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How to control cellulose bulging in 24″ wall cavities

PETER G ENGLE PE | Posted in General Questions on

I don’t have much experience with installation of dense packed cellulose, so I’m drawing on the GB brain bank.  I’m working on a Pretty Good House build in VT, CZ6.  The walls are double 2×4 stud @ 24″ on center, 10.5″ thick, insulated with dense-packed cellulose.  Our insulation contractor says that he needs to staple the netting to the sides of the studs (effectively reducing R-value).  Even then, we need to install 1×3 strapping on the walls to hold the cellulose in place and prevent the bulging from making drywall installation possible.  He’s also recommending 5/8″ drywall.  This causes several issues for us, with some of the wall thicknesses being fixed by finishes and equipment.  Increasing the interior wall thickness is going to be very difficult.  Does anyone have any experience and/or suggestions?

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  1. Trevor_Lambert | | #1

    Can you provide some more detail on the wall? Is the structural wall the inner or outer 2x4 wall? Where is your air barrier and vapour retarder layer? Where are you at in the construction process?

    1. Expert Member
      PETER G ENGLE PE | | #12


      Sorry for the slow reply. I posted the question and then got called away for a few days.

      The structural wall is the outer wall. We are using taped ZIP as the water and air barrier. We will be installing one of the smart membranes on the inner surface of the inside studs as a vapor retarder - we haven't decided which one yet. The walls are framed and sheathed and we are just starting rough-in for utilities, so we've got a few weeks to make decisions about the insulation. Thanks.

  2. jollygreenshortguy | | #2

    15-20 years ago people worried about 1/2" drywall being too flexible for 24" stud spacing. That's really an overblown worry though. At this point there must be many thousands of homes built that way. 5/8" on the ceiling is an option. But you can also go with 1/2" Type C on the ceiling, which is stiffer and also more fire resistant. If the ceiling is supporting insulation above then I'd consider it. But if the ceiling is not, then normal 1/2" will also be fine.
    As far as the cellulose insulation, again, people have been installing it between 24" studs for a couple of decades now. Frankly, I think your installer is just telling you he wants to do it the way he is comfortable with and making it sound as if it's the only right way to do it.

    In general I'd encourage you to buy a recognized name brand drywall, such as US Gypsum. A lot of the material on the market is cheap Chinese manufacture with air pockets in it. Put the cost of labor into the best quality material you can afford. It's no more labor to put up quality material than cheap material and sometimes it's less.

    To consider for the ceiling but not necessary, Type C.

    This is the standard 1/2" US Gypsum panel and is perfectly adequate for walls and ceilings.

  3. Expert Member
    Michael Maines | | #3

    "Lip stitching" is the typical way I've seen double-stud walls insulated. The tiny amount of insulation space lost doesn't matter a bit. It will actually still insulate, as there will be little space and little temperature differential.

    I will design 24" o.c. when my builder requests it but otherwise I stick with 16" o.c.. For 24" o.c., you might consider netting the exterior studs into their own cavities, reducing the depth of the interior dense-pack.

    On a few occasions I have had walls dense-packed behind Intello with vertical 1x3s at each stud. Intello doesn't stretch so it won't bulge inward enough to affect your drywall. It is challenging to dense-pack behind it but if you're in VT, you probably have installers trained by Bill Hulstrunk on how to do it.

    After the cellulose is installed, it should be rolled with a metal roller that flattens it out pretty well.

    1/2" vs 5/8" drywall is a matter of preference. I have builders who are fine with 1/2" at 24" o.c.. You might want to include extra blocking where you'll have cabinets and trim, including baseboards. If you are on a body of water or have a design that allows a lot of raking light into the room, and/or you're using relatively glossy paint, you might be happier with 5/8" drywall but for most situations, nobody will notice if it's 1/2".

    1. freyr_design | | #7

      Do you ever have issues with dense pack ceilings? or do you always use furring strips?

      1. Expert Member
        Michael Maines | | #9

        The main problem I see at ceilings is that the cellulose is almost never installed densely enough. It should feel like a very firm mattress, with almost no give, but most installers leave it feeling like a fluffy pillow, which will result in settling.

        Here in northern New England we always strap ceilings so bulging is rarely an issue.

  4. user-5946022 | | #4

    Consider instead damp applies sprayed cellulose for the exterior walls.
    The damp applied at my exterior walls sprayed against the exterior zip sheathing worked very well.
    The interior wall dense pack installed for sound abatement was installed between netting...It immediately settled, and the drywall on the 16" oc interior walls with the dense pack bulges.

  5. norm_farwell | | #5

    Yes, bulging over 24” cavities can be troublesome. The following can help:

    1) Some types of webbing stretch more than others so try to get one that is heavier and stretch resistant. As mentioned above, membranes like intello don’t stretch at all.
    2) Try to make sure your webbing is installed well and stretched tight (lip stitching is not a problem). Any looseness will bulge more easily.
    3) Don’t overpack—with stretchy webbing ramming the tube in and out will cause more bulging than necessary. Once you reach density move on with the tube—don’t keep stuffing.
    4) Densepack bulges can be pushed back in plane, and once compressed they’ll stay put. A roller works but i find the ergonomics uncomfortable. I prefer to lean in and use my body weight with fists or palms to push because the pressure is more concentrated. It’s physical work but sometimes it’s just part of the job.

    If it were me I’d forego the strapping. We use 5/8 drywall when we’re 24” OC..

    High quality insulation is worth the extra trouble. Good luck!

  6. neilt | | #6

    I was also concerned about the extra pressure applied to the drywall with dense packed insulation on 24" oc but also wanted to reduce studs in the wall. So the last couple of houses I built I framed the walls at 19.2" oc. It was an easy compromise that still works out for plywood sheets and reduced the concern about deflection from 24" spacing. All tape measures have 19.2" centers marked on them so layout is easy. The biggest disadvantage is that you can't purchase insulation batts for the spacing but it you're using dense packed cellulose it won't matter. I also used 19.2" spacing for floor trusses so studs are point loaded over the joists. Might be worth considering!

    1. user-723121 | | #8

      Did a lot of 19.2 layout over the years, good to bring this up. Good for floor systems as well, subfloor can be a bit springy between 24"oc joists.


  7. GBA Editor
    MIKE GUERTIN | | #10

    Peter: I'm not entirely clear on "Even then, we need to install 1×3 strapping on the walls to hold the cellulose in place and prevent the bulging from making drywall installation possible." Were you planning on strapping the walls or was this something the insulator is telling you must be done to restrain the cellulose? Or is the insulator just recommending inset stapling (lip-stitching) and 5/8" gypsum?

  8. Expert Member
    PETER G ENGLE PE | | #11

    My installer is recommending lip stitching, 1x3 horizontal strapping AND 5/8" drywall. I've since contacted several other installers and most agree in general that the 24" centers need some sort of additional support for the Insulweb, even if they differ on some of the details. Because of the space constraints, I am considering adding an additional 2x4 stud in between the already framed 24" OC studs on just the interior wall. It won't have much thermal effect and should effectively reduce the "span" of the Insulweb to 12". Because of the various windows, doors, structural loads and such, there aren't all that many clear 24" stud spaces that we will have to retrofit and this seems like the best solution.

    1. conwaynh85 | | #13


      I have done 1x3 horizontal strapping on walls with cellulose in a previous house and am doing it again on the next. I Installed the netting before interior strapping and interior framing, so the sequence was unusual, but it allowed me to reduce wall nailers were interior walls intersected with exterior walls and makes interior framing really easy. Also, any cabinet blocking on exterior walls is removed and replaced with a little extra strapping. It all held the insulation back well while making other framing much easier.

  9. PAUL KUENN | | #14

    You will have no bulging issues with Intello face stapled to the studs.

    1. Expert Member
      PETER G ENGLE PE | | #15

      But Intello isn't porous enough to allow the air used in blowing the cellulose to escape. Our plan is to use Insulweb to hold the cellulose during installation, and then installing Intello as our vapor retarder. If the cellulose is already bulging, the Intello is not going to help.

      1. Expert Member
        Michael Maines | | #17

        There are ways to dense-pack behind Intello or other airtight membranes; cellulose guru Bill Hulstrunk has taught many installers to use a double-wall installation wand that allows air to evacuate, and/or to leave out one or more strips of membrane to be patched in after cellulose installation. But from what I've seen, your plan is simpler and more likely to have good results.

  10. dfvellone | | #16

    My wall plan was similar to what you’re up against (24” centers, dense pack cellulose) and although I ultimately chose to go with mineral wool. I used 5/8” imperial plaster base on the 24” centers (1/2 on partition walls) and could not imagine using 1/2 on 24”
    centers regardless of managing bulging, but particularly if there was any at all.

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