Cellulose is a relatively low-cost insulation product, is easy to install, and is not subject to convective heat loss. Studies show that cellulose (as well as rock wool) is more resistant to airflow than fiberglass because it has greater density. Cellulose (as well as rock wool) also may be more effective at reducing air leakage and associated heat loss because its higher density causes it to settle and seal more around rafters and in corners.2
Cellulose loose-fill insulation settles more than rock wool or fiberglass loose-fill insulation. The proportions are about 20 percent for cellulose, 2 percent for rock wool, and 4 percent fiberglass. Therefore, install about 20 percent more blown-in cellulose insulation to offset this settling. Cellulose manufacturers are required by federal law to state “settled thickness” on their bags. Because this can be confusing to consumers, many cellulose producers also specify the “installed thickness.”2
Blow-in methods are also referred to as dense-packing. Dense- pack cellulose is installed at densities of at least 3.5 lb/ft2 and up to approximately 4.0 lb/ft2. The insulation flows from a high-velocity insulation blower at 100 ft/s. The air that is trapped between the cellulose fibers contributes to the insulating value, approximately R-3.8 per inch. Technique is important in work with a higher-density installation. The hose has a tendency to become clogged if the installer is inexperienced.
New cellulose insulation technologies are increasing the use of lower-density cellulose produced by “fiberizing” newspaper. Fiberizing breaks the raw material down into individual fibers that are fluffier. This modification means that the product is cleaner, creates less dust, and has a slightly higher R-value.