GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter X Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

How to add cellulose to a wall with existing fiberglass insulation?

NY_ART | Posted in GBA Pro Help on

I have a 1968 back split in zone 6A (Central NY). The 2×4 exterior walls appear to be insulated with fiberglass. I would like to blow cellulose into the cavities to help reduce air leakage. I know I can blow the walls without removing the existing fiberglass. Should I blow between the fiberglass backing (both foil faced and kraft paper) and the inside sheetrock or keep the vapor barrier in place and blow the cellulose between the exterior sheeting and the fiberglass batt?

Thanks in advance for your help.

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. GBA Editor
    Martin Holladay | | #1

    Although it doesn't make a big difference how you do this, I suppose that it would be best to install the cellulose on the exterior side of the fiberglass batts.

  2. NY_ART | | #2

    Is there a reason why most videos demonstrating adding cellulose to a previously insulated wall, show the cellulose being installed between the warm-side vapor barrier and the inside sheet rock?
    Just trying to understand the science behind the solution. This is sort of a one way trip for me. After the cellulose retrofit, phase II will probably involve adding exterior insulation.


  3. GBA Editor
    Martin Holladay | | #3

    I think the answer to this question hinges more on your installation method than building science. On some jobs, it's easier to insert the cellulose hose through the interior drywall. On other jobs, it's easier to remove some of the siding and drill a few holes in the exterior sheathing.

    The facing on your fiberglass batts isn't an air barrier, and your wall should be able to dry in both directions, regardless of the location of the kraft facing or foil facing.

  4. Expert Member
    Dana Dorsett | | #4

    Since the fiberglass is bonded to the facer, when dense-packing from the interior you have to slit the batt where you've drilled to be able to slip the tube all the way though the batt, between the batt and the sheathing. You can't push the tube between the facer & fiberglass, but with only the air blowing (no cellulose) the blast of air keeps pushing the fiberglass away from the sheathing as you go along. As long as you don't have 1001 nails to hang up on, it's not really that hard, though not a simple as when filling empty cavities.

    Slipping the tube between the facer and the gypsum as in the videos is probably pretty easy from either side (I've never done that way, but I'm a hack, not a pro), but a single-stage blower (like mine) may not be able pack all corners of the cavities adequately if you go this route. (Have links to those videos? I'd reallly like to see!)

    Dense packing over batts from the exterior is somewhat more difficult, since the bend of the tube keeps pushing it into the fiberglass where it can hang up, rather than onto the harder easier-slipping surface of the sheathing.

    Doing multiple holes per bay and just packing as best you can without inserting a dense-packing tube in there can still provide substantial reduction in air leakage, and is easier for a newbie/DIY to manage.

    Foil facers aren't air barriers, but can still be vapor barriers unless it get's really shredded, not just burst or slit. There is probably at least some risk to foil-faced rigid foam over those sections, but EPS or rigid rock wool would be fine (and maybe XPS if you limit it to a single inch.)

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |