Cellulose insulation is made primarily from recycled paper products, mostly old newspapers. According to the U.S. Department of Energy, it contains 82% to 85% recycled paper content by weight, making it one of the most recycled building materials available. The remaining 15% to 18% consists of chemical additives that make the paper fire-resistant, mold-proof, and unappealing to pests.
Recycled Newsprint: The Core Material
The base of cellulose insulation is post-consumer newsprint, though other paper products like cardboard and office paper sometimes make it into the mix. At a manufacturing facility, the paper is shredded into small fibers using industrial hammer mills until it reaches a fluffy, cotton-like consistency. This fiberized paper traps air in tiny pockets, which is what actually slows heat transfer through your walls and attic.
The finished product delivers an R-value of 3.2 to 3.8 per inch, meaning a 10-inch layer in your attic provides roughly R-32 to R-38. That puts it in a similar performance range to fiberglass batts, but with a significantly lower embodied energy. Fiberglass requires melting raw materials at extremely high temperatures and spinning them into fibers. Cellulose, by contrast, takes paper that would otherwise sit in a landfill and mechanically shreds it, a far less energy-intensive process.
Fire Retardant Chemicals
Shredded newspaper burns easily on its own, so manufacturers treat it with a blend of chemicals that make the finished product resistant to flames and smoldering. The most common additives are boric acid, borax (sodium borate), and ammonium sulfate. Most manufacturers use a combination of borates and ammonium sulfate, and some add small quantities of phosphate compounds.
Each of these chemicals works a bit differently. Boric acid and borax decompose when exposed to heat, forming a glassy protective layer on the surface of the fibers that blocks oxygen from reaching the paper. This starves the fire while the decomposition process absorbs heat, cooling the material. Ammonium sulfate takes a different approach: it releases non-flammable gases when heated, diluting the oxygen around the material and promoting charring that acts as a further barrier.
The industry standard for cellulose insulation, ASTM C739, requires every product to pass tests for critical radiant flux (resistance to ignition from a heat source) and smoldering combustion before it can be sold. These aren’t optional benchmarks. They’re built into the specification for any cellulose insulation used in homes and commercial buildings.
Mold, Insect, and Rodent Resistance
The same borates that provide fire protection pull double duty as pest and mold deterrents. Boric acid is toxic to insects when ingested, so cellulose-treated wall cavities are inhospitable to termites, carpenter ants, and other wood-boring insects. Rodents also tend to avoid borate-treated material. On the biological side, borates inhibit fungal growth, which means cellulose insulation resists mold even in moderately damp conditions. The ASTM standard specifically requires fungi resistance testing before a product qualifies for sale.
This is a meaningful advantage over untreated fiberglass, which doesn’t deter pests or mold on its own. That said, no insulation is a substitute for proper moisture management. If your attic or wall cavity has a persistent water intrusion problem, the insulation type matters less than fixing the leak.
How It Gets Into Your Walls and Attic
Cellulose insulation comes in two main forms: loose-fill and dense-pack. Loose-fill is blown into open attic spaces using a machine, settling into gaps and irregular framing better than pre-cut batts. Dense-pack cellulose is blown into enclosed wall cavities at higher pressure, packing the fibers tightly enough that they resist settling over time.
There’s also a wet-spray method used in new construction, where water is added to the cellulose as it’s sprayed into open wall cavities. As it dries, the material stiffens and locks in place, making it highly resistant to settling. For any application, proper density at installation is the key factor in long-term performance. Cellulose that’s blown too loosely in an attic can settle over the years, reducing its effective R-value. A properly installed dense-pack wall, by contrast, holds its position for decades.
Dust and Health Considerations During Installation
Blowing cellulose insulation generates a significant amount of airborne dust. A study by the National Toxicology Program found that the respirable particles (the smallest ones that reach deep into your lungs) don’t actually contain much cellulose fiber. Instead, they’re composed mainly of the fire retardant chemicals, particularly aluminum sulfate, boric acid, and sodium sulfate, along with trace amounts of clay.
Workers who install cellulose regularly report nasal symptoms (35%), eye irritation (35%), and morning phlegm production (25%). The eye and mucous membrane irritation appears to be linked to the boric acid content. The good news is that the NTP study found little evidence of serious lower respiratory conditions tied to cellulose exposure. In animal testing, cellulose dust caused a mild inflammatory response in the lungs, but it was nonprogressive, meaning it didn’t worsen over time.
For homeowners, the practical takeaway is straightforward. If you’re doing a DIY attic blow-in, wear a proper dust mask or respirator, safety goggles, and long sleeves. Once installed, cellulose sits behind drywall or under attic flooring and doesn’t release fibers into your living space under normal conditions. The dust concern is almost entirely limited to the installation process itself.
What Sets It Apart From Other Insulation
Cellulose insulation’s composition gives it a few distinct characteristics worth knowing about. Its high recycled content (82% to 85%) makes it the clear winner on sustainability metrics. Fiberglass, even brands using some recycled glass, requires far more energy to manufacture. Spray foam, made from petrochemicals, has an even larger carbon footprint.
The density of cellulose also makes it better at reducing air leakage than fiberglass batts, which can leave gaps at seams and around obstacles like wiring and plumbing. Dense-pack cellulose in a wall cavity essentially fills every void, creating a more uniform thermal barrier. It also has modest sound-dampening properties, since the tightly packed fibers absorb vibration more effectively than lighter, airier materials.
The main trade-off is moisture. Cellulose absorbs and holds water more readily than fiberglass or foam. In a well-detailed wall or attic assembly with proper vapor management, this isn’t a problem. But in a poorly sealed space with chronic moisture issues, wet cellulose can lose its insulating value and, despite its mold-resistant treatment, eventually degrade. The material works best when paired with good building practices: air sealing, vapor barriers where climate demands them, and adequate ventilation.