Can cellulose be too fluffy?
During one of my frequent visits to this area of the site, one of the discussions revolved around, to quote Martin “the slight performance degradation of very fluffy insulation on the attic floor due to wind-washing and internal convection currents, especially in very fluffy fiberglass insulation at very cold temperatures”. My question comes from my recent experience of removing my cellulose insulation and blowing it back in after spraying ccSPF on the attic floor. The re-used cellulose insulation seemed much fluffier than the supplemental cellulose I installed. Can the cellulose be too fluffy and result in the internal convection currents like occurs in loose fill fiberglass?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Peter,
Over a period of several months, newly installed blown-in cellulose on an attic floor will settle and become less fluffy. This process is normal and is anticipated by all cellulose installers. Once the cellulose has settled, I have never heard of problems with performance degradation due to cold-weather convection for cellulose.
If you are worried about fluffiness, we all know the simplest solution -- just blow a little more insulation on top...
Thanks Martin. I expect the cellulose to settle as you indicated. My observation was comparing the apparent density of the re-used cellulose in relation to the density of the new cellulose, expecting that the two would have relatively similar settlement rates. My thinking is that the re-used cellulose would not settle to a density that would be similar to that of the new, and have slightly degraded thermal performance. Just wanted to ask the question first before I went though the effort of installing more.
Thanks again.
Any time you run the material through the blower it mixes in more air. The fresh cellulose packed in the bags went into the hopper at ~10lbs per cubic foot to be agitated broken up and blown at about 1.2lbs density in an open-blow. The recycled goods it went into the hopper at about 1.5-2lbs density and gets more air blown into it unless you readjust the air on the blower, so it's probably well under 1lb density and will settle fairly rapidly.
By virtue of it being flakes of paper it's air-retardency against internal convection is still pretty good though, nothing like low-density fiberglass. And as it settles it will become even more convection-retardent.
The settled density of either the new or old depends on the annual moisture cycling it experiences, as well-studied& documented by a Danish researcher named Torben Valdbjørn Rasmussen. (google him if you want to read the tedious scientific details- much of it is online and in English.) But how long it takes to get there depends on the initial installed density.