# Installing drywall over drywall



## Termite (Apr 13, 2008)

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.


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## Termite (Apr 13, 2008)

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.


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## iminvalid (Apr 20, 2008)

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!


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## troubleseeker (Sep 25, 2006)

thekctermite said:


> 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.


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## Termite (Apr 13, 2008)

troubleseeker said:


> 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.


Hmmm. I'm sure the extra mass certainly doesn't hurt. They often do two layers in multifamily construction for fire assemblies, but rarely for sound attenuation. Sound attenuation/party walls almost always have staggered studs so each wall face has its own studs, or they use hat channel or resilient channel to break contact between the solid surfaces.


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## Nestor_Kelebay (Jun 17, 2008)

thekctermite said:


> Hmmm. I'm sure the extra mass certainly doesn't hurt. They often do two layers in multifamily construction for fire assemblies, but rarely for sound attenuation. Sound attenuation/party walls almost always have staggered studs so each wall face has its own studs, or they use hat channel or resilient channel to break contact between the solid surfaces.


That's true, but short of rebuilding the wall, adding mass in the form of heavy drywall to that wall is the most effective way of reducing sound transmission through the wall from the neighbor's condominium.

That is in fact the basis of the "Mass Law" which is the most fundamental principle of sound transmission through buildings.

PS:
What's the "Mass Law"?
the Mass law states that for every doubling of either the mass of the wall or the frequency of the sound hitting that wall, the sound transmisstion through the wall is reduced by 6 decibels, or to 25 percent of it's previous level. (it's either 25 percent or 50 percent, I forget which now.)

The mass law is the direct result of the extremely simple way noise travels through walls and ceilings in a building. Basically, a sound wave hits the wall and causes the wall to move. The movement of the wall recreates a new sound wave on the other side of the wall. IT IS THE REPRODUCED WAVE WE HEAR, NOT THE ORIGINAL.

If you double the mass of the wall, then a heavier mass won't move as far under an applied force, and the reproduced sound wave will have a lower amplitude (pronounced "quieter") than the lighter wall would have recreated.

Also, the heavier the wall, the more inertia it has, and the less able it is to change it's direction of motion. So, as you increase the mass of the wall, you reduce it's ability to recreate higher frequency sound waves, and the result is reproduced sound waves without the higher frequencies in them.
This is why we hear BOOM-BOOM-BOOM from a loud stereo late at night from another apartment rather than the music the way it plays on a stereo. The inertia of the walls and ceilings are effectively filtering out the midrange and treble frequencies, leaving us with only the low frequencies. The lighter the walls and ceilings are, the less midrange and treble gets filtered out.

If I ever bought a condo, I would pay extra for one with concrete floors and ceilings, and a concrete block fire separation wall around every condo. The mass of the walls, floors and ceilings would ensure good privacy between condos.


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## Termite (Apr 13, 2008)

Nestor_Kelebay said:


> That's true, but short of rebuilding the wall, adding mass in the form of heavy drywall to that wall is the most effective way of reducing sound transmission through the wall from the neighbor's condominium.


Once again, I don't disagree that adding mass would help. BUT, it would be more effective to add that 2nd layer on top of some resilient channel applied over the existing rock. Then, you're adding mass to the wall assembly as well as reducing hard surface contact. I base that on years of experience with the design and installation of separation/party walls in multifamily structures. 

If the ENTIRE mass of the wall were doubled, a sound transmission reduction of 5-6db can be anticipated. That's less than a whisper. That is rather insignificant, given the cost and effort involved.


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## Nestor_Kelebay (Jun 17, 2008)

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?


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## Termite (Apr 13, 2008)

Nestor_Kelebay said:


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


Practical solutions to problems have apparently gone out the window...:laughing:


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## AtlanticWBConst. (May 12, 2006)

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)...


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## gollybonker (Dec 3, 2012)

*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.






AtlanticWBConst. said:


> 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.
> 
> ...


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## ToolSeeker (Sep 19, 2012)

There is always the option of quiet rock. It has the damping qualities of I think 7 sheets of drywall but is only 1/2" thick I think. Being used a lot in hospitals and schools, downside not cheap.


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## jeffnc (Apr 1, 2011)

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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## jeffnc (Apr 1, 2011)

Nestor_Kelebay said:


> That's true, but short of rebuilding the wall, adding mass in the form of heavy drywall to that wall is the most effective way of reducing sound transmission through the wall from the neighbor's condominium.


Not at all. There are much better products than plain drywall to reduce sound.


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## jeffnc (Apr 1, 2011)

Here's another product, and this page has many examples of wall construction and drywall type, with the associated STC.

http://www.acousticalsurfaces.com/soundbreak/soundbreak.htm?d=14


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