Feature Article

Cellulose Insulation: An In-Depth Look at the Pros and Cons

Should we be recommending cellulose to our clients? If so, on what basis? If not, why—what are its drawbacks?

Cellulose insulation has been the darling of the green building movement because of its recycled content, low embodied energy, low-tech processing, and excellent energy conservation performance. But concerns are also raised about health risks for cellulose installers and occupants of cellulose-insulated buildings. In fact, some healthy home advocates strongly discourage its use. Should we be recommending cellulose to our clients? If so, on what basis? If not, why—what are its drawbacks? EBN took an in-depth look at these questions, and we report here on our findings.



Cellulose insulation is a fairly simple material produced from one of our biggest solid-waste products: newspaper. The 70 to 80 cellulose insulation manufacturers in North America purchase old newspaper that has been collected through recycling programs. There are two stages of production, according to Doug Leuthold of Advanced Fiber Technology, a company that makes cellulose insulation manufacturing equipment. In the first, the newspaper is chopped into pieces from one to several inches in diameter. The second stage (finishing mill) differs considerably from one manufacturer to another. The oldest and most common finishing mill is a hammer mill in which swinging metal plates attached to a rotating shaft beat the newspaper in a chamber until the pieces are small enough to fall through a screen. The maximum size of cellulose particles produced in the hammer mill is typically 38” to 14”. Disk refiners are a little different; the chopped newspaper is shredded as it falls between two rotating plates. A third, newer, process is known as fiberization. Instead of cutting the paper, fiber­ization actually disaggregates it back into individual fibers. In the fiber­ization process developed by AFT, high-pressure air is used to “blow particles apart,” says Leuthold. “You end up with a fluffy product that looks like the padding in disposable diapers, except that it’s gray.” Fiberization produces lower-density cellulose with a number of advantages.

Once the newspaper has been adequately shredded, chemicals are added to provide fire-retarding and mold-inhibiting properties to the insulation. The most common chemicals used today are boric acid, sodium borate (borax), and ammonium sulfate. Boric acid and sodium borate have the advantage of not only providing fire retardancy, but also adding mold, insect, and rodent resistance to the insulation. Aluminum sulfate has been used in the past as a fire retardant, but is rarely if ever used today, according to Daniel Lea of the Cellulose Insulation Manufacturers Association, or CIMA. Most manufacturers use a mixture of borates and ammonium sulfate, and some add small quantities of several phosphates. According to Dave Yarbrough at Oak Ridge National Laboratory, there is a trend in the industry to replace some of the borate with ammonium sulfate, because the latter is less expensive. After the insulation is mixed with fire-retardant chemicals—and in some products dry binders—it is bagged and shipped to building supply outlets or installers.



Installation of NatureGuard® cellulose insulation

Photo courtesy Louisiana-Pacific Corp.
There are a number of ways to install cellulose insulation. The oldest and simplest use is loose-fill cellulose in attics. The insulation is blown or poured into the attic space where it provides about R-3.7 per inch. Dry-blown cellulose is also installed in walls as a retrofit insulation material. Holes are typically drilled through the exterior sheathing after removing several sections of siding, and the cellulose is blown in.

Wet-spray cellulose, as the name implies, has water added during installation to make it stick when blown into wall cavities (binders are sometimes used as well). Conventional wet-spray cellulose using a hammermill product is usually installed quite wet—sometimes with more than 100% water on a “dry-weight” basis (weight of water divided by weight of dry cellulose), or about four gallons of water per 30-pound bag. By using fiberized cellulose instead of hammermill cellulose, the water content can be significantly reduced. American Environmental Products (AEP), a Virginia cellulose manufacturer, has gotten the moisture content down to about 28% (dry-weight basis), according to Ivan Sandau of AEP. The company offers cellulose either with or without a dry binder. The binder, described as an organic product approved as an agricultural food additive, is activated upon contact with water.

Another relatively new formulation of cellulose insulation, referred to as

stabilized cellulose, is used in attics. This product has a binder in it and is applied with a small quantity of water. The binder prevents settling, which may otherwise reduce the installed thickness of loose-fill cellulose insulation by as much as 25%. American Environmental Products, one of several manufacturers producing stabilized cellulose, achieves a 1.3 lb./ft3 density with its stabilized attic insulation.

Two other approaches used for walls do not require water. In the dense-pack process, cellulose is blown into closed wall cavities at a relatively high density of 3 to 312 lbs./ft3. Because of the high density, settling does not occur. With the other approach, installers use forms to blow dry cellulose into open wall cavities. The forms, which are propped against the inner side of stud bays, hold the insulation in place as it is installed, and the insulation stays in place after forms are removed and until the inner wall surface is installed.


A Market for Recycled Newspaper


Markets for recycled newspaper, 1992

Sources: American Forest and Paper Association and CIMA
North America produces roughly 13 million tons of newspaper each year—about 100 pounds per person. Fifty-five percent of this is currently recycled (1992) according to the American Forest and Paper Association; the rest accounts for about 4.6% of municipal solid waste. While these statistics are a big improvement over ten years ago when newspaper accounted for 8% of our municipal solid waste, we still landfill or incinerate a huge amount.

There are various ways old newspaper can be recycled. The biggest use, turning it back into new newspaper, requires significant processing (de-inking and bleaching, for example), which takes a lot of energy and produces contaminated water that must be treated. Recycling old newspaper into cellulose insulation is much simpler: de-inking is not required, there is no pollution generated in the process, and very little energy is used.

According to CIMA, 414,000 tons of cellulose insulation were produced in 1990. Because the finished product is approximately 80% recycled newspaper by weight, this means that cellulose insulation currently provides a use for roughly 330,000 tons of old newspaper. This represents 4.6% of total recycled newspaper (see pie chart). On a volume basis, EBN estimates that cellulose currently has about 10% of the fiber insulation market (fiberglass, mineral wool, cellulose). Increasing the market share of cellulose insulation—and thus increasing the use of recycled newspaper—will further strengthen markets for recycled newspaper, which will improve the economic viability of recycling programs. (Today, municipalities often have to pay to get rid of the newspaper they collect through recycling because the markets for recycled newspaper are so weak.)


Limited Boron Supplies?

While the primary ingredient in cellulose insulation is in bountiful supply, the second-most-used ingredient may not be. In North America, borax crude was first mined in Death Valley, California, in 1883, using 20-mule-team wagons (made famous by the television program "Death Valley Days"). A far larger borate ore deposit was discovered in 1925 in the Mojave Desert near Boron, California, and production today is about 10,000 tons per day from an open-pit mine. Since then, no additional borate reserves have been discovered. A source in the company estimated that reserves will last only about 50 years at today’s consumption level. U.S. Borax currently accounts for about 50% of world borax production; the only other major source is in Turkey.


Low Embodied Energy

Because cellulose insulation production is such a simple, low-tech process, manufacturing plants can be small and widely scattered. That means transportation energy can be kept low. By contrast, fiberglass manufacturing is far more complex and requires heavy capitalization and centralized production (i.e., longer shipping distance).


Comparing the Embodied Energy of Cellulose and

Other Insulation Materials

Comparing the Embodied Energy of Cellulose and Other Insulation Materials

Embodied energy figures (other than cellulose) from a 1991 study by Franklin Associates for the Society of the Plastics Industry

Embodied energy figures (other than cellulose) from a 1991 study by Franklin Associates for the Society of the Plastics Industry

The only estimates of embodied energy in cellulose insulation EBN found were from Canadian manufacturer Therm-O-Comfort Co., Ltd. In a 1991 letter to the Canadian Standard Association, Therm-O-Comfort estimated primary energy consumption for cellulose insulation to be 85 kWh/ton (145 Btu/lb.). This figure does not account for the fire-retardant chemicals or transportation energy. Relative to most industrial processes, borax mining and refining is a “low-energy” process. Even with a conservative (high) estimate for fire retardants, embodied energy for cellulose insulation would be only about 750 Btu/lb. This figure is less than one-seventh the energy used to produce fiberglass, and one-30th that required for polystyrene (see table).


Energy Efficiency

In addition to the environmental benefits relating to resource use and embodied energy, cellulose insulation also has performance advantages over most other fiber insulation materials. Cellulose insulation has a higher R-value than standard fiberglass insulation, though high-density fiberglass can provide higher R-value than cellulose. With attic applications, loose-fill cellulose also blocks air convection within the insulation—a process that can significantly reduce the effective R-value of loose-fill fiberglass in very cold regions (see below). Settling, however, is more of a concern with loose-fill (non-stabilized) cellulose than with fiberglass insulation. In wall applications, wet-spray cellulose fills around wires and pipes, sealing the cavity more effectively than fiberglass batts, and settling does not occur. In fact, because of the very good air barrier provided by wet-spray cellulose, many installers suggest that a polyethylene vapor barrier is not required.

Unlike with fiberglass, the R-value of cellulose insulation does not increase as the density increases. Loose-fill cellulose in attics, at a typical density of about 1.5 lbs./ft3 insulates to between 3.6 and 3.8 per inch according to Dave Yarbrough at Oak Ridge National Laboratory. At higher densities, such as wet-spray wall applications (typically 2-3 lbs./ft3), the R-value is slightly lower: 3.5 to 3.6 per inch. With fiberglass, the R-value increases at higher densities up to about R-4 per inch.

In side-by-side tests conducted at the University of Colorado School of Architecture and Planning, two identical test buildings were built on insulated platforms. Cellulose insulation increased the air tightness by 74% over the uninsulated building, while the fiberglass insulation increased air tightness by 41% (neither building had a vapor barrier). The heating tests showed that the cellulose-insulated building used 26% less energy than the fiberglass-insulated building. Because these were short-term tests, it is not known whether settling of the loose-fill cellulose insulation over time would affect its energy performance.


Comparisons between loose-fill cellulose and loose-fill fiberglass attic insulation in very cold conditions have also been made both by the University of Illinois and by Oak Ridge National Laboratory. Both studies showed that at very cold temperatures loose-fill fiberglass loses up to 50% of its R-value, while loose-fill cellulose and fiberglass batt insulation do not. It is worth noting, however, that the extra heating costs from convection in loose-fill fiberglass in even an extreme North Dakota climate will only increase annual heating costs by about 2.4¢/ft2 of attic at an R-19 insulation level and 1.4¢/ft2 at an R-38 insulation level.


Health Concerns

Just how safe cellulose insulation is for installers and homeowners has been the subject of considerable controversy in the past few years. Some healthy house proponents argue that the chemicals found in the insulation and the cellulose fibers themselves are harmful and potentially even carcinogenic. Let’s take a look at these chemicals.

Cellulose insulation is typically about 20% fire-retardant chemicals by weight. The most commonly used fire retardants in cellulose insulation are boric acid, sodium borate, and ammonium sulfate. Informational materials published by Owens Corning Fiberglas claims that borates and boric acid are “known to cause reproductive disorders in rats. Medical authorities also warn that borates and boric acid may cause toxic poisoning in humans if absorbed through a cut or scrape. Ingesting as little as 1/8 ounce of these chemicals can be fatal to infants.” An article in the British Journal of Industrial Medicine (February 1993) by J. M. G. Davis goes into greater detail about health concerns:

“Boric acid itself is a toxic material and can be lethal to humans when ingested in gram quantities. It is not considered that the inhalation of cellulose insulation dust could approach this lethal toxicity but the heavily impregnated respirable cellulose dust will liberate the readily soluble boric acid in significant amounts in lung tissue. Symptoms of sublethal toxicity to boric acid include abdominal pain, liver, kidney, and lung dysfunction and severe exfoliative dermatitis.”

A front-page article in

Energy Design Update (June 1993), however, brings into question the validity of Davis’s article by noting that he is “under retainer to fiberglass and rockwool manufacturers and that in fact ‘one of those groups’ suggested he write the journal article.” A letter from Davis is quoted in the article as follows: “I would certainly emphasize that I do not feel that most cellulose fiber manufacturing or products present any hazard at all, if only because most will not liberate respirable fibers in significant amounts. Even in the most questionable case of shredded paper insulation materials, where the chemicals I listed [boron fire retardants] do occur, I fully acknowledge that there is no definite evidence that sufficient [amounts] can be inhaled to be harmful” (bracketed items from EDU article).

Concerns have also been raised about ink residues in cellulose insulation. There are two basic types of ink used in newspaper printing, according to Don Hensel, Environmental Manager at the Newspaper Association of America (NAA). Black inks have traditionally been a mixture of 60-80% petroleum-based mineral oil, 15-20% pure carbon black, and small amounts of other additives. In recent years, soy-based inks have gained in popularity. With these inks, oil made from soy beans replaces the mineral oil; the carbon black is still the same. The primary concern with inks has always been colored inks, which were traditionally made with toxic heavy metals such as cadmium and lead. According to Patricia Penza of the General Printing Ink Division of Sun Chemical, however, colored pigments weren’t used in the news industry until about ten years ago. By that time, she said, the health hazards of lead and other heavy metals were well known and NAA (then the American Newspaper Publishers Association) banned their use in newspaper inks.

Dr. John Comerford at the College of Agricultural Sciences at Penn State University has studied the safety of using shredded newspaper as animal bedding. He found no detectable levels of 16 different hydrocarbons in blood and liver samples. Of the metals cadmium, copper, lead, and mercury, only copper was found in measurable levels. Copper levels were “well below the toxic level” and higher levels were found in a control group of cattle that were bedded on sawdust. As for the newspaper bedding itself (shredded newspaper), cadmium, copper, and lead were found at levels less than 1100 the levels permitted in feed.

Insulation expert David Yarbrough of Oak Ridge National Laboratory dismisses most of the concern surrounding toxicity of cellulose. “I think that’s a major red herring,” he said. “The installer in an attic certainly is exposed to quite a bit of dust, …[but] as far as occupants are concerned, to me it’s a nonsense issue.” He noted that these health concerns were originally prompted by asbestos and that the concerns bled over into mineral fibers. The mineral insulation industry, “in a logical counter argument, said you ought to look at the cellulostic fibers and the chemicals and dust and all that’s associated with it,” added Yarbrough.

EBN’s recommendations on the health concerns raised with cellulose are as follows: 1) don’t eat it, 2) installers should always wear proper respiratory protection, and 3) as with other fiber insulation materials, it should be installed with a continuous air-tight barrier between the insulation and the living area (i.e., in most of the country, that means installation of polyethylene vapor barriers on walls and ceilings).


Fire Safety Concerns

Potential combustibility of cellulose insulation has long been an issue of concern, given the inherent combustibility of its primary raw material. As mentioned, various chemicals are added to provide fire-retardant properties. The biggest concern with these chemicals—all of which are readily water-soluble—is that they might leach out or somehow dissipate from the insulation over time. Concern has been particularly great with borates.

The North American Insulation Manufacturers Association (NAIMA), a trade association representing the fiberglass and mineral wool industries, has publicized the results of several reports showing that the fire resistance of cellulose insulation drops over time. Specifically, NAIMA cites a study by an association member of 24 loose-fill cellulose insulation samples from attics in six states that had been in place for at least two years. “Of the 19 samples tested for critical radiant flux [a value that influences the ability of flame to spread across insulation], 10 failed to meet the ASTM C 739 criterion.” Even more damaging is an ongoing study by the California Bureau of Home Furnishings and Thermal Insulation (CBHF) to measure the long-term performance of cellulose insulation relative to flammability. Over three years of testing, boric acid and borax levels were found to drop and the insulation samples “failed to meet the critical radiant flux requirements of ASTM C 739,” according to the NAIMA report.

The CBHF does not seem overly concerned about these results, however. In an October 1992 letter to a cellulose insulation manufacturer (provided to EBN by CIMA), the chief of the CBHF said that “…the long term aging studies show the samples maintain their ability to pass the smoldering test normally administered on new materials. We consider the smoldering test a much more important test than the critical radiant panel in predicting in-field fire performance…. We have not received a significant number of reports from California fire departments indicating that insulation materials constitute a fire hazard or major consequence.”

Several studies by researchers at Tennessee Technological University (TTU) and Allied Signal Corporation provide evidence that the fire-retardant chemicals do not disappear from cellulose insulation except at much higher temperatures than would commonly be found in attics. The most thorough and widely quoted study was done by David Yarbrough of ORNL and N. Chiou of TTU and published in

Energy and Buildings in 1990. Enough vibration to simulate 672 years of use in an attic was found to cause no measurable settling of boric acid or borax in test samples. As for evaporation (sublimation) of boric acid from cellulose, the study found that at very high temperatures (90°C or 194°F) and 100% relative humidity, the loss of boric acid was significant, but the loss was negligible when the temperature is lower 70°C (158°F), even at 100% humidity and air exchange rates of 2.0 attic changes per hour. “It appears that it would take 300 years or more at 70°C, 100% relative humidity, and air exchange rates from 1.0 to 2.0 attic volumes per hour to lose enough boric acid to significantly affect the combustion tests,” according to the article’s authors. Separate studies of ammonium sulfate by David Yarbrough and Allied Signal Corporation reached similar conclusions: that loss was not significant except at very high temperatures.

EBN considers the potential loss of fire-retardant chemicals to be the most significant concern relating to cellulose insulation. Further research on this concern is clearly needed, but the apparent lack of building fires in which cellulose insulation has been implicated gives us confidence that cellulose insulation is safe enough for use.


Disposal of Cellulose Insulation

When the useful life of any insulation material comes to an end—i.e., when the building is demolished or the insulation removed during renovation—what happens to it? Cellulose is not readily reusable as an insulation material. Even if it weren’t so messy, given the concerns about fire-retardant chemicals with new cellulose insulation, the acceptability of fire retardants in old cellulose should be highly suspect. In most cases, the material is either landfilled or incinerated. If landfilling is the disposal method, cellulose insulation does pretty well because of its inherent biodegradability. As the cellulose decomposes, however, the borate and ammonium sulfate fire-retardant chemicals will remain. In older landfills, these water-soluble chemicals, dissolved in rainwater (leachate), can permeate through the underlying soils. According to a Material Safety Data Sheet from U.S. Borax, “although boron is an essential micronutrient for healthy growth of plants, it can be harmful to boron-sensitive plants in higher levels,” and it is toxic to some fish at levels of 1 mg per liter. Overall, the toxicity of the borates in cellulose insulation is low enough that cellulose insulation is not considered a hazardous material even in California, which has the nation’s most stringent standards. “You can dispose of it in any landfill,” according to Jerry Pepper, manager of environmental affairs at U.S. Borax.


Corrosion Concerns

If ammonium sulfate gets wet or thermally decomposes, it can produce sulfuric acid, which is corrosive to metals. There have been anecdotal reports of copper pipes and steel truss fasteners in attics corroding when in contact with cellulose insulation that has gotten wet. With the rising popularity of wet-spray cellulose for wall applications, the issue of corrosivity is particularly significant. Many wet-spray cellulose installers specify material treated only with boric acid and borax to eliminate concern about corrosion. According to David Yarbrough, however, all cellulose insulation must pass corrosivity tests, and if properly installed, any commercially available cellulose should be all right. To improve resistance to corrosion, some manufacturers may add corrosion inhibitors.


Does Wet-Spray Cellulose Insulation Dry Out?

With wet-spray cellulose insulation concern is often expressed that moisture from the insulation might remain trapped in the wall cavity and result in rotting or mildew growth. The concern is greatest in situations where there is an effective vapor barrier on both sides of the wall cavity—for example, when foam sheathing or plywood is used on the exterior and a polyethylene vapor barrier is used on the interior. There have been a number of horror stories, such as a public housing project in New England where in one building, even after 112 years, the insulation moisture content was found to be 30-60% (Energy Design Update, July 1989). In this case, it appears that the insulation was installed very wet (up to 200% moisture on a dry-weight basis—five to six gallons of water per 30-pound bag), and the wall system impeded drying (interior polyethylene vapor barrier and meticulously installed extruded polystyrene on the exterior). An experimental study of different wall systems by the Canada Mortgage and Housing Corporation in Newfoundland (1986 and 1987) found that even after two years, wet-spray cellulose had not dried and the moisture level in studs was about 60% (Energy Design Update, November 1987).

A study in Calgary, Alberta, on the other hand (where the climate is drier), showed that wet-spray cellulose dries out quite well. One wall of a test house was configured to test for the effect of poly vapor barriers and the tightness of the exterior sheathing. Even the wall section with a poly vapor barrier on the interior and well-sealed plywood sheathing on the exterior dried to acceptably low moisture levels within 120 days (Energy Design Update, October 1989).

Clearly there is cause for concern in humid climates. Wet-spray cellulose should only be used in situations where adequate provision has been made for drying of the insulation. Joe Lstiburek, of Building Science Corporation, who co-authored the

Moisture Control Handbook for the U.S. Department of Energy, says that when using wet-spray cellulose the wall must have adequate drying potential. In northern climates, it must be able to dry to the exterior. That means a moisture-permeable exterior sheathing such as one-by lumber, asphalt-impregnated fiberboard, or an exterior gypsum board. In warm climates where there will be central air conditioners operating during the summer, the wall can be designed to dry to the interior by leaving out the interior vapor barrier, says Lstiburek.

If vapor retarders are to be used on both sides of wet-spray insulation, provision needs to be made for the insulation to dry out before the wall system is closed in. Depending on the climate conditions, low-water-content wet-spray cellulose insulation can dry at a rate of up to about one inch of depth per day, according to Ivan Sandau of American Environmental Products.

Along with allowing wet-spray insulation to dry, keeping it dry is also very important. Cellulose can absorb moisture. If soaked, it can compress. Even if only moistened, its R-value will drop, reducing energy performance, and the resultant humidity can permit mold growth or rotting of wood framing members. Proper detailing to prevent migration of water or water vapor into the insulation—from either the inside or outside—should be followed.


A Boost From


Cellulose insulation is receiving a big image boost from Louisiana-Pacific on two accounts. First, L-P is the first company to market cellulose insulation nationally; its Nature Guard™ product is produced at four plants around the country and actively promoted through advertising campaigns and at trade shows. Second, at the end of July, L-P launched a program to guarantee low heating bills for homes insulated with their cellulose. Under the Snug Home program, a home must be tested for air tightness, and the leakage ratio must be between 2 and 3 (leakage ratio is defined as the Effective Leakage Area, divided by the square footage of the building envelope, divided by 100). If the home meets the air tightness requirement, L-P reviews the plans and determines the expected heating load, adds in a comfort margin, and provides a written warranty listing the maximum yearly heating costs for a period of three years. L-P will pay any difference in heating cost. While this program only covers Nature Guard, publicity about the program will increase awareness of cellulose insulation across the board, helping out all producers.


The Bottom Line

After thorough review, we at EBN have concluded that properly installed cellulose insulation is acceptable from a health standpoint. We found no significant risk of indoor air quality problems, combustion, or moisture damage if appropriate installation procedures are followed. The most significant concern—apparent loss of certain fire-retardant properties—calls for additional research but does not appear so significant as to suggest a moratorium on use. Given its environmental advantages over most other insulation materials (low energy production, high recycled material content, and biodegradability), EBN believes that cellulose insulation should be a preferred insulation material for environmentally concerned builders and designers. This is not to say that cellulose is the perfect insulation material; it is not. Care must be taken in its use to ensure proper performance and a long, safe operating life. A checklist for cellulose installation follows.


Alex Wilson

For more information:

Advanced Fiber Technology

4710-L Interstate Dr.

Cincinnati, OH 45246


American Forest and Paper Association

1250 Connecticut Ave., N.W., Second Floor

Washington, DC 20036


Cellulose Insulation Manufacturers Association

136 S. Keowee St.

Dayton, OH 45402


Newspaper Association of America\

11600 Sunrise Valley Dr.

Reston, VA 22091


North American Insulation Manufacturers Association

44 Canal Center Plaza, Suite 301

Alexandria, VA 22314


Therm-O-Comfort Co., Ltd.

85 Forest St.

Aylmer, Ontario N5H 1A5


U.S. Borax Inc.

26877 Tourney Rd.

Valencia, CA 91355


805/287-5455 (fax)



Published September 1, 1993

(1993, September 1). Cellulose Insulation: An In-Depth Look at the Pros and Cons. Retrieved from https://www.buildinggreen.com/feature/cellulose-insulation-depth-look-pros-and-cons

Keep exploring our featured topic:

Letting Floodplains Do Their Job

Read more