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Dense pack cellulose in 2×6 exterior wall…how long does it take to dry?

Frank O | Posted in Green Products and Materials on

My exterior walls were insulated with damp, dense pack cullulose in July of this year. This week I drilled a 3″ hole through the wall from the interior for a mechanical entry and discoverd the last inch (against the plywood) of the cellulose still somewhat damp with the plywood damp. There was no mold or discoloration as the cellulose had completely filled the cavity…but it was disconcerting to feel the moisture inside my wall cavity (exterior sheathing plywood, typar, cement board, paint).
– how long will it take to dry?
– besides the usual concern for mold and mildew, are there any other concerns with this slow rate of diffusion (interior 1/2 drywall, vapor barrier paint)?

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  1. Riversong | | #1

    How long it takes to dry depends on the installed moisture content, the ambient conditions at the time of installation, and the vapor permeance of both the exterior and interior surfaces.

    You don't say where you are, but if the heat's been on for some time and the insulation has not yet dried, it may have been too wet to begin with, closed in too soon, or your wall assembly doesn't sufficiently breathe, particularly to the outside.

    It takes a new house, built with "kiln-dried" lumber a full year to dry out. Concrete foundations and slabs, grouted tile, paints and other water-based finishes all give off large amounts of moisture. A typical 2200 SF house with 17,600 BF of lumber can release 300 gallons of water just from the framing. Damp spray cellulose in the walls can release another 50 gallons.

    It's best to keep the walls open until the cellulose moisture content drops to 15% (it's typically installed at 25%-30%)

  2. J Chesnut | | #2

    Hmmm. . . So still wet towards the exterior side.
    So on the surface there seems to have been more drying potential to the interior over the late summer and fall. If you have vapor barrier paint (?perms) to the inside and plywood (~1 perm?) to the outside does this make sense?
    Cellulose is described to have the ability to distribute moisture, so why in this circumstance can you detect wetness only on the exterior 1"?
    I would want to know which exterior wall this is? North? South? On a South wall I might expect more drying potential to the outside because of the sun's heat unless the circumstances are allowing for inward solar vapor drive.
    What about the possibility of moisture entering the assembly from the exterior at this location?

  3. Frank | | #3

    Seattle, WA. We are just beginning our heating season. I'm on a wooded lot and do not get significant solar to the siding. No evidence of exterior water intrusion or plumbing leaks.

    For drying, I'm running a HRV, dehumidifier, wood stove when occupied, low level slab radiant unoccupied. My humidity level hovers at 50 when dehumidifier is running. 2.14 ACH50. Without the dehumidifier, 60 RH.
    It seems my recourse is to continue to dry to the interior...and with the vapor retarding drywall texture and paint - this may take a while as suggested in post 1.

  4. David Meiland | | #4

    I wouldn't be too worried about that. If only the last inch is damp then the rest has dried. The only reasonably effective thing you could do would be to turn off the HRV, light the woodstove, and run the dehu non-stop... probably not worth it. I assume you get a lot of the same wind we get up here (I'm 75 miles north and the wind always seems to come from the general direction of Seattle) so the rate of natural drying will probably be increasing anyway. If you like buying tools, a good moisture meter (or two) will let you monitor things from either side of the wall.

  5. frank | | #5

    Thanks David...why turn off the HRV - to keep more heat in the house? Hope your staying warm this weekend - snow here.

    Robert - anything else to do to ensure drying?

    thank you for the feedback

  6. David Meiland | | #6

    Frank, my thought on the HRV is that if you were running the dehu(s) full-time you would soon have the interior air drier than the exterior. To clarify, I would use one or two Drizair 110 or similar units that remove a lot of moisture. I can't comment on the efficacy of this now that the drywall is done, but in a new house I start drying right after we install windows and doors, and continue as needed thru drywall finishing or even interior painting. Reality is, from your description it sounds like the walls are actually fairly dry.

  7. Frank | | #7
  8. Riversong | | #8

    Just keep the interior warm and dry and be patient.

  9. Greg | | #9

    Hello Frank,
    Just read your post and wanted to clear up one statement you made. Wall spray stabilized Cellulose insulation is installed with water in open cavity walls. This is not dense pack. After teh cellulose is installed, the sheet rock or sheathing is installed. Dense pack is installed dry without water through access holes cut into the exterior or interior of the wall and blown in. Dense pack specification are 3.5 PSI fill. very seldom can a stabilized wall spray celluolse installation reach the density of a dense pack. The main reason is spraying in the cellulose does not have a closed cavity to pressurize. The second is the fact that the density is not needed since the cavity of the wall is open and the fill is visibly inspected. The adhesive binds the cellulose to the wall to prevent settling. To assure a dense pack wall doesnt settle the appropriate pressure must be achieved.

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