Some loose-fill insulations installed in attic cavities will lose some of their installed R-value over time because of settling. Installers need to refer to the “installed thickness” specifications. Researchers say that it is possible, however, to install loose-fill insulations in wall cavities without settling if the cavity is completely filled with insulation at the proper density. A general density guideline for walls is roughly 3.5 lb/ft3 of wall cavity for cellulose and 1.5 lb/ft3 for fiberglass or rock wool. These specifications are roughly two or three times the density of horizontal applications.2

Density measurements may not be practical for do-it-yourself installers. Another guideline to ensure that wall cavities are being filled at a density sufficient to prevent settling is based on the quanti­ty of material. For example, if installing in 8-ft walls with 16" on-cen – ter wall cavities and 2 X 4" framing, use roughly one 30-lb bag of cellulose or about 15 lb of fiberglass or rock wool for every three wall cavities to be filled.2 (These quantities are for general information only. Consult the manufacturer’s literature for a specific application.)