There are several performance characteristics to consider when selecting an insulation material. Among the most important to compare are insulating capacity, weight, convective heat loss, settling and loss of insulating capacity, fire resistance, and moisture resistance.

Insulating Capacity: A material's resistance to heat flow is expressed as its R-value. The higher the R-value, the better the material insulates, and the lesser the thickness you will need. (However, in an open, unrestricted attic application, the height limit of insulation thickness is of no great concern. But if you use your attic for storage, heavy objects will compress insulation and decrease its benefits.) Different insulations also have different densities, or weights. There are weight limits for certain ceiling types (see the chart and the section on Weight that follows).

Weight limits and other factors at R-38 insulation levels are shown in the chart on this page for the three primary types of loose fills. (R-38 is a commonly recommended ceiling insulation level in many parts of the United States.

Weight: Ceiling drywall can sag under heavy loads, such as those sometimes created by insulation. One drywall manufacturer recommends loads of no more than 1.3 pounds per square foot (6 kilograms per square meter) for 1/2-inch (1.3-centimeter) ceiling drywall with framing spaced 24 inches (61 centimeters) on center. The limit increases to 2.2 pounds per square foot (11 kilograms per square meter) for framing spaced 16 inches (41 centimeters) on center and for 5/8-inch (1.6-centimeter) drywall.

Loose-fill cellulose and Rockwool, being heavier materials, could cause the ceiling to sag if installed at R-38 on 1/2-inch (1.3-centimeter) ceiling drywall with framing spaced 24 inches (61 centimeters) on center (see chart). Therefore, when deciding whether to use these materials for new construction, consider switching to 5/8-inch ceiling drywall or, if possible, changing your ceiling framing widths to 16 inches on center.

Some cellulose and Rockwool insulation manufacturers include weight limit information on the bag. Because fiberglass is much less dense, its weight on ceiling dry-wall is not a concern.

Convective Heat Loss: Convection is heat flow caused by air cur-rents. Convective heat loss in insulation is rare, but it can occur when large temperature differences above and below insulation create tiny air currents (called "convection loops") within the insulation. Studies have shown that convective heat loss can occur with lighter density loose-fill fiberglass at the very low attic temperatures possible in extremely cold climates. Depending on the attic temperature, the insulation's measured R-value could decrease by as much as 50%.

To minimize these convection loops and their associated effects, some researchers suggest installing blown-in cellulose or a fiberglass blanket on top of the loose-fill fiberglass. Another solution is to purchase one of the currently available "convection blanket" products that can inhibit this convective heat loss.

Cellulose and Rockwool are more resistant to airflow than fiberglass because they are denser. They may also be more effective at reducing air leakage and associated heat loss, because their higher densities cause them to settle and seal more around rafters and in corners.

Sprayed-in-place foam insulations are an alternative to loose fills in some applications. They offer higher R-values at lower thicknesses than loose fills and, when properly installed, can help stop air leakage.

But no insulation, by itself, provides an effective air retarder because it cannot completely block airflow. Installing an air retarder along with your insulation and using caulking and weather-stripping seals all gaps and greatly reduces air infiltration into your home (see the section on Air Retarders that follows).

Settling and Loss of Insulating Capacity: Many loose-fill insulations installed in attic cavities will lose some of their installed R-value over time because of settling. Cellulose loose fill settles more than Rockwool or fiberglass loose fill-about 20% compared to roughly 2% to 4%. Therefore, install about 20% 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 "installed thickness" on their bags. Regardless, installed thickness can be estimated by adding 20% to the stated settled thickness, but be sure not to exceed previously mentioned weight limits.

Researchers say that it is possible to install loose-fill insulations in wall cavities without settling. If the cavity is completely filled with insulation at the proper density, no significant settling should occur. A general density guideline for walls is roughly 3.5 pounds per cubic foot (17 kilograms per cubic meter) of wall cavity for cellulose and 1.5 pounds per cubic foot (7 kilograms per cubic meter) for fiber-glass or Rockwool. These specifications are roughly twice the density of horizontal applications.

One expert suggests this easy-to-follow guideline to ensure that wall cavities are being filled at a density sufficient to prevent settling. Use roughly one 30-pound (13-kilogram) bag of cellulose or about 15 pounds (8 kilograms) of fiberglass or Rockwool for every three wall cavities you fill. (Assumptions: 8-foot [2.4-meter] walls, with 16-inch [41-centimeter] on center wall cavities, and 2x4-inch framing studs.)

Fire Resistance: Loose-fill insulations offer very good resistance to fire. Although fiberglass and Rockwool are naturally fire resistant, cellulose's fire resistance is achieved by adding chemicals. To ensure that it does not present a fire hazard, cellulose must pass tests established by the Consumer Product Safety Commission.

Moisture Resistance: The average household generates a considerable amount of water vapor each day through activities such as cooking, laundry, and bathing. This vapor migrates into insulated cavities and, if it reaches the dew point (the air temperature at which water vapor cools enough to condense), it converts to liquid within the insulation. This reduces the insulation's effective R-value.

All loose-fill insulations are permeable to water vapor. Permeability is the extent to which water vapor can pass through a given material. Fiberglass and Rockwool absorb about 1% of their weight, and cellulose absorbs 5% to 20% of its weight. However, any insulation can absorb large amounts of water if exposed to extremely high humidity.

Higher levels of outdoor moisture can also penetrate into insulated cavities. If your roof leaks, for example, 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.

If insulation is saturated only one time, it will eventually dry and regain most of its original R-value. However, loose-fill insulations that are repeatedly saturated will lose much of their R-value. Moisture also causes additional problems, such as mold and mildew growth.

Before You Install Insulation: Upgrading or Repairing Other Building Components - There are other home weatherizing and sealing measures to complete before you undertake any insulation project. A tight, well-sealed home is more energy efficient and needs less insulation to keep you and your family comfortable. Tests have shown that far more cold air infiltration and heat loss result from improperly sealed windows, doors, ducts, light switches, and outlets than from insufficient insulation coverage or performance.

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