Cellulose-blown fiber insulation is now being manufactured in a different, more-efficient manner. Most large manufacturers are employing a new method called fiberization. The primary difference in the process is the grinder. This expensive piece of equipment uses an oscillating disk method to create the cellulose. The old manner of preparation (the original hammer mill style) is no longer a cost-effective option for most manufacturers. The oscillating disk method blows apart and expands the fiber using air instead of hammer-cutting.

Our weatherization division dictates that contractors perform dense-pack wall insulation. The insertion of cellulose in an extremely tight pack is sometimes more than 3.5 lb ft3, which uses a lot of material. Using low-density cellulose to perform high-density packing uses nearly twice as much material than before. And since fiberization is designed and intended to be less tight, compacting the product may defeat the purpose of this ostensibly efficient material.

Should we continue dense-packing using this low-density product since higher density cellulose is no longer available? If so, will the added material cost supersede the savings gained by the reduced infiltration? If manufacturers are in business to move product, why would they develop a way to move less?

John Quimby
Technical Supervisor
Home Weatherization Program
Kansas City Housing and
Community Development Department

Editor's Note: We referred reader John Quimby's questions to David Yarbrough, a professor and chairman of the department of chemical engineering at Tennessee Technological University in Cookeville. His response follows:

While cellulose is now manufactured for installation at a lower weight, manufacturers neither suggest nor recommend density. By law, the labeled density for attics is specified by the blower-cyclone-shaker test (in ASTM test standard C 739). However, there is no ASTM test to fix the density in wall cavities. When it comes to wall installations, a key issue is settling of the insulation after installation because uninsulated regions in a wall cavity greatly reduce thermal resistance. The hammer mill cellulose installed at above 3 lbs per ft3 performed well in vibration tests conducted at Tennessee Technological University (it didn't settle).

The new product does have a relatively low settled density in attics (determined by the blower cyclone-shaker test) but this may or may not be relevant to wall performance. There is a possibility that fiberized loose-fill cellulose insulation can be installed in walls at lower densities and without subsequent settling, but this has not been demonstrated yet. Until significant data are presented, contractors should continue installing loose-fill cellulose in wall cavities at densities above 3 lbs per ft3. A change to lower wall cavity densities awaits valid data. Finally, if the cellulose contains adhesive, the settling properties are quite different and it may be possible to install it at a lower density. But again, more data are needed.