How to drywall and caulk? Step Sequence....

[Quint]

Hi Stuart, thanks for your answer. To be clear, I'm not talking about the outside layer on a wall that will be hidden. I'm talking about the inside layers of sheetrock on the same wall. In other words, if there are two layers of sheetrock sandwiched together on one wall, as is typical when soundproofing, does the inside layer need to be taped and floated in addition to caulking or can it just be caulked and tape and mud just applied to the outermost layer for aesthetic reasons? Is there a sound related reason to tape and float inside layers or do they only need to be caulked?

[Zenon Marko]

However, I'm curious about this:

Drywall-corner-transition-leaf-not-decoupled.jpg

What's going in there? What's the light grey and dark grey, and the black channel-thingy? To me it looks like your inner-leaf is coupled to your outer leaf there.

Stuart -

Thank you for all the detailed answers! Working through the others.
Quick reply to this.
That is the inner leaf soffit. (There is also an outer-leaf soffit...I can upload the full drawing if you wish, but perhaps that would make more sense in my studio build thread).

I was planning to attach the inner leaf soffit from the outer leaf (ceiling joists and masonry wall) with decoupling clips, either with :

Kinetics Unibrace-L

or

one of the variants of RSIC-DC04

The alternative would be to couple the inner soffit to the outer leaf, and use something like RSIC-1 or IsoMax clips between it and the drywall, as will be done with the ceiling.

[Zenon Marko]

Are there any links to info on the relative isolation provided by 3 layers of sheetrock/drywall/gypsum instead of 2? I seem to recall that it becomes a matter of diminishing returns, but can't find the source.

Here's your answer: Don't guess! do the math to make sure that you really will get the correct MSM resonance frequency, and the amount of isolation that you need, at the frequencies where you need it. Here's what you need to know:...

- Stuart -

Thank you for the equations!
I can determine the mass of my new (to-be-constructed) inner-leaf walls; the mystery is the mass of the existing outer-leaf walls. 3 of those are masonry (partially-filled hollow cinderblock, cell walls at least 1" thick); 1 is a foundation "party" wall to the next building, an office tower. The inner surface of that party wall is 2 layers of old brick. Not sure how to estimate the weights in any of these walls...

Best regards

[Zenon Marko]

Here is a drawing I created in attempt to capture the differences.

Either 1 or 2 is fine. Definitely not 3. Personally I prefer #2 but renumbering the layers, so your layer 2 is first, then your layer 1, then your layer 4, then your layer 3. If you do it in that order, then the drywall on the wall gives you a small "ledge" to rest the ceiling drywall while you are maneuvering it into place.
- Stuart -

Stuart -

Why do you recommend against #3? I'm curious as knightfly (above) described a "all wall layers first" method, which I hope is what I drew there as option #3, as possibly avoiding a future sag/coupling problem.

Thank you again

[NickMac10]

Hi all,

Apologies if this is a silly question or if I've missed the answer somewhere above, but with regards to the backer rod when installing the drywall...as per the pic below from the original post, why would you not place the layers flush and then caulk/seal along the joins? The backer rod doesn't have much mass, so I would have thought this would make these joins an easy point for sound to get through?


Thanks in advance,
Nick.

[Soundman2020]

The backer rod doesn't have much mass, so I would have thought this would make these joins an easy point for sound to get through?

The backer rod is to get an extra independent seal in there, in case the caulk doesn't make a good seal, for whatever reason. Caulk could shrink or crack over time, or might not have stuck completely, leaving a tiny air path. The backer rod is a secondary seal that helps with that.

Yes, the backer rod is low density, but the caulk is HIGH density! It's actually two to three TIMES the density of drywall, so filling just half the depth of the cavity already gives you MORE mass than just the drywall alone. So there's no problem here. The caulk has more than enough mass to match the drywall, and the backer rod gives you an extra seal.

Plus, backer rod is much cheaper than caulk! That's another really good reason...


- Stuart -

[NickMac10]

That's great. Thank you.

One last question...it was suggested to me that silicone could be an alternative to caulk, as it can be more flexible.

What are your thoughts on this? Is it lower density/not right for the job? Or are they interchangeable?


Thanks,
Nick,

[Soundman2020]

Any type of caulk can work: silicon-based, latex-based, butyl based, acrylic, etc. The properties you want most in the caulk are:

- It flows easily into the gaps and cracks where you want it to go
- It does not "run" or drip once it is in place, even on vertical surfaces, or overhead surfaces
- It sticks like crazy and never lets go
- It sticks like that to all of the materials you'll be using
- It does not shrink as it cures
- It does not crack as it cures
- It does not get hard as it cures

In other words, it must remain soft and flexible, even when fully cured, without shrinking, drying, cracking, or pulling away form the surfaces.

If you use silicon, latex or acrylic, DO get the colored ones, NOT the transparent ones! The colored ones have pigments in them that greatly increases the density, which is what you want. Transparent ones don't have those, so they are lighter. (Yes, "white" is a color here....)

So anything that fits the above profile will work. Personally, I've had good results with Sika brand F11 bathroom and kitchen caulk/adhesive.


- Stuart -

[Quint]

I've had great results with GE Silicone 2. It meets ALL of the requirements listed in the previous post. It's 100% silicone and has a lifetime warranty. It remains nice and rubbery soft and doesn't shrink at all. I used the white color.

[NickMac10]

That’s great. Very helpful indeed.
Thank you both.

Nick.

[SGleason]

why would you not place the layers flush and then caulk/seal along the joins?

I'm also thinking that it's not a bad idea to take away the physical sound transmission path between drywall planes through the use of a resilient caulk.

I have also always thought that the backer rod prevents the acoustic caulk from migrating deeper into that joint over time and losing its connection with the drywall.

Steve