Improving a staircase wall...

[audio_alan]

Hi all, long time no post...

I first came on here a few years ago with an extreme lack of knowledge and construction skills, some crazy ideas, and lots of questions. I decided to take a step back, buy a couple books, lurk for a while, and regroup. For the last couple years I've been busy (off and on) working on the basement studio on things I was confident that I would do "correctly". Things are coming along, but as I solve one problem, I seem to discover new sources or sound leaks in my house. Making "soundproof" duct boxes and adding 5/8" sheetrock with green glue attached to the first floor's subfloor has made big improvements already. For a while now, I've known that my livingroom staircase/wall (which shares the same wall on the staircase that goes into the basement) is a huge source of sound transference. I determined there was no insulation on the inside of the wall, and if I recall correctly it's only 1/2" sheetrock on both sides of a 2x4 wall. As the noise coming out of the basement gets reduced, that wall by comparison seems to get louder!

As part of fixing the wall's sound problem, I'd also like to run conduit for coax, speaker wire, ethernet, etc to/from the basement, as well as hang some big CD racks on the livingroom wall. At the risk of hearing "You did it all wrong, rip everything out!", I thought I'd come here and ask a couple questions....

So far I have done the following:

I demo'd the sheetrock on one side of the wall (the side facing the livingroom).
Added a layer of 5/8" sheetrock between the studs with green glue to the side of the wall that still has the sheetrock (on the basement stairwell side).
Added 2 layers of 5/8" sheetrock with green glue to the footer plate of the wall to dampen it somewhat.
Put putty pads on the outlet boxes and basement stairwell light.
Installed 2x3 nailer boards to the studs and footer, so I can inset 1/2" plywood.
Finally, I caulked just about every crack I could find.
(see picture for the current state of the wall)

The reason for the inset plywood is 3-fold. #1 More MASS! #2, I want a strong base for the CD racks to screw into. #3 (the reason for it being inset instead of flush) I don't want to make the wall any wider than it is, because the architecture of the room almost demands that it stays the same size or things will start to look funny.

My next steps I was thinking about doing are: install the insulation, screw the 1/2" plywood to the nailers, caulk, and add a finish layer of 5/8" sheetrock over the plywood (with green glue, of course). After mudding/taping/painting, I thought about hanging a 1" rigid fiberglass panel on the wall (to dampen vibrations between the wall and the CD racks), and finally hang the racks by screwing through the rigid fiberglass panel, through the sheetrock, and finally into the 1/2" plywood for a strong connection. I also thought about installing a heavy-duty shelf underneath the CD racks so the majority of the weight is sitting on the shelf, instead of hanging on the wall. There should be very few screws that actually need to penetrate into the plywood if I build the shelf. Hopefully that won't compromise the integrity of the wall too much.

That leads me to my questions:

I was thinking of using the 1 inch "smurf tube" (I think they call it) that you can get at H.D. for running the cables. Other suggestions? I've read that PVC transfers too much sound.
Besides the conduit, is there anything more I should do to the inside of the wall before I seal it up?
Should I put random cross-bracing between the studs (cats, I think they're called), to reduce resonance in the wall chambers?
Is hanging the CD racks on the wall a really bad idea? Other ideas? The main reason I wanted to hang them is because they rock back and forth on the carpet, and I don't want them to tip over from someone bumping into them...
Any other comments on what I'm trying to accomplish?

Your help would be greatly appreciated!

Thanks!

Alan

[audio_alan]

A couple more details that are probably pertinent:

In the basement, I plan to decouple the ceiling and walls with sound clips and hat channel, double 5/8" sheetrock, green glue, insulation, and lots of caulking (of course!). Once the basement ceiling/walls are finished, I'd imagine I'll see a good reduction in sound coming from that upstairs wall. Even so, I'd like to error on the side of caution and do whatever I can to the wall upstairs to improve the noise situation. If horizontal bracing inside the wall will help, I'll do it, or if not I'll skip that step, etc. I thought about putting rigid fiberglass inside the wall, but from other posts I've read, it sounds like that wouldn't be worth the expense for the limited improvement. About the only thing I can't do is make the wall wider, as I explained above, because it will make the symmetry of the room look wrong. I think even a couple inches would be obvious, hence insetting the plywood into the wall face. I'm hoping that doesn't make the wall cavity too small (2.5").

Regarding the 2.5" cavity, I was thinking about putting R13 in there, but R13 is 3.5". Should I try to peel off an inch of the R13 to get it down to 2.5", or will compressing it only an inch be ok?

I also plan to put a "soundproof" door at the bottom of the basement stairs, again to keep sound from directly vibrating the basement staircase wall which connects to the staircase wall in the living room.

I'd like to get the wall closed up this week so my girlfriend doesn't kill me because the living room looks like a construction zone. Does anyone have opinions or answers to my questions, or am I that far off in left field?

Thanks,

Alan

[Soundman2020]

Hi Alan, and Welcome Back to the forum!

I have NO idea how I missed your new thread for so long! I just happened to find it more or less by accident... sorry about that !

At the risk of hearing "You did it all wrong, rip everything out!", I thought I'd come here and ask a couple questions....

Well, the problem is that you did it all wrong, so rip everything out...

Added a layer of 5/8" sheetrock between the studs with green glue to the side of the wall that still has the sheetrock (on the basement stairwell side).
Added 2 layers of 5/8" sheetrock with green glue to the footer plate of the wall to dampen it somewhat.

I do hope you realize that Green Glue is not glue at all? You cannot use it as an adhesive... that isn't what it is meant to do, despite the name. So you need some other method of actually attaching the drywall: The normal way of doing that is with cleats pressed up against the drywall, and nailed sideways into the studs. It looks like you did actually do that, but it's hard to be sure from the photos... ?)

My next steps I was thinking about doing are: install the insulation, screw the 1/2" plywood to the nailers, caulk, and add a finish layer of 5/8" sheetrock over the plywood (with green glue, of course).

That should work fine, provided that you also nail the drywall in place.

I thought about hanging a 1" rigid fiberglass panel on the wall (to dampen vibrations between the wall and the CD racks), and finally hang the racks by screwing through the rigid fiberglass panel, through the sheetrock, and finally into the 1/2" plywood for a strong connection.

Rubber would probably be better for that: Some small Sorbothane pads would do the job. Or Neoprene. Or EPDM.

I've read that PVC transfers too much sound.

Any type of tube will transport sound, to varying degrees, since the sound moves through the air inside the tube, as well as vibrations moving through the tube itself. If you use PVC, you can avoid transmission through the tube itself by cutting out a small section and wrapping the gap with rubber. In all cases, regardless of the type of conduit you use, seal both ends of the tube by stuffing with fluffy insulation, then caulking over that.

Should I put random cross-bracing between the studs (cats, I think they're called), to reduce resonance in the wall chambers?

Not necessary: the insulation will damp the resonance.

In the basement, I plan to decouple the ceiling and walls with sound clips and hat channel, double 5/8" sheetrock, green glue, insulation, and lots of caulking (of course!).

Good plan!

If horizontal bracing inside the wall will help, I'll do it, or if not I'll skip that step, etc.

Additional bracing is not necessary, acoustically. As long as you have good insulation in the stud bays, that will damp the resonance well.

I thought about putting rigid fiberglass inside the wall, but from other posts I've read, it sounds like that wouldn't be worth the expense for the limited improvement.

Insulation is critical! Without that, there is nothing to damp internal resonance. You need it. It can be either fiberglass insulation (30 kg/m3) or mineral wool (50 kg/m3). Rigid or semi-rigid is not necessary, but would be good if you can afford it.

or will compressing it only an inch be ok?

Never compress insulation in a wall: just lay it in. Compressing creates the potential for flanking paths, and changes the density if the wool. Higher density is worse for low frequencies...

- Stuart -

[audio_alan]

Thanks for the reply, Stuart!

Yes, I realize that green glue is not an adhesive (but thanks for checking! ). You're correct, I installed 2x3 nailers/cleats inside the wall cavity to firmly hold the the additional layer of sheetrock (w/gg) inside the wall. I skipped the bracing, but installed regular R-13 insulation. Unfortunately, I did compress the insulation a little when I inset the plywood against the cleats. Perhaps I should have pealed off an inch of the insulation, but I didn't. However, I'm already hearing a major reduction in the sound being transferred through the wall, and I still don't have the final exterior layer of sheetrock on the wall yet! (Any yes, I'll be using drywall screws to attach it, not just relying on the green glue...). So hopefully I didn't hurt the overall performance of the wall too much with the minor insulation compression.

Doing the math inside the wall -- 5/8" sheetrock, plus 2.5" cleats, plus 1/2" plywood = 3 5/8", making the plywood stick out 1/8" past the 3.5" studs. To fix this, I installed joist gasket tape on the studs (and caulked everything really well), so there won't be a 1/8" gap between the final layer of sheetrock and the studs. Perhaps that was overkill, but I didn't want any strange resonances going on. If you didn't follow that, see the attached pictures. You can see how the plywood sticks out just a little before I applied the tape...

Regarding the conduit, I decided against it. I ended up drilling holes just large enough for coax and ethernet cables, and ran a few extra cables in case one goes bad. It's a gamble, but I think the chances of all those cables going bad is slim to none. I then caulked the holes several times to make sure I had a good seal around the cables. In the long run, I think this solution will perform better than having a 2" conduit going through the floor.

About preventing the CD racks from vibrating on the wall, I've also decided against installing the racks on the staircase wall. This wall is too critical to my soundproofing project to have any more penetrations in it than necessary. I'm going to hang the racks on another wall that is less conducive to sound transfer...

Other details: I installed putty pads around all the outlet boxes and coax/ethernet box (and put insulation in the coax box). Also, to ensure the plywood panels were perfectly flat in relation to each other, I constantly checked the alignment of each inset panel to the next panel. I should have a perfectly flat wall with no variations in the sheetrock making contact with the plywood beneath.

I hope to find time to install the remaining 5/8" sheetrock on the wall in the next week or two, and will post more pictures and observations on the improvements of sound isolation at that time.

One question about installing the sheetrock. Should I leave a 5/8" gap at the bottom of the wall and fill it with backerrod (so it doesn't come in direct contact with the floor)? Or should I just run it all the way down to the subfloor? Thanks.

Alan

[Soundman2020]

Looks like you are doing things right!

One question about installing the sheetrock. Should I leave a 5/8" gap at the bottom of the wall and fill it with backerrod (so it doesn't come in direct contact with the floor)? Or should I just run it all the way down to the subfloor? Thanks.

I'd leave a gap, then fill it. It won't make a huge difference by any means: just very small. But adding up a whole lot of "very small" difference will lead to a large difference!


- Stuart -

[audio_alan]

Looks like you are doing things right!

Thanks, I sure hope so! The one thing I should have done differently was use OSB or sheetrock instead of plywood for the layer on the cleats. But, I had read that plywood was better to screw into (because OSB can have irregularities and voids that plywood doesn't, and sheetrock doesn't offer much support for screws at all). However, since I'm not screwing into the wall with the racks like I originally planned, I probably could have gone with one of the more mass-ladden options. Oh well, live and learn. Even so, the added mass on both sides of the wall, plus the green glue, plus caulking the existing holes, plus insulation and putty pads should give me huge improvements over what was there before (a hollow wall with 1/2" sheetrock with uncaulked electrical holes!). Saying it out loud, no wonder why that wall transferred so much sound!

I'd leave a gap, then fill it. It won't make a huge difference by any means: just very small. But adding up a whole lot of "very small" difference will lead to a large difference!

Sounds good, I'll leave a small gap at the bottom. Thanks for the quick answer on that.

Alan