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Celllulose settling

_matt_p | Posted in General Questions on

Hi, at what density does blown in cellulose insulation not settle any more over time? Does it have to be 3.5 pcf for bay that is 11 inches deep? Or would something like 2 pcf or 2.5 pcf be fine from a settling perspective as well? Thanks ! Matthias

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  1. GBA Editor
    Martin Holladay | | #1

    According to most sources, cellulose has a "settled density" of about 1.5 pounds per cubic foot. This is the density of open-blown cellulose that has been allowed to settle over time, after the effects of gravity, vibration, and humidity swings have been allowed to occur. (That is the settled density of newer fiberized cellulose. Older varieties of cellulose -- called hammer-mill cellulose -- have a settled density of 2.3 pounds per cubic foot.)

    For a closed cavity (rather than an attic floor), cellulose installers are trained to aim for at least 3 pounds per cubic foot if you want to avoid the possibility of settling. Typical dense-packed installations aim for 3.5 pounds per cubic foot.

  2. Expert Member
    Dana Dorsett | | #2

    Deeper wall cavities such as 10"+ double-studwalls usually need a higher density than 3.5# to avoid settling, but it's somewhat climate & stackup dependent too. Creepage from normal seasonal moisture cycling into/out of the cellulose (which is climate variable) is a significant driver of settling, but for a given climate & stackup there is a density above which it essentially stops.

    For open blown attics 2lb density should be good for at least a couple feet of depth, but maybe not three, depending on the degree of moisture cycling. (But in an ~R100+ attic with 30"+ of cellulose would be hard to measure the performance hit from settling.)

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