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Technical Pages

Performance and Safety Factors

When you consider performance and safety factors, the balance swings to cellulose building insulation.

Thermal Resistance (R-Value)

Thermal resistance (R-Value is one of several factors that contribute to the performance of the "thermal envelope" of a building. It is a mistake to consider only R-Value when specifying insulation, but R-Value is important. Understanding what R-Value actually means may be the biggest challenge for a buyer or specifier.


The R-factor of cellulose insulation is approximately 3.8 per inch and it does not vary significantly over a wide range of densities. In an attic, 10 inches of cellulose insulation will have an R-value of about R-38, regardless of the density of the material. "Fluffing" cellulose by adding excessive air during installation does not change the R-factor of the insulation, so it is easy for an inspector or homeowner to check the R-value of the installation. As long as the insulation maintains the required thickness it will have the specified R-value. (Refer to the section on Settlement for more information.) In addition to maintaining R-value over a wide density range, cellulose insulation also maintains R-value under cold conditions. At an attic temperature of 20° F below zero the R-value of cellulose insulation is higher than at 70° above zero. It's important to emphasize that while R-value is important, many other factors are nearly as significant in determining the real world thermal performance of buildings. Focusing on R-value to the exclusion of these can lead to poorly performing buildings.

Fiber Glass

Fiberglass R- value changes with density. Light, fluffy blown- in fiberglass usually has an R-factor of 2.2 per inch, or less. The dishonest practice of "fluffing" blown- in fiberglass both reduces the R- factor of the material and increases the amount of settlement that will occur. Depending on the density of the material, 10 inches of blown fiberglass usually has an R-value of about 22. Under winter conditions the R-value of fiberglass is further reduced. The actual R-value of blown- in fiberglass in an extremely cold attic may be up to 50 percent lower than stated. The more you need it the less insulating performance it delivers. The stated R-value of fiberglass batts is based on full thickness. Batts stuffed into wall cavities are often compressed to less than full thickness and lose R- value and increases the amount of convection heat loses throughout the fiberglass.

Settling & Loss of R-Value

All loose-fill insulation settles after installation. Cellulose insulation is always specified and sold at settled density, so compensation for settling is built into the bag count and material weight columns of cellulose coverage charts. Other loose-fill insulation materials may or may not compensate for expected settlement in coverage chart data.


The Federal Trade Commission R-Value Rule and accepted industry standards all require cellulose insulation to be specified and sold at settled density. The bag count and weight of material columns on cellulose coverage charts give precise and accurate information on the amount of material that must be installed to produce the specified R-value. It is not necessary to install more material than is indicated by these columns to compensate for settling. Compensation for settlement is built into the coverage chart. In fact, it is commonly believed the test specified results in over compensation for settlement.

Fiber Glass

There are no industry or government standards that address settlement of fiberglass insulation, in spite of the fact that several studies indicate blown fiberglass settles. The FTC R-Value Rule acknowledges that fiber glass settles, but contains no provisions requiring manufacturers to account for loss of thickness after installation. Settlement will be greatest (and the R-value less) if the material is deliberately fluffed during installation, an all too common occurrence. ASTM has adopted a test method that can accurately determine the installed thickness of any loose-fill insulation. If manufacturers use this test accurate coverage charts can be developed.

Air Infiltration

Uncontrolled leakage of air through exterior walls and ceilings of homes is almost as important as R-Value in determining how much energy will be required to heat and cool a building. This factor is all too often totally ignored in specifying insulation.


Cellulose insulation, either spray-applied or dense-packed in walls, is very effective at sealing buildings against air infiltration. And air infiltration is almost as important as R- value in the thermal performance of a building. Scientific studies have confirmed anecdotal reports and "conventional wisdom" about the superiority of cellulose at tightening buildings. Research shows cellulose to be up to 40% better than fiber glass at controlling air infiltration. Infiltration of unconditioned ambient air means that heating and cooling systems must expend more energy to compensate for the infiltration. Many authorities believe insulating a building with cellulose makes air barriers (housewrap) unnecessary. Canadian engineers tested a new cellulose-insulated home for air tightness, then slit the polyethylene air/vapor barrier in about 20 places and retested the building. There was absolutely no change in measured air leakage. Smoke pencil testing at the slits showed "not a breathe" of air leakage. No attempt was made to tighten the exterior sheathing and the siding had not yet been installed when the test was performed.


Fiber Glass

Fiber glass is used as air filter media, and fiber glass in walls and ceilings behaves much as the fiber glass in an air filter. Air rushes right through it. When fiber glass- insulated walls are opened the batts are usually found to be covered by dust, just as an air filter would be. Additional materials to control air movement are essential in fiber glass insulated building assemblies. In addition, extreme care must be taken to seal all areas around pipes, windows, electrical boxes, and along studs in fiber glass- insulated walls. High R-values won't assure comfort or energy savings if cold air (or hot air) can leak into the building around and through the insulation. It is possible, but expensive to build tight fiber glass insulated assemblies. By using overlapping foam a separate air barrier, and extensive amounts of tape, caulk, injected foam sealant, and other materials, fairly tight walls and ceilings can be constructed under controlled conditions. The extra materials and the painstaking attention to details add considerably to the cost of the building. Why not just use insulation that "automatically" tightens walls and ceilings?


Hot air rises, and when this happens cold air rushes in to replace it. When this occurs in insulation installed in an attic the air circulation carries heat through the insulation, reducing its effective R-Value. Under cold winter conditions the R-Value loss can be significant.


The R-value of insulation materials tends to increase slightly as the temperature difference between the hot side and the cold side of the insulation increases. With cellulose insulation this is exactly what happens. Scientists at Oak Ridge National Laboratory have reported that "R-values [of cellulose insulation] measured under winter conditions increased as the temperature difference across the insulation increased." Based on air permeability, the Oak Ridge scientists have calculated that cellulose insulation will not lose R-value due to convective heat loss at temperatures as low as 40° F below zero. This means that cellulose insulation maintains its resistance to heat transfer under virtually all weather conditions that occur in North America.

Fiber Glass

Maintaining R-value at below freezing temperatures is a problem for light, fluffy blown fiber glass, because of a phenomenon called convective heat loss. At about 32° F air begins to circulate within the insulation. These airflows carry heat through the insulation, reducing its effective R-value, often by a very significant amount. Studies at Oak Ridge National Laboratory showed loss of nearly 15% of R- value at 20° F. At 18° F below zero the insulation had only about 60% of its nominal R-value. A layer of cellulose on top of the fiberglass has been found to be effective at controlling convective heat loss.

Water Vapor Sorption

Moisture is one of the more misunderstood aspects of building shell performance. Different insulation materials exhibit different moisture handling characteristics. These characteristics must be considered in designing insulated assemblies.


No insulation "attracts" moisture, but various materials exhibit different moisture handling characteristics. Cellulose insulation is a "storage layer" in an assembly. This means it can safely hold moisture that might otherwise move into more vulnerable parts of the assembly and still maintain its thermal resistance. Exfiltration of moisture-laden air into walls and ceilings is the major moisture transfer mechanism. The low air permeability of cellulose all but eliminates this means of moisture movement.

Fiber Glass

Glass fibers do not absorb moisture, however, moisture can condense in air spaces within the insulation and migrate to parts of the insulated assembly that may be damaged. Especially vulnerable are bottom plates in walls insulated with batts, which may exhibit the so-called "thatched roof effect" of condensed moisture trickling down the vertically-oriented fibers. Fiber glass is susceptible to air ex filtration, the major cause of moisture migration into walls and ceilings. Vapor retarders, air barriers, and careful caulking and sealing are necessary to prevent moisture buildup in walls in colder climates, adding to the cost of the home.

Fire Safety

"Fire Safety" is a complex matter that can’t be defined by a single factor or material characteristic. It’s a mistake to assume that a "noncombustible" building material is necessarily more "fire safe" than a combustible material. Most building materials are classified as "combustible" and some combustible materials actually offer greater fire protection to building occupants than "noncombustible" materials.


In the United States cellulose insulation must meet the strict flammability standards of the Consumer Products Safety Commission. Fire retardants are applied during the manufacturing process to insure fire safety. Cellulose insulation products routinely qualify for Class I flame spread ratings. Some products are actually approved as flame barriers and fire stops. In several demonstration burns, buildings with cellulose have retained structural integrity significantly longer than buildings with other fiber materials. In one demonstration the ceiling of a fiber glass- insulated building collapsed 22 minutes after fires were ignited. A ceiling with cellulose insulation stayed in place for 70 minutes --an important margin of safety for building occupants and fire fighters. The National Research Council Canada added scientific support to the burn demonstrations with a study that concluded fiber glass reduced the fire resistance of insulated assemblies while cellulose improved fire resistance 22% to 55%. NRCC tested a cellulose insulated floor/ceiling assembly in 1995 and found it to have approximately 50% higher fire resistance than a fiber glass- insulated assembly. The cellulose assembly resisted direct fire exposure about 30 minutes longer than the fiber glass test assembly. A cellulose- insulated wall tested in 1999 by Omega Point Laboratories was found to be 46% to 77% more fire resistant than an uninsulated wall. As a result of this fire resistance, under some fire conditions, cellulose gives building occupants more time to reach safety and fire fighters more time to save the structure.

Fiber Glass

Fiber glass is an inorganic material, and is therefore noncombustible. This does not mean fiber glass provides greater fire safety. The National Fire Academy notes: "It is critical to recall that noncombustible does not mean 'safe.' It certainly does not mean 'fireproof.' The concept of fireresistance goes beyond that of no combustibility. It refers to the capacity of a material or construction to withstand fire or give protection from it." Fiber glass does not measure up to either standard. The open structure of fiber glass makes abundant amounts of oxygen available to wood and other combustible materials in ceilings and walls. Assemblies insulated with fiber glass are much less fire resistant than walls and ceilings insulated with cellulose, as studies by the National Research Council Canada proved. In a paper presented at an ASTM symposium a prominent fire protection expert noted that: "standard fiber glass has an operating temperature of 3500 F. Temperatures above that tend to make it shrink. Before fiber glass loses its physical characteristics, it can contribute to the fire. Generally speaking, fiber glass does not provide adequate protection in a fire. A panel composed partly of fiber glass can lose its physical properties within 10 minutes, depending on the extent of the fire." Facings of fiber glass batts are usually combustible and may accelerate the progress of a fire, especially if batts are inset stapled, a practice that creates miniature "chimneys" lined with paper or petrochemicals with absolutely no fire retardants.

Corrosion Resistance

Corrosion problems associated with insulation are not common, but they can occur given the right circumstances with any insulation.


Federal and industry standards for cellulose insulation specify strict corrosiveness requirements. Many laboratories and other organizations that certify building materials require periodic corrosiveness testing of cellulose insulation as a condition for gaining and maintaining certification or labeling privileges.

Fiber Glass

Fiber Glass is a relatively inert substance that is not inherently corrosive. Some fiber coatings and materials used in batts may be potentially corrosive, but few problems have been reported.


Concerns about mold and indoor air quality have become a hot topic in the building industry. It’s important to understand that insulation alone will not cause mold to grow. Knowing the causes of mold growth is instrumental in preventing the unwanted fungi from becoming a problem in your home.

Cause and effects of mold

Mold spores are surrounding us constantly, these tiny "seeds" are just waiting for the right conditions to grow. Trying to keep mold spores out of your house is virtually impossible. You bring them inside whenever you have been outside. Kids playing outside can be exposed to hundreds of mold spores and bring them in as unwanted houseguests. Firewood, Christmas trees, and gardening mulch can liberate thousands of spores into your home. Open windows in your house are a virtual revolving door for the transfer of mold spores. So, if we can’t control the mold spores from entering our homes then how do we stop them from ever growing?

Mold needs three conditions to support growth. The first is a temperature range of about 47 to 120 degrees F. Unfortunately, almost all areas in your home are within the temperature ranges needed to facilitate mold growth. This would include attics, basements, living rooms, crawl spaces, and most other areas in your home. The second condition mold needs to support growth is a source of nutrients. Some suitable organic materials in your home which could provide nutrients for mold growth include: carpet, fabric, upholstery, paper and paper products, cardboard, ceiling tiles, drywall, insulation, wood, and wood products, dust, paints, and wallpaper. Once again, there is not much you can do to stop mold from having a source of nutrients in you home. The key to stopping mold in your home is to control the third condition that mold needs to support life, moisture. Most cases of unwanted mold in homes can be traced to a moisture problem. Roof leaks, plumbing leaks, condensation, and other forms of moisture will trigger mold growth in as little as a few days. If the moisture problem is not fixed mold can grow at amazing speeds. Relative humidity levels of about 40 percent or


more will allow water vapor to condense on surfaces that have a cooler surface temperatures then the humid air.

The best way to prevent mold from ever growing inside your home is to control the moisture in your home. Always vent dryers, kitchens, bathrooms, and other sources of moisture to the outside of your home. If you ever have a roof leak or plumbing leak, fix it immediately. In most cases, mold may be removed by thorough cleaning with bleach and water.


Cellulose Insulation will not promote the growth of mold. However, mold can grow on just about any substrate if the conditions are just right. Under extreme conditions mold can grow on cellulose insulation, however if that were to occur mold would likely be growing on everything else in sight.

Most problematic moisture (i.e. moisture which is necessary for mold growth) that condenses in wall cavities arrives via air infiltration. Cellulose insulation helps provide reduced air inflitration which lowers your chances for mold. Control moisture and you help to control mold. 

Fiber Glass

The kraft faced backing on fiber glass batts along with the dust that settles among it's strands can provide a source of nutrients for fungi growth. Additionally, lighter density glass allows for air movement and thus moisture migration which can support mold colonies.

Image below is of mold growing on fiberglass duct lining.

             Mold on fiberglass lining

Recycled Content & Environmental Issues

Today recycling and environmental concerns are mainstream issues. Even people who do not identify themselves as "environmentalists" want to make environmentally responsible buying decisions.


Cellulose insulation is an inherently recycled material with approximately 80% recovered content. Most of the recovered content of cellulose insulation is post consumer waste. In addition, cellulose insulation has low embodied energy -- less than 750 BTU/lb by one conservative estimate --which results in less air pollution from the manufacturing process and greater energy efficiency.

Fiber Glass

The recovered content of fiber glass ranges from zero to about 30%. In many cases factory waste accounts for much of the recovered content. Fiber glass is produced in furnaces that burn natural gas and release CO2 and other greenhouse gases into the air. Estimates of the embodied energy of fiber glass range from about 8,500 BTU/lb to more than 12,000 BTU/lb.

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