Dense-pack cellulose is installed with a powerful truck-mounted blower that mixes the fiber with air and distributes it through a large-diameter hose. The same setup is used to blow loose-fill material into attics, but the technique differs: Instead of allowing the cellulose to pour out of the hose into open space, it’s forced into framing cavities under pressure, resulting in a firm, compact material that can’t settle and is highly resistant to air infiltration.
Some installers try to dense-pack closed cavities that contain existing fiberglass, but I find that this always leaves some voids. In that situation we’ll remove a 2-foot strip of drywall at the bottom or a strip of sheathing on the outside, pull out and discard the batts, and then net the opening before blowing the empty cavities.
Blowing dense-pack cellulose into closed cavities also is slowed by the need to seek out and fill every framing cavity, including oddly shaped or unexpected ones behind blocking or diagonal braces. A light touch is needed because trying to force in too much material in too little time can bow out existing drywall or plaster.
In new construction and gut retrofits, we staple non-woven polyester netting over the entire frame and blow the cellulose behind the netting, making it easy to inspect the material for voids. The permeable netting gives the air in the cavities an easy route of escape, making it possible to fill the cavities quickly.