Cellulose insulation is made primarily of recycled paper, mostly old newsprint, treated with fire-retardant chemicals. According to the U.S. Department of Energy, the recycled paper content typically makes up 82% to 85% of the finished product, with the remaining 15% to 18% consisting of chemical additives that protect against fire, mold, and pests.
Recycled Paper: The Core Material
The base of cellulose insulation is post-consumer waste paper. Newsprint is the dominant source, though other paper waste like cardboard, office paper, and shredded documents can also go into the mix. The EPA’s Comprehensive Procurement Guidelines recommend a minimum of 75% post-consumer paper content for cellulose insulation products, and most manufacturers exceed that threshold comfortably.
This high recycled content is one of the main selling points. Rather than sitting in a landfill, old newspapers get a second life as a building material that can last decades inside your walls or attic. The paper fibers themselves are what give cellulose its insulating ability: when shredded and fluffed up, they trap millions of tiny pockets of air that slow the transfer of heat.
Fire Retardants and Protective Chemicals
Shredded paper by itself would be a fire hazard, so manufacturers add chemical treatments that make up roughly 15% of the product by weight. The three most common additives are boric acid, sodium borate (borax), and ammonium sulfate. A study by the National Toxicology Program confirmed that the major chemical components in cellulose insulation, including aluminum, boron, calcium, sodium, and sulfur, trace back to these fire-retardant compounds, specifically aluminum sulfate, boric acid, and sodium sulfate.
These chemicals do more than prevent fire. Boric acid and sodium borate also inhibit mold growth and repel insects and rodents. This triple function makes borate-based treatments especially popular among manufacturers, since a single additive handles fire safety, pest control, and moisture protection all at once. Ammonium sulfate provides additional fire resistance but doesn’t offer the same pest-deterrent benefits.
How It’s Manufactured
The manufacturing process is straightforward compared to many insulation types. Collected waste paper is fed into industrial shredding machines that break it down into small pieces. Those pieces then go through a fiberizing process, often using hammer mills, that tears the paper into fine, fluffy fibers. The finer the fibers, the better the insulation traps air.
Once the paper has been adequately shredded and fiberized, the fire-retardant chemicals are blended in. The finished product is then compressed and bagged for shipping. Because the raw material is waste paper rather than mined minerals or petroleum-based chemicals, the manufacturing process uses significantly less energy than producing fiberglass or foam board insulation.
Loose-Fill, Dense-Pack, and Wet-Spray
Cellulose insulation comes in three main forms, and each involves slightly different materials or application methods.
- Loose-fill is the most common type. It’s blown into attics and open cavities using a machine, settling among joists and rafters. It delivers about R-3.5 per inch of thickness.
- Dense-pack uses the same material but is blown at higher pressure into enclosed wall cavities. The tighter packing pushes the R-value up to about R-4 per inch and creates a more airtight seal around wires, pipes, and other obstructions.
- Wet-spray adds water (and sometimes a small amount of adhesive binder) to the cellulose as it’s blown into open wall cavities. The moisture activates the binder, which causes the cellulose to stick in place and resist settling. Once it dries, it stays put without netting or other supports.
The core composition is the same across all three types. The difference is in how much material is packed into a given space and whether moisture or adhesive is introduced during installation.
Thermal Performance Compared to Other Insulation
Cellulose’s R-value (its resistance to heat flow) falls between R-3.5 and R-4.0 per inch depending on density. That puts it in a similar range to fiberglass batts, which also land between R-3 and R-4 per inch. Where cellulose often outperforms fiberglass is in real-world conditions: because blown cellulose fills gaps and conforms around obstacles, it reduces air leakage more effectively than batts that may leave gaps at edges or around penetrations.
Settling Over Time
One characteristic of cellulose insulation is that it settles after installation. The fluffy fibers gradually compact under their own weight, which reduces the total thickness and lowers the effective R-value. Manufacturers account for this by listing both the initial installed thickness and a minimum settled thickness on product labels. Installers are required to include the settled thickness, coverage area, R-value, and number of bags used on receipts so homeowners have a record of what was put in.
If you’re getting cellulose installed in your attic, the installer will typically blow in extra material to compensate for this expected settling. Dense-pack and wet-spray applications settle much less because the material is either compressed from the start or held in place by a binder.
Dust and Installation Precautions
Cellulose insulation generates fine paper dust during installation. OSHA sets workplace exposure limits for cellulose dust at 10 milligrams per cubic meter for total dust and 5 milligrams per cubic meter for the finer respirable particles that can reach deep into your lungs. For anyone in the room during installation, wearing a dust mask or respirator is a practical necessity. Once the insulation is in place and undisturbed, it doesn’t release significant dust or volatile organic compounds into your living space.
The borate chemicals in the insulation are low in toxicity for humans at the concentrations used, which is part of why they’re preferred over older chemical treatments. The primary concern is with the dust itself during the blowing process, not with ongoing chemical exposure after installation.