Axiom Home Page Forums General Discussion Advice from Axiom Owners Drywall

I just tore apart a room that was two layers of drywall with construction adhesive (probably Liquid Nails) between them. The room was used for recording drums, and with the door closed (he screwed and glued a layer of drywall to the door too) you couldn't hear anything outside of it. Huge pain to deconstruct the thing though.

What about carpet underlay between sheets of drywall,below and even between the floor joists, or on the sides of the floor joists? I considered installing it against the sub floor but there's way too many nails and screws for that (its an older house, must have had squeakier floors at one time).
I know where I could pick up 3 or 4 new unopened rolls for free.

Nick, not to disrupt the flow of info here.... But I wanted to tell you that if I ever have the opportunity to build a home theater, I'm putting you on speed dial.

Thanks Mark! I think....

OK, here is the scoop on the 1/2" and 5/8" (or two other different thickness of drywall used together). The idea is that each thickness has a natural resonance frequency based off of its thickness and mass. In turn, each layer then also blocks different frequencies and so forth. It sounds good doesn't it? There might be some science behind that somewhere that says "sheet thickness 1 blocks A to B frequencies well" and "sheet thickness 2 blocks B to C frequencies well" and so combining them together you should get frequencies A to C blocked well... However, it doesn't work that way exactly. At least not in a measurable way.

However if IS proven that the more mass you have (regardless of thickness of individual sheets) the more mid to lower frequencies you are stopping. So if you can put up two layers of 5/8", then that is better than a layer of 1/2" and another layer of 5/8" which is better than two layers of 1/2" and so forth.

So don't get hung up on the *need* for 2 different thicknesses. The key is just getting as much mass as possible. If that ends up being two 5/8" layers, excellent, if it is a layer of 5/8" and 1/2" I would put up the 5/8" first. This will be quite a bit heavier, and if you are going to use Green Glue, it will be easier to work with the lighter 1/2" layer as the second layer.

As for the clips and channel. My ceiling has two layers of 5/8" drywall with Green Glue in the middle, all hung from the hat channel and clips. Kind of scary, but they are rated for a lot more. I think that each piece of hat channel would have only needed another clip or two maybe tops per 12 foot length (spaced 2 feet apart) to hold an entire 3rd layer with lots of extra wiggle room. With my two 5/8" layers, I am NOT at their rated capacity yet. Also, I am getting no cracks in the drywall, and it is really solid. My biggest issue is that my ceiling height was already only 7 feet 10 inches or something like that. Adding clips and channel was going to drop it a couple of inches (maybe 3.25" or something) plus the two layers of drywall, so I had to make a number of "stringers" to tie my ceiling joists together, but these stringers were actually recessed up from the bottom of the ceiling joists. These stringers are what I connected my clips and then the channel to. I saved some space there. Not sure what your ceiling height is, but mine doesn't feel low at all, even when standing on my 12" riser for the second row.

Here is a picture from The Soundproofing Company that shows the recessed stringers...


As you can see, the channels are spaced 2 feet apart (except for the outside edge) and the clips are every 48" (plus one around the perimeter). If you go this route, we can map out an exact clip count and orientation, or call someone like The Soundproofing Company and they will calculate the number of clips you need.

Now, if you REALLY want to try to do something about the sound in your ceiling, you can do this:

Basically it is the same as above, but two layers of green glue and two layers of 5/8" drywall up attached to bottom of the *floor* above the theater. For me, I found that the joists were never very straight making the cutting of drywall to fit a pain, plus the added cost and effort was beyond my personal desired "bang for my buck."

One thing to notice, in these pictures, you see just regular pink fluffy insulation (the cheap stuff) shown. The stuff I used was yellow, but it was still the cheap stuff. I used John's Manville brand R19 for the walls and I believe the ceiling (or at least that is what I saw in one of my construction videos).

In this video of my theater, at about 2:35 into it, I start showing the clips and channel.

Go to 2:35 for clips and hat channel


I did not go with clips/channel on the walls, but built a staggered stud wall which gets me really close the the same effect as using clips/channel. I am getting an estimated 66 STC for my walls and ceiling (because I am using clips/channel up there) which is a good, high rating for sound proofing. That is with cheap R19 insulation, two 5/8" layers of drywall and one layer of Green Glue.

Going without the clips (two layers of 5/8", Green Glue, and R13 insulation) yields about 52 STC

Using something else besides Green Glue (the stuff REALLY is THAT good) like 1/2" Sound Deadening Board between the two layers of 5/8" drywall (and R13 insulation) yields just 44 STC. OK. That is still pretty decent, but still considered just a "moderate" improvement. It isn't until you start using Green Glue that you really start to improve the low frequency soundproofing, and also reduce "flanking" (sound traveling between two or more different surfaces like wall to ceiling, ceiling to the ceiling in the room next door, etc.

Here are some pretty pictures and more details.
Soundproofing Walls - Different Configurations.

And here is one with even more pictures and information specifically for ceilings.

Soundproofing a Ceiling - Different Methods

I did SPC Ceiling Solution #3.

I reference The Soundproofing Company a lot. I am in no way associated with them. I just had a good experience with them for their customer service in answering my questions, and their great pictures and diagrams that get referenced a lot on the internet (just like I did above).

They explain things very easily on their web site, and it is worth poking around there for a while, even if you buy stuff from someone else or don't buy anything at all.

Carpet underlay? Like the pad?

I've read that people have tried that and that the foam based stuff did nothing, zip, zero to help, as you needed to either get really thin stuff so that then you screwed down the top layer of drywall, the thickness of the foam didn't cause the screw to just pop through the outer layer of drywall as you were screwing it on, or you had to go with really dense rubber stuff, still fight that issue of screws pulling in, and really just ending up with a headache and almost zero improvement even using measurement devices.

The Green Glue isn't a glue at all. It stays sticky, never dries (ok, I am sure that in 200 years or something it would start to dry out), and being semi-fluid, it will always maintain its absorptive properties. Again, there is a reason that people use this over any other method. It IS messy, but doing other things with thicker materials like carpet pad will just cause headaches potentially during construction and worse of all, provide little to no improvement.

Look at it this way. Even the insulation in the wall cavity allows sound through it if the wall cavity is stuffed with it. That is why ideally you would want an air gap in addition to the insulation. The sound that gets past the Drywall/Green Glue/Clips/Channel/Whatever then gets absorbed by the insulation, however that is only at the frequencies that the R19 (or whatever you use) insulation absorbs. It takes that sound energy and turns it into heat due to the slight vibration of the insulation (at a microscopic level). If that slightly vibrating insulation is also touching the drywall on the other side of the wall, some of that sound vibration of the insulation transfers to that outer layer of drywall and into the next room.

My using a foam or rubber, or whatever product, you are only trapping a very small frequency range, but most of the sound is still just passing from one layer of drywall to the next because you would have to sandwich the stuff so tightly in order for the top layer of drywall to actually be secured.

You are sort of talking about doing "Wall Solution 3" from the link I put above.

Soundproofing Walls

However you wouldn't get nearly as good of STC because option #3 is using Mass Loaded Vinyl. That stuff is HEAVY and is really there like a super dense 3rd layer of drywall (figuratively).

If I had to guesstimate, I would say that two layers of 5/8" with a layer of carpet underlayment between them, you would probably get just 43 STC (maybe even just 42 STC like not having anything in the middle of the two layers).
Same thing, but with Green Glue --> 52 STC and improved low frequency performance and reduced flanking.

Again, those guys don't make Green Glue, so it isn't just marketing. It really does work.

I will reread the websites posted.
Without the T bar I have about 7'8" to work with.
I do wonder about the T bars ability to act somewhat like the channel and clips. I accept that there's no mass there but it should isolate the floor joist from transferring sound through the T bar because they're not connected.
I'll be taking back the 5" Roxul today.
The Safe and sound is 3" (R12 x 2?) and would allow me two air gaps, 3 if you count the gap between the T bar and floor joist.
I think this is the best I'll get.
The room next to it has a 7'6" ceiling measured from the bottom of the bar. Every inch is going to count.
We met up last night after my last post and decided to build the 2nd wall in the adjoining room. The drywall will get removed from both sides, both walls will be insulated with and air gap between them.
We confirmed that the two heat ducts that are for this room will be replaced with the insulated accordion style.
I'll get on the green glue, I'll need it for the shared walls.
Just curious, it would seem the insulation in the exterior walls is around R12, the room is cold most of the time. Would adding the second layer of drywall add any R Value?
Again I have considered removing the drywall and re insulating. I am a little scared of where this is heading though.
Better pay down my CC now.

Adding more drywall may or may not impact the R value in the cold room. I honestly don't know the answer to that, and would only be speculating. I would *think* that it would help, but not sure how much as the cold would still transfer through the drywall due to direct contact (that is where air gaps and "airy" insulation helps out).

I don't think that it would be worth the effort just for temperature control to add the drywall and have to finish it (mud/tape/paint/etc). I don't think that it would hurt anything from an R value perspective, but again only talking about temps, it won't help much/any. Sound, different story.


EDIT: I had to look it up. From the National Gypsum website.

Drywall thickness and R Value:
1/2" -> R0.45
5/6" -> R0.56

Not going to make much difference at all for R value help.

EDIT #2: OSB is a little better at R0.51 for 1/2", but still nothing great.

FYI that you could use OSB for your first layer if you wanted to. It weighs almost the exact same as drywall so you have good mass, and you can then screw the drywall anywhere you want because it won't have to hit a stud, but it costs about 50% more than the same essential thickness of drywall and OSB has to be cut with a power saw, vs a box cutter and straight edge.

Oh, and just remember that when you do put up your two layers, do one layer horizontal and the other layer vertical so that you have a more rigid, and less common "edge points" where the horizontal edges of one layer overlap the vertical edges of the other layer. You can use regular clear (not colored) long life 100% silicone caulk on the edges and seams of the bottom layer to get you REALLY close to the more expensive acoustical sealant. Don't use it on the top layer as 100% silicone doesn't allow mud/tape/paint/primer/anything to stick to it. Or just use it if you have any gaps and call it pretty darn good.

EDIT #3: Don't get the lightweight drywall either. Go for the heavy stuff (like 54 pounds for a 1/2" thick 4'x8' sheet and 70 pounds for a 5/8" thick 4'x8' sheet). You want mass, and now you can also see why that little extra 1/8" thicket 5/8" drywall works better than the 1/2" at 15 pounds more per 4'x8' sheet.

Don't be afraid of the weight. I used two layers of the 5/8" drywall but in 4' x 12' lengths. So each sheet was about 100 pounds and really big to work with, but it also meant fewer seams, and each sheet that went up covered more area faster.

EDIT #4: Check out local drywall supply shops before going to a home improvement store. I had all of the drywall for our entire basement delivered exactly where I wanted it INTO my house for a LOT less than buying them, even at the "bulk" rate at the cheapest home improvement store and I would have had to haul them home myself and unload them myself too.

I saved time, money, my arms/back/legs, and I got "fresh" drywall that wasn't brittle from sitting in a home improvement store for a long time.

Thanks for all your help.
Its lots to chew on. I have today to decide.

Nick,
how does spray foam insulation compare with the pink stuff, in terms of sound abatement?

Scott

You know sir, that is a great question that I will admit I haven't looked into. I know that the spray foam insulation is AWESOME for its thermal properties.

EDIT: Found this from this site:

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8) How about Spray Foam Insulation. I heard that is a great soundproofing product?

Save your money and use cheap (about 3-4 times cheaper) fiberglass or cellulose insulation. Use the extra money on an extra layer of drywall and quality damping compound. Insulation absorbs some sound waves and that's it. The fiberglass will absorb as much if not more than the spray foam. This has been confirmed to us by many builders. As a side note one thing we do hear from contractors and we have noticed it as well during sound control installs that we have done is that contrary to "science soundproofing" mineral wool and cellulose are better at controlling sound than fiberglass. Either way if you are on a budget fiberglass will do the job.

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So it doesn't sound (no pun intended) like it would be baaaad per se, but just that it is too expensive to use solely for soundproofing. The biggest issue I could see with it is that it expands and can fill a wall cavity, and an air gap of a couple of inches (if possible) is actually a good sound insulator. So if a wall was filled with spray foam, then I would think that it would be worse as you are basically "coupling" the inside and the outside of the wall.

Not sure how much control you have either if you could spray and fill only about half of a wall cavity's depth, then I think you would be OK and certainly more air tight.