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Insulating a Double-Stud Wall With Rockwool

Peipaul | Posted in General Questions on

I am constructing a double stud wall and would like to insulate it without blown in cellulose using 3.5 ” rockwool in the outer cavity 2×4 wall with 3.5 rockwool
placed on the horizontal in the centre and then 3.5 rockwool on the second portion of the double wall giving me a total wall of 10.5″. Has anyone done this
and will it work.
Thank you

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Replies

  1. Doug McEvers | | #1

    Yes, I used this insulation sequence on a number of double wall homes years ago with fiberglass. Make sure your wall is airtight and it will perform.

    1. Peipaul | | #3

      Thanks for the reply Doug

  2. John Woloshyn | | #2

    I did exactly this with 3.5” Rockwool on our own house. I built it with a layer of 1/2” plywood on the exterior side of the interior wall. The exterior wall is also sheathed in 1/2” ply. The inner sheathing layer is detailed as the air and vapor barrier(retarder). We used Joe Lstiburek’s “ideal wall” design as the basis for our own. We are in zone 7.

    1. Peipaul | | #4

      Thank you John.

    2. Donald Christensen | | #7

      John,
      Which wall is load bearing? Do both walls sit on the foundation (or floor framing), or, asked in another way, is the outer wall cantilevered? Also curious how you connected the inner sheathing air barrier at the floor and ceiling. I read Joe Lstiburek's write-up on this assembly, but he is kind of vague on some of these details, with the outer wall just hanging there. Also curious where you mounted the windows. Thanks.

      1. John Woloshyn | | #9

        I made the exterior wall load bearing. It is set flush with the foundation framing (its a PWF foundation) so it is not cantilevered like the Lstiburek wall. Regarding the air barrier continuity, I sealed the perimeter of each sheet of 1/2” plywood to the studs and plates with a hybrid elastomeric caulking. I taped horizontal seams with 3M 8067 air barrier tape. I used 11” strips of 1/2” plywood under the bottom plates to tie the two walls together. I sealed the bottom plates of the walls to the plywood strips with the same elastomeric sealant. I sealed the plywood strips to the subfloor with heavy beads of acoustical sealant. At the top of the walls, I built them with a single top plate and tied them together with a single layer of 11” strips of 3/4” plywood. Those I sealed to the top plates with the elastomeric sealant like I did at the bottom of the wall. Then I added a second layer of 11” x 3/4” plywood strips to the first layer, but I sandwiched a strip of 6 mil poly between the top and bottom layers of 3/4” plywood. I sealed the poly to the lower layer of plywood with acoustical sealant. The 6 mil poly strips overhung the walls to the interior so that I could tie my 6mil ceiling vapor barrier to the strips. I have flat 10 ft ceilings throughout. It’s all on one floor. I also added a ceiling service cavity to limit the number of vapor barrier penetrations. My windows were flanged, so they got mounted at the exterior sheathing.

        1. Donald Christensen | | #11

          Thank you John for the additional details. Great solutions, using widely available materials.

  3. Patrick OSullivan | | #5

    I have not built this way before, but I know that the hygrothermal performance of double stud walls is sometimes talked about in way to suggest there could be problems. But, folks that have built them with cellulose have had lots of success. The question is whether the moisture adsorptive properties of cellulose give it a working advantage over mineral wool.

    Put another way, you need to figure out if there's an excessive risk of wetting in this assembly.

    On the plus side for this assembly, mineral wool through and through inherently solves for the fire blocking you'd otherwise need to prevent fire from spreading along the length of the stud wall.

    1. Expert Member
      Malcolm Taylor | | #12

      Patrick,

      "you need to figure out if there's an excessive risk of wetting in this assembly."

      That's something I've asked a couple of the double-stud builders here on GBA (like Dan Kolbert) who have data from sensors in their houses. How much do these assemblies rely on the moisture buffering characteristics of cellulose for their performance? I suspect the answer is climate dependent, and relies on maintaining high levels of air-sealing - but beyond that I haven't heard a good answer.

      1. John Woloshyn | | #13

        Malcom, that is my takeaway as well from Ben Bogie’s research/experience on their on projects. They have done fairly extensive wall cavity and exterior sheathing wetness monitoring, I understand, from reading Ben’s blog/articles comments that it has to be very well air sealed to be durable. That makes sense to me. Dr. Joe Lstiburek says this assembly works too. Other people, with credentials, and with way more knowledge than me have already figured this stuff out.

        1. Expert Member
          Malcolm Taylor | | #15

          John,

          But all those examples still leave the question begging as to whether a well air-sealed double wall with good drying to the outside is risky when it uses batts. I've seen lots of articles saying they work with cellulose, but none that I can remember which directly address how much the moisture buffering contributes to their performance. Maybe it's a non-issue, but it certainly hasn't been put to bed yet.

          1. John Woloshyn | | #16

            Fair enough Malcom. I will point to Dr.Joe’s paper on Double Wall Construction on BSC from 2014 where in the “ Thermal Control” section, he says that the thermal insulation can be either “fibreglass batt or blown in cellulose”. I am taking, by extension, that Rockwool insulation could be substituted for fibreglass unless there is some inherent quality in fibreglass batts that I am not aware of, but I don’t think there is.

          2. Expert Member
            Malcolm Taylor | | #17

            John,

            I have belaboured a point that I suspect is entirely inconsequential - and it's good to know Joe L. has endorsed the approach. I'd be comfortable using batts in double walls, with a good air-barrier and rain-screen cavity. Out here on the coast I won't get the chance though. Our climate is too mild to justify that level of wall insulation.

  4. Greg Houston | | #6

    That's what they did here. sorta.

    https://www.youtube.com/watch?v=kQt-XnmffiA

  5. Expert Member
    Akos | | #10

    Having insulated staggered stud walls, I can tell you it is a pain. If you are building this type of wall, try to line up the studs so you don't have to wiggle the insulation around studs to install it.

    I would also use batts for steel studs for the middle layer. These are actual 16" wide and you can install them in the vertical orientation and there will be no gap between them. Much easier than trying to thread a horizontal batt behind a 2x4.

    As for moisture in double stud walls, the consensus seems to be that OSB sheathing can be made to work but plywood/fiberboard/gypsum is a better choice.

    1. John Woloshyn | | #14

      Akos, the middle layer of insulation is a continuous layer. I don’t think it matters what dimension the batts are because there are no framing members in the cavity. The batts in the center cavity are installed without any gaps between them. Also, if the air barrier is on the exterior side of the interior wall, there is no benefit to aligning studs for the inner and outer walls. You only need to align openings for windows and doors.

      1. Expert Member
        Akos | | #18

        I understand there are no framing members in the middle, but you still have to get the insulation in there.

        Assuming all framing is done first, if you go with standard batts, you have to install them horizontally and thread them behind the inner studs. This is not a lot of fun.

        My suggestion was to go with the wider batts as they can be installed in the standard orientation through the space between the studs in the in the inner wall but because they are wider, once past the stud they would expand and end up touching just behind the studs.

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