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Installing drywall over drywall

13K views 15 replies 8 participants last post by  jeffnc 
#1 ·
ok so im renovating a condo and sound"containing" it to help it sell

i already have 1/2 inch drywall up, and im adding 5/8" plus green glue

my question is:

1) do i still have to line up / hang new sheets on the studs or can i just go anywhere?

2) what length drywall screw do you think i need?

any other tips? thanks
 
#2 ·
You should still make every effort to ensure that your seams land on framing members, but you can certainly install another layer of rock. You need to try to get at least 1" of the screw into the joist, so buy your screw length based on that.

I'd use a chalkline to mark out the joist layout on the first layer of rock, and also mark the location on the wall with masking tape.

For locations where you can't seem to find the joist, or where seams just don't hit wood, use "laminator" screws. They're special drywall screws that are used for screwing drywall to drywall, and they actually work pretty well. You won't find them at average hardware stores or box stores...You'll have to go to a sheetrock supply place to get them. It is really worth buying a few pounds of them.
 
#3 ·
By the way, adding more rock probably won't make a big difference in sound transmission. In order to soundproof a wall, you need airspace with no solid contact from one side of the wall to the other. It might be worth your trouble to consider using hat channel furring strips on the ceilings to space the 2nd layer of rock down off the first layer.
 
#5 · (Edited)
By the way, adding more rock probably won't make a big difference in sound transmission. In order to soundproof a wall, you need airspace with no solid contact from one side of the wall to the other. It might be worth your trouble to consider using hat channel furring strips on the ceilings to space the 2nd layer of rock down off the first layer.
While I agree that this will not greatly reduce certain types of noise transmission such as direct contact noises or very deep base vibrations, the extra mass does dampen ordinary airborne sounds like voice and normal household sounds.
 
#4 ·
thanks kc... i would lose too much space doing another wall, and this was the best option

i have a friend who used the green glue and it really cut down(almost eliminated) the noise from the next unit, its amazing stuff

ill go check out those screws,thanks for the tip!
 
#9 ·
Iminvalid:

Do you have the space to construct a brick wall in front of the wall you were intending to hang new drywall onto?

Would you be interested in doing that just to reduce noise transmission for the neighboring condo?
 
#11 · (Edited)
FWIW - Once again, I agree with KCT. Particularly here, about the best construction method to reduce sound transmittance.

We have been involved with multiple projects that have been designed by engineers to reduce sound transmittance through walls. Our job was to supply the materials and build these actual sound reducing walls according to the design plans. Several of these projects have been in medical buildings and hospitals.

One in particaular, was an entire wing of Doctor's executive offices at a major regional-hospital remodeling and expansion project. The wing also contained multiple conference and meeting rooms. Confidential discussions in privacy was more than a preference, it was about legality. The engineer-designed partition walls were not built with Mass as the primary sound reducing material. They were as KCT described: Multiple layers of sound reducing sheetrock, with staggered stud arrangements, inter-woven "sound attenuation" blankets (mineral wool insulation), channels placed over these, with additional layers of sound reducing sheetrock. All facets of the areas had sound dampening caulk and pads at floor, ceiling, and outlet boxes (as these, and other connected metal components like venting) will transmit sound.

I am not going to get into the technical details, like decibal counts, or STC levels with this (no offense to anyone). The point being, that in the construction community, the best methods & designs, are not speculated, they are being put into plans, built, and tested...and they work.

With standard buildings and structures (as opposed to buildings with specific purpose-design), when you create air chambers/pockets and variable layers of sound reducing materials, these greatly reduce sound transmittance, over the use of mass, alone, in wall construction.
This is speaking on first hand involvment with such, and not based on, reading about things on the internet (no offense DIY Chatroom)...
 
#12 ·
Whatever

No offense to the so-called experts who often quote marketing spiels from "sound attenuation blankets" companies, but the apartment where I lived with the best sound-proofing had a single-layer concrete wall. We remodeled a house according to the air pockets, staggered studs, and sound blankets (as described above), but all this expensive effort had barely any noticeable improvement over simply adding more drywall. If I ever build a house or apartment on my own, I will go straight for concrete.
 
#14 ·
You want drywall screws to go about 3/4" into the studs. More than this is not only overkill, but also can result in failures because of how wood studs expand and contract. So that means 1 1/4" screws for 1/2" drywall. For 5/8" drywall over 1/2" inch, you want about 3/4" beyond the total width, or 2" screws. Going into the studs, of course.

However, don't expect this to work very well. Very little bang for all that money and effort. If I were you, I'd install an acoustic drywall solution. There are several available, including patches you apply between your drywall layers, or drywall with damping built in. For example

http://www.quietrock.com/drywall/special-panels/remodeling.html

It lists standard 1/2" drywall at about 40 STC (roughly 40 dB sound reduction) and 1/2" QuietRock 510 at about 50 STC. If you know your decibals, that means twice the sound reduction. If you put those both on the same wall I'm not sure what the reduction would be, but I'm pretty certain it wouldn't be 60. It would probably still be around 50 STC, or slighly higher.

You can listen to their claimed sound reduction on the right side of this page.
http://www.quietrock.com/drywall.html
 
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