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Insulating a double stud wall cavity

StephenW81 | Posted in Green Building Techniques on

Assuming: 2×4 double stud wall with 3-4″ between the inner and outer (load bearing) walls and with the air/vapor barrier (e.g. Intello Plus) on the outside of the inner wall, and dense pack cellulose or blown-in fiberglass installed in the cavity.  The outer (load bearing) wall is installed first and completed with structure of the house, and the inner wall is installed later.

How would you design/install the inner wall to ensure consistent blown-in insulation density while avoiding damage to the vapor barrier?

It appears that for double wall construction where the air/vapor barrier is not between the walls, the recommendation is to install netting between the studs in every other bay.  However, this would not be possible with the air/vapor barrier on the outside of the inner wall.  

I had thought of installing plywood gussets every 16-24″ along the outside wall studs in every every other bay and then installing netting on the gussets, leaving ~1/4″ spacing between the edge of the gussets and the inner stud wall to reduce the potential for damaging the vapor/air barrier and to provide a minimal thermal break.  However, I’m not sure whether the ~1/4″ open space would be large enough to cause problems with blowing the insulation dense enough. In addition, it seems like there is still a substantial risk of damaging the air/vapor barrier when raising the inner wall, if the wall slipped a little.

What is the best way to accomplish this? 

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Replies

  1. Expert Member
    MALCOLM TAYLOR | | #1

    Stephen Sheehey is your man. His blog is here on GBA, or at this link: https://stephen1147.wordpress.com

    1. StephenW81 | | #2

      Malcolm,

      Thanks for the response; very interesting information at the link you provided. I note that it looks like they did not provide any netting to create insulation bays, and experienced difficulty in getting the correct density.

      I also found a video from the Cold Climate Housing Research Center where they suggest it is very difficult to get the correct density without using netting to create insulation bays when using blown-in fiberglass (which is what I will likely use due to lack of dense pack cellulose availability): https://www.youtube.com/watch?v=R2Vixw_lSHg

      I'm curious if anyone has a resolution for this potential issue, or whether they just blow insulation into the entire cavity and it works fine regardless of what some people say.

  2. Expert Member
    Rick Evans | | #3

    Stephen,

    Do you want the intello to be a vapor retarder or an air barrier?

    If you want it to be an air barrier, your approach may be tricky. How will you tape the seams of the intello at wall intersections? All of the interior walls that you stand will have to run the length of the entire exterior wall. Otherwise, you will want need to caulk the wall connections. Also, how will you tie this wall into your ceiling air barrier and slab? This is easy with a single story slab on grade but more difficult with floor joists.

    If the intello is to serve as a vapor retarder then perfect air tightness isn't an issue. In that case, I wouldn't bother putting it at mid wall. I would add it on the inside of the interior wall and add drywall over it. It will get poked with screws ,etc but that is okay for a vapor retarder.

    Regarding insulation:

    The insulation contractors can "poke through" the intello and dense pack behind it. They will need to do it carefully as Intello is so air tight that it can create air pockets. Additional slits will need to be made to help equalize the pressure. No netting is needed. Make sure you tape up the holes afterwards.

    The other approach is eliminate the mid wall vapor retarder and put it on the inside of the inner wall. That way, you can net the walls and then dense pack the cavity as normal and then add your Intello. To protect it over time, add 2x2 or 2x3 strapping over the intello. This is the best option if you want the intello to serve as your air barrier. Air leaks move a lot of vapor! So I would go this route.

    I suspect that mid-wall vapor retarders/air barriers are best achieved when the interior wall is the load bearing wall.

    1. StephenW81 | | #4

      Rick,

      The Intello (or equivalent) would be both air and vapor barrier. The issue of air sealing when installed on the outside of the inner wall was addressed in another thread ( https://www.greenbuildingadvisor.com/question/double-stud-wall-vs-2x8-with-strapping ). I am convinced that it can be done successfully, although it will certainly require great care.

      I have considered installing the Intello on the inside of the inner wall with strapping over it (as advocated by 475 for instance), but that results in a thicker wall and more lumber use. That doesn't preclude building the walls that way (and I do intend to build the ceiling that way), but I would prefer not to.

      Using the outer wall as the primary air barrier is a thought. I had intended to make the outer wall as a secondary air barrier for redundancy and to protect insulation from exterior air intrusion. However, I planned to use the interior wall as the primary air barrier.

      I believe that all of this can be addressed in one way or another. The primary concern that I have is that when blowing in dense pack fiberglass, the installers will have a difficult time getting the correct density if cavity bay width is 50' vs 2-4'. That is a problem identified by the Cold Climate Center but challenged by others, including yourself.

      Thanks for your input.

      Steve

      1. Expert Member
        MALCOLM TAYLOR | | #5

        Steve,

        It's not something I know much about, but recently when I asked Dan Kolbert (and others chimed in) about the importance of moisture buffering by the insulation in double-walls, he replied he thought it made a significant contribution to keeping the walls safe. That may inform your choice between cellulose and fiberglass.

        1. StephenW81 | | #6

          Malcolm,

          Yes, I am aware of the moisture buffering argument for using cellulose. The primary issue is that the nearest dense pack cellulose installer that I am aware of is in Boise, ID, which is ~180 miles from the build site. The other point is that northeastern Oregon is a relatively low humidity environment compared to the Midwest, New England or Portland/Seattle/Vancouver where it seems like most double stud wall houses have been built.

          Steve

  3. Expert Member
    Rick Evans | | #7

    I don't think you will have any issues with fiberglass. My walls are double stud, (12" total) with fiberglass. The installers had never filled such a big cavity but they did just fine (with a little supervision from my wife and I).

    Cellulose isn't as "fluffy" as fiberglass. And the density needed is twice what you need for fiberglass. The CCHRC, if I remember correctly, was filling a 18" - 24" cavity! They probably need at least 4 pound density to prevent settling with cellulose. With your plan, you would only be filling a 6.5" - 7.5" cavity.

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