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Insulating exterior wall in 1950s home: risky?

J_Adam_G | Posted in General Questions on

I’m converting the spare bedroom in my 1958 brick ranch into a music studio. Part of the project involves soundproofing, and I’ve removed the drywall and have begun putting Rockwool Safe ‘n Sound insulation into the stud cavities. After I finish with the insulation, I’ll be putting a layer of mass loaded vinyl (MLV) over the insulated walls to add additional mass and block more sound. After that, I’ll hang the drywall. This is standard procedure in many soundproofing projects.

There is one exterior wall in this room, and I’m concerned about insulating it. The house has no insulation in the walls currently. It’s just the brick on the outside with a layer of black felt-covered sheathing on the other side. Anyone know what they would have used for sheathing in 1958? This sheathing material seems solid, but it does have minor damage in places. It’s not wet; it just has some dented areas where there are small holes. The wood framing is good, sturdy, dry wood: truly high quality lumber from the 50s.

If I were to fill the exterior cavities with Rockwool, add the MLV (which is just heavy plastic) and then hang drywall, would I be creating a serious moisture intrusion problem. I want you to say I’m not, but I’m worried that I am! I live in central North Carolina, so it’s hot and humid outside for most of the year.

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  1. 1869farmhouse | | #1

    Mass loaded vinyl is going to act as a vapor barrier. The wall assembly just needs to be able to dry to the interior or the exterior. If the MLV is facing the inside, just make sure there isn’t anything preventing vapor from evaporating out via the exterior. Otherwise, you’ll be good in my opinion. Assuming it’s not getting wet repeatedly, in which case you should fix that anyway!

  2. J_Adam_G | | #2

    Thanks, Austin. My concern, however, is that moisture won't be able to exit the structure via the exterior. If there's plastic between the drywall and the insulation (the MLV functioning as a vapor barrier), won't that cause condensation to form on the insulation-facing side of the plastic? Then the mineral wool might hold moisture and rot the framing or lead to mold growth over time. This is my concern.

    Or maybe that wouldn't happen? I'm thinking of this article:

    Here's how the wall layers would break down:

    Brick exterior -- > Black felt-covered sheathing board material (not sure what it is) --> Insulation-filled wall cavities --> MLV (plastic layer) --> Drywall

  3. Expert Member
    AKOS TOTH | | #3

    Provided you have a gap between your brick and the sheathing and there is some WRB in there, you should have no issues in insulating in warmer climates. Insulating old houses is problematic is colder places with softer brick as they will now be significantly colder and can start spalling. You still should check your window flashing details. Water leaks there are generally not a problem with un-insulated walls but create a moldy mess with insulation.

    MLV is great for absorbing sound for very specific application. For example, the best thing hung over a fence around a generator. For it to work, it needs to be floppy (official engineering term) not sandwiched between layers.

    For soundproofing walls, all you really need is mass, for that you can't beat 5/8 drywall or 3/4 OSB. MLV is expensive mass. How well your wall works depends a lot on how well you can air seal both sides. Without proper air sealing you can loose most of the effectiveness of high STC assemblies.

    Insulation inside walls helps but makes very little difference in STC. If you want good sound isolation you need to decouple the two sides. Generally the best is a double stud wall. I've tried a number of assemblies over the years, very cheap and effective wall is 3 5/8" metal stud with 5/8" type X and batts and a 1 5/8" metal stud wall with 5/8" type X spaced 1/2 away. With the perimeter sealed only a bit of bass will make it through.

    For a home studio, your best bet is resilient channel or hat channel with isolation clips with two layers of typeX. Make sure you air seal (this includes any device boxes) and if there are any device boxes in the same cavity, move one of them to a different stud bay.

  4. J_Adam_G | | #4

    Thanks, Akos! This gives me more confidence in sticking with the plan. I have no idea if there is a WRB in there. Would the black felt sheathing board stuff count, or does there need to be something between the brick and that material? I don't know if they were using WRBs in 1958 and I'm not sure how to check either.

    I appreciate the advice re: decoupling and resilient channels and such. The only reason I'm not doing that is that this is already a really small room and I can't afford to lose any space. I've accepted that it will be semi-soundproofed and not perfectly soundproofed. As long as my kid can sleep while I jam, it's fine. Doesn't have to be totally silent.

    1. Expert Member
      AKOS TOTH | | #5

      You can carefully cut a small opening into the sheathing in a location away from a window and preferably under an overhang. Typical WRB of the time would have been felt paper. Lot of times the fiberboard itself was designed to be the WRB. This mostly works provided you keep liquid water out of of your walls.

      RC adds 3/8" to depth, an extra 1/8" for a single layer of type X and you are not loosing that much interior space. This would an OK wall to take the edge off, good enough if you have a standard solid slab door. Skip the MLV, you are just spending money for something that doesn't make much difference.

    2. Expert Member
      BILL WICHERS | | #6

      If you don’t want to lose the space for a double stud wall or resilient channel, consider a double layer of 5/8” type X drywall instead with green glue between layers. This will work better than a layer of MLV. MLV isn’t really the best option inside of walls and is rarely used inside walls professionally.

      From best to worst in terms of sound blocking performance, double stud walls are best, then walls with resilient channel, then double drywall. That’s a simplified list, but gives you the basic idea. You don’t lose much space with the double layer of 5/8” drywall and green glue.

      You can also build a staggered stud wall which is similar to a double stud wall but in less space (and with a bit less performance too). An example is a 2x6 top and sill plate with every other stud aligned to the opposite edge of that 2x6. This gets you decoupling between wall surfaces, but doesn’t leave as much room (only about 1.5” between each stud and the opposite side drywall) for any other soundproofing materials.


  5. Expert Member
    Dana Dorsett | | #7

    >"If I were to fill the exterior cavities with Rockwool, add the MLV (which is just heavy plastic) and then hang drywall, would I be creating a serious moisture intrusion problem. I want you to say I’m not, but I’m worried that I am! I live in central North Carolina, so it’s hot and humid outside for most of the year."

    It's right to be VERY concerned about creating a moisture problem with the MLV in a brick clad house! It's a problem with brick structures even in colder climates that have high summertime moisture. (The Canadian NBC prescribes an interior side vapor barrier, and has created a lot of problems when brick houses are remodeled with interior side studwalls + vapor barrier.)

    The black sheathing is asphalted fiberboard, which is extremely moisture tolerant. Since there's a layer of #15 felt (a classic WRB) between the fiberboard & brick it doesn't need another WRB. Many homes were built with just the asphalted fiberboard- no felt, due to the high moisture tolerance of the fiberboard. But the vapor permeance of the felt and fiberboard together are well above 10 perms, probably north of 20 perms when the humidity levels in the brick cavity are high.

    The LAST thing you want to do with an air conditioned room on a brick clad house in in hot humid climate is to install interior side vapor barrier materials! The wallboard & vapor barrier would be well below the peak summertime dew points, causing condensation to collect inside the wall cavities. Additionally, brick stores a lot of moisture that gets released in intense bursts whenever the sun warms it up, which is why moisture tolerant fiberboard is/was used in lieu of moisture susceptible plank or plywood sheathing. The assembly MUST be allowed to dry toward the interior to keep the moisture risk low. That means no vinyl, no foil, no polyethylene, etc.

    Using a "smart" vapor retarder to moderate the rates of water vapor transfer is fine. Something like 2-mil nylon (Certainteed MemBrain) or asphalted kraft paper would be fine. It's fairly vapor tight when the humidity levels inside the insulation & stud wall are low, but becomes more vapor open than standard latex on wallboard when the humidity levels are high enough to support mold.

    Double-layered wallboard with Green Glue(tm) between the layers reduces sound transfer by quite a bit without increasing moisture risk.

    1. J_Adam_G | | #8

      Dana, thanks for the substantive response! It's interesting that different people, all of whom are presumably experienced, have such vastly different advice: everything from "nah, it ought to be ok" to "no, don't do that! It'll create a big moisture problem."

      With regard to your guidance: You're clearly advising NOT to use the MLV in this way. Would you have the same moisture concerns if I used just the mineral wool insulation without the MLV?

    2. J_Adam_G | | #9

      Update, re: my first response...

      I just talked to a BPI-certified home performance contractor in my area. He says I'm good to put in the insulation and that I SHOULD put up some kind of vapor barrier between the drywall and the insulation.

      I'm getting so much conflicting advice! I'm honestly no more confident about what to do than I was when I started this project. I'm starting to come to the conclusion that the actual answer to my questions about all of this is, "It seems plausible that you should do [fill in the blank with whatever], but nobody actually knows." lol

      And I'm still curious as to whether you think the insulation on its own is ok if I don't put up the MLV.

      1. insaneirish | | #10

        > And I'm still curious as to whether you think the insulation on its own is ok if I don't put up the MLV.

        Yes. Mineral wool is vapor permeable.

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