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Any opinions on long term durability issues with either closed cell spray foam or dense pack cellulose for wall insulation?

user-914555 | Posted in Energy Efficiency and Durability on

I have read that eventually the spray foam will shrink and that the cellulose will settle. It seems that if both of these are true in would be easier to fix the cellulose by blowing more in at the top of the wall cavity. Whereas, correcting gaps from shrinking foam would require removing all the interior drywall.

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  1. GBA Editor
    Martin Holladay | | #1

    Although there have been cases of spray foam shrinking away from studs or rafters, these problems only occur if the installer made an error. Usual causes include an improper mix of A and B chemicals, or spraying under cold conditions.

    The fact that cellulose insulation settles is well known. If the cellulose is being blown onto an attic floor, all cellulose installers plan for settling by installing more insulation than the desired settled thickness.

    If the cellulose is installed in closed cavities, it should be installed using the dense-pack method. If the installer is experienced and able to achieve an installed density of over 3 pounds per cubic foot -- 3.5 pounds per cubic foot is preferred -- then the cellulose won't settle.

  2. user-914555 | | #2

    Thanks Martin,

    We will be adding a second story during the middle of winter in Minneapolis. I think this would qualify as "under cold conditions". Once the house has exterior sheathing and windows are installed would the interior be warm enough to prevent the shrinking. We are also adding a new furnace dedicated to the 2nd story addition. We could wait until after the new furnace is installed and running before spraying foam. What is the proper timing for installing the spray foam during winter construction?

  3. GBA Editor
    Martin Holladay | | #3

    As long as you can heat your job site, you should be able to install spray foam. But all of these questions should be directed to your spray-foam contractor.

  4. Riversong | | #4


    It's not just the interior air temperature but - more importantly - the substrate temperature, If the sheathing is cold, it will inhibit foam expansion and adhesion. Additionally, less than optimum conditions can result in prolonged off-gassing, which can be severe enough that some residents have suffered medical problems and have been forced to vacate their homes. Also, KD lumber shrinks and warps as it acclimatizes to the heated environment - usually taking a full year - and this can create gaps as well.

    There is no healthier or more ecological insulation than cellulose, as long as it contains only borates for fire retardant (no ammonium sulfate). Cellulose also contributes to the durability of the wooden structural frame because of its unique ability to absorb, redistribute and release incidental moisture, as well as its resistance to fire, insects, rodents and mold.

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