The word cellulose comes from the French word for a living cellule and glucose, which is sugar. Insulation is low-thermal-conductivity material used to separate the internal climate and sounds of a building from the external climate and sounds. Cellulose insulation is plant fiber used in wall and roof cavities to separate the inside and outside of the building thermally and acoustically.
Cellulose is the oldest building insulation material. Many types of cellulosic materials have been used, including newspaper, cardboard, cotton, straw, sawdust, hemp, and corncob. Monticello was insulated with a form of cellulose. Modern cellulose insulation, made out of recycled newspaper by using grinding and dust-removing machines and adding a fire retardant, began to be used in the 1950s and came into general use in the United States during the 1970s.
The market for insulation increased following the oil embargo of 1973–1974. The embargo caused energy costs for heating to skyrocket across the nation, leading to increased interest in energy conservation measures. Insulation gained significant national attention as a cheap and available technology to increase the energy efficiency of homes. In 1977, following a particularly severe winter, a tax credit was given for homeowners who installed insulation.
While in 1976 there were roughly 100 cellulose insulation firms with 125 plants, by 1978 there were more than 350 firms with more than 500 plants. Cellulose insulation was produced locally by small manufacturers who purchased ready-to-operate machines and offered a cheap and easy low-tech production process. In spite of some constraints created by a shortage of boric acid for use as fire retardant, cellulose captured an increased share of the market due to lower costs and its suitability for retrofits. Meanwhile fiberglass and rockwool producers found it difficult to keep up with the demand for insulation from their customers.
Due to complaints by retailers, contractors, and consumers about price, safety, and quality control problems, the federal government began enacting insulation standards beginning in 1978. There was a great concern that the growth in cellulose manufacturers was leading to improper or insufficient treatiment of insulation against the threat of fire, even though reliable national statistics did not exist. This concer led to the Federal Consumer Products Safety Commission (CPSC) passing 16 CFR Part 1209, which sets safety standards covering four product attributes for cellulose insulation only: settled density, corrosiveness, critical radiant flux, and smoldering combustion. Another regulation passed was the R-value Rule, which places clear limitations on the claims that manufacturing and marketing firms can make about their product.
The effect of regulations by the CPSC put most of the small producers of cellulose insulation out of business. The costs incurred by increasing fire testing made cellulose more expensive, and the bad publicity helped decrease demand. The smaller producers were either unable to meet the testing requirements or they merged with other small manufacturers. In 1985, after further studies, the CPSC asked Congress to repeal the flammability standard. By 1991 only 61 cellulose producers remained in the US.
The fiberglass industry meanwhile benefited from most of the regulations passed by the federal government. The heavy lobbying by the more centralized fiberglass and mineral insulation manufacturers helped pass the tough fire standards for cellulose insulation. These standards were reinforced by technical bulletins published by the Mineral Insulation Manufacturers Association (currently known as the North American Insulation Manufacturers Association), which promoted fire hazard claims against cellulose insulation. These claims were not independently verified, faced little scientific review, and were misleading and untrue.
Currently cellulose insulation has increased again in use in the US. Part of the reason for this growth is that some study results have suggested that cellulose may actually protect a building from damage in a fire better than fiberglass because cellulose is denser than fiberglass and doesn’t allow the oxygen necessary to burn structural members. Several National Research Council Canada (NRCC) studies have backed these claims. Another major reason for the comeback of cellulose might be the increased interest in green building. Cellulose has the highest recycled content of any insulation material and also has less embodied energy than fiberglass and other furnace-produced mineral insulations.
Products
Four major types of loose-fill cellulose products have been developed under a variety of brand names. These are generally characterized as dry cellulose, spray-applied cellulose, stabilized cellulose, and low-dust cellulose. These types are used in different parts of a building and for different reasons.
Dry Cellulose (Loose Fill)
For use in retrofitting old homes, dry (or loose-fill) cellulose is blown into holes drilled into the tops of the walls. It can also be blown into a new wall construction by using temporary retainers or netting that is clamped in place then removed once the cellulose has reached the appropriate density. This form of application does settle as much as 20%, but the stated R-value of the cellulose is accurate after settling occurs. In addition, a dense-pack option can be used to reduce settling and further minimize air gaps. Dense-pack places pressure on the cavity, and should be done by an experienced installer.
Spray-Applied Cellulose (Wet-Spray)
Spray-applied cellulose is used for applying cellulose to new wall construction. It is ismilar to dry cellulose, but water is added during spraying. In some cases the installation process might also include mixing in a very small percentage of adhesive or activating a dry adhesive present in the cellulose. Wet-spray allows application without the need for a temporary retainer. In addition, wet-spray allows for an even better seal of the insulated cavity against air infiltration and eliminates settling problems. Wet-spray installation requires that the wall be allowed to dry for a minimum of 24 hours (or until a maximum of 25% moisture is reached) before the insulation is covered.
Stabilized Cellulose
Stabilized cellulose is used most often in attic and roof insulation. It is applied with a very small amount of water to activate an adhesive of some kind. The process reduces settling and decreases the amount of cellulose needed. This type of application can prove advantageous in that it reduces the overall weight of the product on the ceiling drywall, thereby helping to prevent sagging. The application is ideal for sloped roofs and has been approved for 5:12 (41.66%) slopes.
Low-Dust Cellulose
The last major type of cellulose insulation on the market is the low-dust variety. Nuisance levels of dust are created during application of most types of dry insulation, causing the need for simple dust masks to be worn during installation.The low-dust type of cellulose has a small percentage of oil or similar dust-dampener added. The low-dust variety may be particularly appropriate for homes that harbor people who are sensitive to newsprint or paper dust (though new dust will not be created after installation).
Advantages
Thermal Performance
The thermal performance of loose-fill cellulose compares favorably to other types of insulation. The thermal conductivity of loose-fill cellulose is approximately 40 mW/(K*m) (an R-value of 3.8 per inch), which is about the same as or slightly better than that of glass wool or rock wool. The thermal conductivity rating does not represent the whole picture of thermal performance, however. Other important aspects incude how well the building envelope is sealed from air infiltration, convective airflows, and thermal bridging.
Cellulose is very good at fitting around items in walls like pipes and wiring, leaving few air pockets that can reduce the overall efficiency of the wall. It also seals walls from air infiltration while providing the density to limit convection. The University of Colorado School of Architecture and Planning did a study that compared two seemingly identical test structures, one with cellulose and the other with fiberglass. The cellulose structure had used 26.4% less energy to heat. It also was shown to tighten the structure more than 30%. Subsequent real world surveys have shown cellulose performing 20–30% better than fiberglass at reducing energy used for heating.
Compared to foam insulation, cellulose has a lower R-value per inch, but is much less expensive; foam has a higher cost per equivalent R-value.
Sound Insulation
Noise reduction is achieved in three ways with cellulose. First, cellulose completely fills cavities, leaving few air pockets for sound to travel in. Second, the cellulose materials have the ability to trap air. Third, and most significantly, cellulose is approximately three times denser than fiberglass. The density of the material helps deaden the sound traveling through walls and between floor levels.
Several installation options allow walls to have a Sound Transmission Class (STC) of 50 or greater. As a comparison, walls with fiberglass batts have an STC of 36–39, depending on stud and screw spacing.
Mold and Pest Control
The borates in cellulose insulation provide superior control against mold, insects, and pests such as rodents. Some studies have shown that even improper installation and several months of water saturation did not result in mold.
Fire Retardation
The borate treatment also provides cellulose with the highest (Class I) fire safety rating. This is an advantage over foams and fiberglass. In addition, fire will not cause toxic fumes.
Vapor Barrier
A vapor barrier may not be needed with cellulose insulation. For example, recent studies have shown that air movement is the primary method by which excessive moisture can accumulate in a mild marine climate, such as that of Portland, Oregon. An insulation that fills the wall cavity completely (such as cellulose or foam) can help prevent moisture problems. Recommendations against using vapor barriers with cellulose insulation are supported by studies, even though they classify cellulose as vapor permeable.
In addition, cellulose acts to distribute moisture throughout the cavity, preventing the buildup of moisture in one area and helping to dry the moisture more quickly. Cellulose manufacturers do not recommend the installation of a vapor barrier with cellulose.
Most city codes will require a vapor barrier for any external wall. Most US cities will consider an appeal of the requirement if proper reasoning is provided. In March 2008 the city of Portland, Oregon, approved an appeal to waive the requirement for a vapor barrier/retarder when using cellulose insulation. The appeal can be viewed in the Portland Bureau of Development Services search form by searching for appeal ID 4996. Fundamental to any appeal is mentioning that recent studies show air movement is the primary problem for vapor, that cellulose is an effective barrier to air movement, and that cellulose acts to diffuse vapor.
Disadvantages
Cellulose has few disadvantages. As compared to other insulation options, the R-Value of 3.6 to 3.8 is very good but not the best (though it competes well in cost per R-Value). Spray foam has many of the same benefits as wet-spray cellulose (such as sealing the cavity), while having an advantage in R-Value and rigidity (however, spray foam has undesirable environmental qualities).
Installation Expertise and Building Codes
In some areas it can be difficult to locate installers who are experienced with cellulose. An experienced installer understands how to correctly dense-pack loose-fill dry cellulose, how to best apply stabilized (partly wet) cellulose on sloped surfaces, and how long to wait for wet-spray cellulose to dry.
As with other non-batt insulation, city and regional/state building codes may not be updated for cellulose insulation. Homeowners should call the city to verify that the insulation will be approved, and it may be necessary to provide product specifications to the city. The installer and the manufacturer should both be willing to handle this process, saving the homeowner any true effort.
Most building codes require a vapor barrier, and it can be hard to convince some cities that one is not required for cellulose. In this case, an appeal (usually a quick process) may be necessary. Otherwise, vapor barriers can be installed. Options for a vapor barrier include plastic sheeting (fairly low environmental footprint due to the thin layer needed) or PVA paint (contains toxins and other qualities undesirable from an environmental standpoint).
Wet-Spray Drying Time
Wet-spray provides the advantage of a better sealing of the insulated cavity and superior rigidity. However, the moisture from a wet-spray insulation requires a longer drying time before the drywall or sheet-rock can be applied. This drying time is usually reduced by the use of large space heaters, which are run for a few days to weeks, depending on ambient humidity. The installer should use a moisture meter to verify acceptable in-wall moisture levels before any drywall is applied. From an environmental and energy efficiency perspective, wet-spray is well worth the extra heating and time.
Termite Infestation
One drawback to cellulose insulation may be the attractiveness of the primary ingredient to insects. Both subterranean termites and drywood termites thrive on cellulose. The insulation, especially if used on lower floors, acts as an attractant to termites. Termites can detect simple cellulose, such as cellulose insulation, from hundreds of feet away. Their preferred food source is cellulose with the least resistance to consumption (paper, for example). Insulated areas should be checked regularly for signs of infestation such as mud tunnels behind walls and between rafters.
However, almost all present-day cellulose insulation contains a borate treatment. The borate will not attract termites or other insects and has been shown to be unsuitable as a food source for termites.
Sustainable and Environmentally Preferable Properties
Cellulose insulation is growing in popularity, in part because it is one of the most environmentally friendly types of insulation available.
Recycled Content
Cellulose is composed of 75—85% recycled paper fiber, usually post-consumer waste newsprint. The other 15% is a fire retardant such as boric acid or ammonium sulphate. Cellulose has the highest recycled content of any insulation available. By contrast, fiberglass has a maximum of about 30% recycled content.
Low-Toxicity and Environmental Impact of Raw Materials
The nonrecycled components of cellulose insulation are still environmentally preferable to the raw materials of most other insulation types, which are often petrochemical-based (this includes foam and fiberglass). Unlike foam insulations, many of which use HFC or HCFC blowing agents that have global warming potential hundreds or thousands of times higher than that of carbon dioxide, cellulose does not produce significant gaseous emissions. Unlike fiberglass, cellulose does not use formaldehyde-based glues, which present a continuing hazard after installation due to off-gassing of formaldehyde.
Cellulose has great advantages for industrial health and worker safety. Toxicity of the raw materials of most insulation types is typically highest during manufacture or installation. Neither is an issue with cellulose.
The sole hazard for cellulose is categorization by OSHA as a dust nuisance, requiring a simple dust mask during installation. This compares very favorably to the potential NIOSH cancer risk of fiberglass.
Embodied Energy
The embodied energy of cellulose is lower than that of all other types of insulation. It requires 20 to 40 times as much energy to produce furnace-made insulation materials than to produce cellulose insulation. Cellulose is made by electrically powered machines, while mineral insulation is made in furnaces. Cellulose is also made with locally available materials (except for the fire retardant), while mineral insulation factories have to ship materials and products over greater distances.
Cellulose insulation does use borates, which are mined in California and Turkey and are a limited resource. However, the manufacture of cellulose insulation uses only small quantities of borates, and borax mining employs one of the cleanest mining techniques available. From a mining and natural resource perspective, cellulose is preferable to foam and fiberglass insulation.
Green Value of Insulation
All insulation helps make buildings more energy efficient. Using cellulose insulation can contribute to obtaining LEED credits in the US Green Building Council certification program. It can earn credit in two categories: the energy and atmosphere energy performance category and the materials and resources recycled content category.
Product Safety
Cellulose insulation can be very dusty during installation, and it is recommended that a standard dust mask be worn while working. The fire retardant boric acid is about as dangerous as table salt. There is slight concern over the offgassing of ink from the newspapers, but the material is sealed behind walls, and no studies have shown offgassing to be an issue.
