The R-value of loose-fill fiberglass, when settled naturally as in an attic at 0.7 lb/ft3, is 2.2 per inch. When installed at a density of 2.0 lb/ft3 in a wall, the R-value is 4.0 per inch. One of the most signifi­cant criteria for achieving the desired R-value is meeting the des­ignated minimum weight per square foot of material. It is also important that the minimum thickness be achieved, since this, along with the required weight per square foot of material, is essential to obtain the desired R-value. As mentioned earlier, the correct values for coverage with each loose-fill material are stated by the manufacturer in a bag label specifications chart.

Fiberglass blown insulation can be purchased installed for about 7 cents per inch-thick square foot. Except for blown cellulose insu­lation, which costs about the same, fiberglass in blown (or batt) form is the cheapest insulation on the market for the insulating value achieved.9