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Character and Types of Loose-Fill Insulation
If you are increasing the insulation levels in your current home, choosing the right insulation material can be challenging. Fibrous loose-fill insulations such as cellulose, fiberglass, and rock wool are options you may wish to consider.
Character and Types of Loose-Fill Insulation
Cellulose loose-fill insulation is made from wastepaper, such as used newsprint and boxes, which is shredded and pulverized into small, fibrous particles. Chemicals are added to provide resistance to fire and insects. Also, less energy is required to produce loose-fill cellulose than to produce other insulations.
Fiberglass loose-fill insulation is spun from molten glass into fibers. The glass is typically melted in high-temperature gas furnaces. Most major manufacturers use 20% to 30% recycled glass content.
Rock wool (or slag wool) loose-fill insulation is similar to fiberglass except that it is spun from blast furnace slag (the scum that forms on the surface of molten metal) and other rock-like materials instead of molten glass. The production of rock wool uses by-products that would otherwise be wasted.
Primary Applications of Loose-Fill Insulations
Insulation materials are compared on the basis of their R-values per unit of thickness, density per unit of volume, and weight per unit of area.
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.
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.
Loose-fill cellulose and rock wool, 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).
Some cellulose and rock wool 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
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 rock wool 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 thickness 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 weatherstripping 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
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 fiberglass or rock wool. 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 rock wool for every three-wall cavity you fill. (Assumptions: 8-foot [2.4-meter] walls, with 16-inch [41-centimeter] on center wall cavities, and 2x4-inch framing studs.)
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 rock wool 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.
See the Vapor Retarders section that follows for steps you can take to ensure that moisture does not create a problem in your insulation.
Upgrading or Repairing Other Building Components
When installing loose-fill insulations, a material such as 6-mil (0.006-inch, or 0.015-centimeter) polyethylene plastic sheeting can be used as a vapor retarder. Paints that act as vapor retarders are also available. These paints may be more practical for retrofitting homes where no vapor retarder exists because they can be installed without removing finished surfaces.
Federal Housing Administration Minimum Property Standards require that any product, including paint, must have a permeability (perm) rating of 1.0 or lower to qualify as a vapor retarder. The lower the perm rating, the greater the material's resistance to vapor penetration. For example, 15-pound (6.8-kilogram) asphalt felt paper has a perm rating of 1.0, while 6-mil polyethylene sheeting is rated at 0.06, and common household aluminum foil is rated at 0.0001.
If the drywall on your ceiling or wall is removed and the insulated area is completely exposed, you can install 6-mil polyethylene sheeting. Be sure that it runs continuously along the surface area of the ceiling and walls, and that no tears occur during installation. Additionally, all penetrations, such as electrical outlets and light switches, should be carefully sealed. There are preformed foam gaskets for use behind outlets and switchplates.
An air retarder is different from a vapor retarder in that it blocks only air, not moisture. The American Society for Testing and Materials specifies that a material must have a perm rating of 5.0 or higher to qualify as an air retarder. Remember, the higher the perm rating of a material, the more moisture can pass through it. An air retarder should have a high perm rating because this allows the escape of moisture that may have migrated into insulated cavities.
Installation often calls for the "two-hole method," which entails drilling two holes spaced vertically between the exterior walls' framing studs. The holes should be 2 inches (5 centimeters) in diameter. Working between each stud, drill one hole 16 inches (41 centimeters) from the top of the wall. Drill the other hole 24 inches (61 centimeters) from the bottom of the wall. The insulation is blown into the holes, and then the installation holes are sealed. Professionals who are experienced at operating the equipment to ensure proper density and complete coverage most commonly do installation. In conventional and cathedral ceilings, insulation is easier to blow in if an access opening through the ceiling already exists. Otherwise, it may be necessary to drill holes in the ceiling or between the roof rafters.
Voids and Gaps
You may create undesirable voids or gaps if you install the insulation at too low a density or if you do not completely fill the cavity. Voids are most likely to occur at the top of wall cavities, above windows, around doorways, and in the corners of ceiling cavities. Voids also occur if the installation holes are improperly located between the vertical framing studs or if there are too few fill holes. Keep in mind, though, that installers' practices may vary regarding the number, location, and size of installation holes.
It may be difficult to achieve recommended R-values with loose-fill insulation in the eave area of the attic. There are insulation techniques that can be used to insulate this area adequately.
Intentional fluffing by unscrupulous contractors has been a problem in some parts of the country. To avoid these problems, compare bids from several contractors to see how many bags they specify. Count the number of bags used during installation, either by you or a contractor, and compare it to the instructions on the bag. The manufacturer should specify the amount of insulation required to obtain a particular R-value per square foot (or square meter) of space.
Electrical devices and recessed lights (except "IC-rated" fixtures) require 3 inches (8 centimeters) of clearance from insulation.
Pipes for kitchen stoves, wood stoves, and furnaces should only be insulated with fiberglass or rock wool because cellulose may smolder if flue temperatures become hot enough.
Insulation fibers can also be drawn into air distribution systems if the ducts are not properly sealed, allowing the fibers to circulate within the living space. Be sure to seal all of your home's ductwork, as well as any other openings where insulation could leak out of the wall or ceiling cavities and into your living space.
Source: U.S. DOE, Energy Efficiency and Renewable Energy Network, Consumer Energy Information: EREC Reference Brief. EREC is operated by NCI Information Systems, Inc. for the National Renewable Energy Laboratory/U.S. Department of Energy. The content of this brief is based on information known to EREC at the time of preparation. No recommendation or endorsement of any non-U.S. Government product or service is implied if mentioned by EREC.