that near the edges you will be using screws where the taping mud gap is located and you risk causing stress, cracking or screw head pull through. If you put the sheets on the correct / normal way you will eliminate this problem for the first layer of the drywall. Then to be perfectly honest with you it would be advisable to trowel in a layer of drywall mud into the taping mud gap before you come with the second layer installation. This will prevent the same gapping problem on the second layer when you put screws through that layer into areas where the tape mudding gap would have been.

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I've already accounted for this by drawing a line parallel to and two inches from the edge so I know not to drive any screws there. The first layers is hung and it looks pretty good. –  oscilatingcretin Nov 6 '12 at 15:33
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