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Community and Q&A

Blowing Cellulose in Staggered Stud Walls

dylan_lamar | Posted in Green Building Techniques on

I’m designing a Net Zero apartment in Albany, NY and planning for a 2×8 staggered stud wall. I’m uncertain about the feasibility of insulating with dense pack cellulose since it’s not easy to create closed cavities. The wall has bearing 2×6 studs @ 24″ o.c and staggered non-bearing 2×4 on the other side of the 2×8 plate.

I’ve thought about netting stapled diagonally from 2×6 to 2×4 to create sides for each cavity (a lot of labor), or perhaps strips of fiberboard tacked to studs and extending back to the wall sheathing. I see from prior article “Details for Insulating a Double Stud Wall with Cellulose” a comment from Jonathan Tauer mentioning installing the insulation in multiple passes so that it starts to “self-dam” itself into cavities. This seems tricky to ensure uniform consistency though.

If anyone has experience with this I’d appreciate your thoughts.

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  1. user-2310254 | | #1
  2. Expert Member
    Michael Maines | | #2

    The only way I've found to do staggered double stud walls is to include a membrane between walls. If you have a good installer, it could be a variable permeance membrane, but it takes more skill to dense-pack behind that than it does behind netting.

  3. mattbrennan4 | | #3

    I've had lots of experience working with contractors on these projects.

    I do not recommend the multiple pass method as it is very slow and you have a very poor idea of how much cellulose is going into each section.

    A better solution would be to 1st determine target density - I spec 3.5lbs per cubic ft typically. Determine how many bags you will need total, and per wall. Keep track of the bags used as you go. This is the easy way to determine that you're using the right amount of bags for the density you are looking for. **If you're really smart you can mark on the walls or studs the number of bags you should have used up to each section and have the person you is putting the bags in keep a tally.

    To your main question, stapling mesh and breaking up the open wall cavity into sections. You're right that takes labour, but not as much as the alternatives. Pneumatic stapler, roll of mesh, scissors/blade) - mindless but fast. In fact much faster than backfilling in every case I've seen. Site prep is key for speed. Have every cavity prepared so those blowing can just go and not have to stop and start.

    If its your first time doing it, expect some growing pains to start as you dial in your machine settings to have enough force to dense pack but not clog.

    1. dylan_lamar | | #4

      Thank you both for the feedback. Matt would you recommend diagonal strips of netting to enclose cavities (1st image attached)? Or running a continuous strip of netting between inner and outer studs (2nd image attached) creating inner and outer cavities as Michael suggests?

      1. Expert Member
        Michael Maines | | #5

        Dylan, I think Matt is saying that you don't need to create insulation "cells" using netting. While I agree that it's a good idea to keep track of how many bags you have used for each section, from what I have learned directly from industry experts such as Bill Hulstrunk is that for large framing bays, 4.0 pcf is a safer target density. At 3.5 pcf there can be settling. In smaller bays, 3.5 pcf is enough because friction against the boundary helps hold things together. Insulator Jon Riley wrote a good overview here:

  4. Expert Member
    Akos | | #6

    Just doing the rough calculation on your assembly R value, the 2x8 staggered stud is not adding all much. Including drywal, siding and air film it is around an R27 assembly.

    Just going with simple wall with 2x8 studs 24OC gets you an R23 assembly for much less headache.

    If you do want higher R values, much better to bump up your bottom plate to 2x10 or 2x12 and dense pack that assembly. That will be significantly more R value for very little extra material and insulation cost.

    If you must have the 2x8 staggered stud wall, I would suggest not staggering the studs so much, even if the studs are touching on the side, it will get you most of the thermal bridging benefit. 2" stagger is about the same assembly R value as a 12" stagger. This way it is much easier to add dividers the wall for insulation fill.

    You can also go with 24OC for the 2x6 and 16OC for the 2x4. This would naturally put a 2x4 right next to a 2x6 every 4' to create the separation for almost no extra labor. The installer would still have to be careful with fill but you are only doing 2 stud bays at a time so it is pretty easy to ensure a good fill. Bonus is that almost any siding can be hung on 16OC.

    P.S. If you are building the staggered stud for sound, dense pack is not the way to go as it directly couples the two layers of drywall.

    1. dylan_lamar | | #8

      Thanks Akos, I'm just seeing your post. Your points are well made. We're looking at a 2x10 wall as well, and based on the labor issues with staggered stud for both framing and insulation, it looks likely we'll move that direction.

    2. Expert Member
      Deleted | | #10


  5. dylan_lamar | | #7

    Thanks so much Michael. This is a 30,000 SF building, so I'm not sure counting bags is going to be practical. Perhaps with a skilled installer they can be sure to hit 4.0pcf by "feel" and then we'll cut some test samples out to weight and verify.
    Part of our goal in moving to cellulose is to reduce the embodied carbon of the building, so I'd like to find a way to make this work at scale.
    I'm also skeptical of getting consistency without enclosed cavities. Does my second image above match the netting approach you recommend in your first response?

    1. Expert Member
      Michael Maines | | #11

      Dylan, yes I think your installer will like your second approach. Installers are trained to install at 3.5 pcf and that's a much softer pack than 4.0 pcf. I've found that 100% of the time (unless it's Jon Riley, the author I linked to above) that you can just plan on saying it's not dense enough. Or you can let them do 3.5 pcf; with the center netting it would probably be fine.

      I agree with Akos' calculations and suggestions, other than sound--a dense-packed wall assembly is incredibly quiet. It seems to absorb almost all sound.

      1. Expert Member
        Akos | | #12

        I kind of remembered that cellulose was a fair bit less effective for double stud walls. I just check the NRC data, it is very slightly less but pretty darn close (p 24):

        Bottom line is that any well sealed and insulated double stud wall is pretty darn quiet no matter the insulation.

        1. Expert Member
          Michael Maines | | #14

          Akos, I have not seen that document, very helpful. I've used this one, from a manufacturer but with testing at an independent lab. I'm not sure when it was published but I saved it in January, 2017.

  6. user-5946022 | | #9

    What about wet sprayed cellulose in lieu of dense pack? Seems like that would solve the issue.

    1. Expert Member
      Michael Maines | | #13

      CL, damp-spray cellulose (my old sales rep trained me out of calling it wet spray) would work but in thick walls it takes a long time to dry and installers are hard to find. There are a lot of cellulose installers around me but none do damp-spray.

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