All loose-fill insulations are permeable to water vapor. (As discussed in Chap. 4, permeability is the extent to which water
Figure 7.2 Cellulose insulation on top of fiberglass batts. (Greenstone)
vapor can pass through a given material.) Fiberglass and rock wool absorb less than 1 percent of their weight, whereas cellulose absorbs 5 to 20 percent of its weight. It is important to note that any insulation can absorb moisture if exposed to extremely high humidity.
If water penetrates a cavity, such as with a roof leak, moisture can accumulate in the attic cavity and wet the insulation to the point that it mats and compacts. Enough moisture penetration could even cause the ceiling to sag under the extra weight. Typically, if insulation is saturated only one time, it will dry eventually and regain most of its original R-value unless permanent compression is present (i. e., matted down.) However, loose-fill insulations that are repeatedly saturated will lose much of their R-value. Mold and mildew growth also can develop under extensive conditions.
Insulation blown into ceiling cavities should cover the top plate of the wall, but be sure the eave vents are not covered. These vents provide necessary ventilation to the attic, and covering them could result in severe moisture problems.