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Community and Q&A

Double-Stud Walls in a Humid Climate

Josh F | Posted in Energy Efficiency and Durability on
Hi everyone,
Long time reader, first time poster.
I live in Austin, TX, and just took the plunge last week of buying 0.75 acres to build myself a house!
The one main downside of the land is that it butts up against some train tracks, so I’m planning on framing the house with double stud walls to increase the overall STC and hopefully reduce some of the train noise.
That being said, I wanted to ask around and see what others thought of my proposed wall assembly. I’m guessing I won’t have any condensation issues being so far south, but Austin is very humid, and knowing I will have probably 8-9 inches of interior insulation and zero exterior insulation, I was curious to see if anyone thought I should be worried about condensation? I’ve read a handful of GBA articles about double stud walls, but it is usually in colder climates.
Basically, from interior to exterior: 5/8″ drywall > 2×4 framing with rockwool > 2×4 framing with rockwool (haven’t decided yet if the walls will share top/bottom plates) > ZIP sheathing > 1″ minimum airgap for brick drainage > brick facade. I’ve included a quick illustration for reference (the view is from the top looking down).
Any feedback would be greatly appreciated!
-Josh

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Replies

  1. Eric Anderson | | #1

    I suspect that you could get good sound proofing with a 2X6 wall, rockwool and R-zip. When I redid my siding, I used R-3 zip, 2x4 wall with Rockwool and my house is amazingly quiet, but I'm not next to a RR trax (but I am in Austin). Spend money on STC rated windows. For my master BR, I used Pella STC 31 glass and it makes a huge difference in sound attenuation. My neighbor has a very old HVAC unit that is so loud you can't carry on a conversation if it is on and you are near it. With the STC glass, I can barely hear it (and it is right outside by BR window). Also, insulate all interior walls with Rockwool safe and sound, this will also help cut down outside noise. Double stud walls are pretty uncommon here, so I figure you will have trouble finding framers that know how to do this but 2x6, 24" OC is normal.

    1. Josh F | | #5

      For the sake of information (especially since you are in Austin too and understand the climate), would you assume the double stud wall I listed would be free of condensation issues (of course, with correct installation)?

  2. Expert Member
    Malcolm Taylor | | #2

    Josh,

    Much as Eric suggested, I'd concentrate on adding components with the aim of increasing the STC rating of the wall (better windows, mass loaded vinyl, layers of drywall, res. bar, etc.) rather than going to double-stud construction.

    To ensure it will act in predictable ways, I'd pick a tested assembly and replicate it exactly.

  3. DCContrarian | | #3

    I assume Austin is cooling dominated? So you want your walls to dry to the interior and have vapor barrier to the exterior. The interior is fine so long as the rock wool is unfaced. On the exterior you want a vapor barrier, probably the easiest way is to use faced rock wool with the facing toward the exterior.

  4. Josh F | | #4

    Thanks for all the feedback everyone!

    I also agree about finding framers to frame 2x6 will be easier than hovering and micromanaging people to correctly frame double stud walls. Plus, with lumber prices so high, the cost of that entire second set of walls can be used for high STC windows.

  5. REBECCA B | | #6

    Austin's climate is pretty forgiving in terms of condensation, north facing walls in winter tend to be your highest risk. Highest humidity months are fall/ spring with lower condensation risk since inside and outside temps are pretty similar. Everyone I've worked with has treated it as a mixed heating/ cooling climate with good air barriers and no vapor barrier. If you go with exterior foam (which even if you go with glass faced polyiso is only nominally vapor permeable) you'll need to watch the balance of fluff to foam but it won't take much to get the balance right and the IRC has some reasonable baseline guidance there.

    Just don't put any vinyl wall coverings on the interior of exterior walls and you'll be fine.

  6. Kyle R | | #7

    Not green, but I would imagine icf would have some sound proofing benefits.

  7. Expert Member
    Zephyr7 | | #8

    Double stud walls are effective for sound reduction, but you have to take care to avoid anything rigid coupling one side to the other. This includes packing them full of insulation -- you need that central air gap.

    I wouldn't use MLV in this type of wall assembly. MLV is a vapor barrier, which can be a problem in some cases, but it's also expensive. I would use additional layers of drywall instead. The result is pretty much the same: you're adding additional mass. I would at least use a double layer of 5/8" drywall on the interior side, ideally using green glue between the layers.

    I don't think your proposed wall assembly will be a problem, but you might find it simpler to use a 2x6 wall as others have suggested, and hang a double layer of 5/8" drywall on resilient or hat channel instead of going with a full double stud wall. Remember that the brick itself is going to help with sound reduction too.

    I would insulate this wall with mineral wool as a first choice, or high density fiberglass batts as a second choice. The extra density of those two materials compared with "regular" fiberglass batts will help with sound reduction since higher density equals more mass.

    BTW, I would use putty pads on any electrical boxes in this wall. Electrical boxes without putty pads sealing them are weak spots in any "soundproof" wall assemblies. The putty pads are usually sold for fire stopping, but they work very well for soundproofing too. An added bonus is the putty pad will also act as an air seal for the electrical box.

    Bill

  8. Steve Knapp CZ 3A Georgia | | #9

    There are a couple of sound-deadening drywalls on the market. I used Certainteed SilentFX (essentially a drywall sandwich with Greenglue in the middle) on a previous house, but we had some quality control issues with the product.

    If I were doing that project over, I'd go with two layers of 5/8-inch drywall. Plus 1 also on using an STC-rated window. We lived in a townhouse with two busy roads on either side side of the community. The noise was unrelenting for 18 hours out of the day and made me really want to replace the crappy shop windows installed by the developer.

    It's also important to air seal the structure as much as possible since any opening is a pathway for noise transmission.

  9. Jon R | | #10

    Best to see some actual STC numbers, but I expect that the windows will be low enough that the wall thickness doesn't matter.

  10. Steve Knapp CZ 3A Georgia | | #11

    Also...

    Vibration may be an issue if you are really close to the tracks. We are in a rental at the moment that is half a block from a major freight line (fortunately no crossings where a horn warning is required). Whenever the trains pass through, I can feel the cars traveling down the line.

    To Jon R's point, you might want to use a Db app or a Radio Shack meter to get a baseline on the noise.

  11. Josh F | | #12

    Thanks so much for all the great info everyone! I really appreciate it.

    I used a Db app the other day at the closest possible place to the tracks and it was a scary 85 Db, but up around where our house would be it was around 70-75 Db. We also are going to make sure to situate the master bedroom at the far end of the property so the house itself will act as a sound barrier for our room.

    The vibration is definitely something I've been thinking about, which is a lot harder to deal with than higher frequencies.

    Double 5/8th drywall and green glue I think could be a good idea even if it is just four our main bedroom.

    Air sealing is going to be a big focus for us, which is partially why we wanted to go with ZIP, as it seems easier to be really diligent for air sealing purposes.

    Building costs right now are a whole other issue though.... hopefully I'm not biting off more than I can chew!

    Thanks again.
    Josh

    1. Expert Member
      Zephyr7 | | #13

      Drywall is the one building product that doesn't seem to have gone up in price.

      BTW, a 6dB sound reduction will seem to be half as loud. Keep that in mind when working on your design. It doesn't take a lot to make a big difference, but it does take a lot to make it SILENT.

      Bill

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