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stressedsnyder | Posted in Energy Efficiency and Durability on

Just received a quote to insulate the 2 x 6 walls of a residence 6″ of open cell spray foam costs a bit less than 6″ of dense packed cellulose.
I am apprehensive about the open cell foam with regard to it holding moisture. ┬áThe exterior wall is 1/2″ plywood with Tyvek stucco wrap and then cementatious lap siding. ┬áThe interior walls are 1/2″ drywall both painted and the kitchen has porcelain tile.

Is my apprehension unfounded?

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  1. GBA Editor
    Martin Holladay | | #1

    First of all, it would be helpful to know your climate zone or geographical location.

    Q. "I am apprehensive about the open cell foam with regard to it holding moisture."

    A. Why are you apprehensive? What type of moisture are you worried about?

    The most common source of water in walls is wind-driven rain. For more information on this issue, see "All About Wall Rot."

    If that's the kind of moisture problem you are worried about, the choice of insulation has nothing to do with preventing this type of problem. What you need are good exterior flashing details, a water-resistive barrier (WRB), and good roof overhangs.

    If you have a problem related to the entry of wind-driven rain, cellulose will be just as much of a headache as open-cell spray foam.

    Note that in some climate zones (Zone 5 through 8, as well as Zone Marine 4), building codes require the use of an interior vapor retarder (for example, vapor retarder paint on your drywall). Regardless of whether you choose to install cellulose or open-cell spray foam, you should comply with this code requirement if you live in a climate zone where it is required. For more information, see "Vapor Retarders and Vapor Barriers."

  2. stressedsnyder | | #2

    Thanks for the info.. I am located in New Orleans.

  3. Expert Member
    Dana Dorsett | | #3

    Termite territory? (Probably yes.)

    Cellulose treated with all borate fire retardents would be worth paying a bit extra for. Borates are toxic to the gut flora that termites need to digest wood- even a small amount of ingested borate becomes fatal to the host.

    Is the house already built? If yes, how are they intending to install the open cell foam? A half-pound pour approach to insulating fully assembled walls with open cell foam has a blowout risk if overfilled, and a risk of voids in the insulation were the pour was partially blocked, then fully blocked by the foam as it cured. If it's still bare studs, open spraying does not have those issues.

    The additional thermal mass of cellulose make a bit of difference in performance relative to open cell foam too, but not worth paying a lot extra for.

    Open cell foam makes the assembly somewhat more air-tight than 3lbs cellulose, but not enough to make an energy use difference.

  4. stressedsnyder | | #4

    Thanks for the reply. It's an open wall renovation with 2 x 6" stud walls. Termites are a problem with drywood termites becoming a huge and expensive problem requiring the building to be tented and fumigated. I'm going with the borate treated cellulose.

  5. Jon_R | | #5

    Don't forget to be very careful about air sealing and tests to verify it.

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