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Blown-in cellulose in a wall with no plywood sheathing?

steve4699 | Posted in General Questions on


I have a house in the SF Bay Area from the 1950s. It’s stucco exterior, but it has no plywood sheathing. It’s simply stucco, then waterproof paper, mesh, and then the inside of my walls.

Is it safe to get blown-in cellulose into my walls? A contractor that I talked to said it was fine as long as there were no cracks in the stucco. Or is there the potential for water intrusion and for the cellulose to absorb it and cause mold or other water-related problems?

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  1. GBA Editor
    Martin Holladay | | #1

    Good question. I think the answer depends on judgment.

    If your house is a one-story house with generous roof overhangs, and the walls stay mostly dry, I think there wouldn't be any problems. A taller house, with stingier roof overhangs, and an exposed location with wind-driven rain, would make me nervous.

  2. charlie_sullivan | | #2

    And which micro-climate in the bay area might matter as well ...

    You'd also have to be careful to only fill gently and loosely, so as to avoid cracking or blowing out the walls.

  3. Expert Member
    Dana Dorsett | | #3

    A 1-story bungalow with 2' over hangs in a wind-sheltered area, no problem.

    A 2 story stucco Spanish colonial with tile roof, no gutters and limited over hang, big problem.

    You'd probably be safer going with 1lb density damp-sprayed fiberglass (JM Spider) which dries about 100x faster than cellulose would if any moisture got in. The R-value would be comparable to 2.5lb -3lb cellulose. The damp sprayed fiberglass has a moisture activated adhesive that limits the amount of settling that would occur even at low density, similar to "stabilized formula" cellulose. Even if it has to be dry-blown to get it into cavities, it doesn't take too many foggy days or rain-wetting of the wall to get it at least partially glued in place.

  4. severaltypesofnerd | | #4

    I had a bad experience with a similar home, in the same climate. The small pre-existing leaks soaked the cellulose. It was really expensive to fix. Quite clearly the pre-existing leaks were causing slow damage before the cellulose, and catastrophic damage after.

    Don't take the risk: improve something else about the home.

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