Cellulose insulation is a loose-fill insulation made primarily from recycled paper, usually shredded newsprint, and treated with fire-retardant chemicals. Installers blow it into attics, wall cavities, floors and other hard-to-reach areas to slow heat transfer and improve indoor comfort. Its thermal resistance is commonly about R-3 to R-3.7 per inch, depending on the product and installation method.
Cellulose insulation is common in retrofit projects because blown-in fibers can fill irregular spaces and enclosed wall cavities more completely than poorly fitted batts.
Cellulose Insulation at a Glance
| Feature | Cellulose insulation |
|---|---|
| Main material | Recycled paper or wood-based fiber |
| Typical form | Loose-fill, blown-in fiber |
| Common applications | Attic floors, enclosed walls, floors and difficult-to-fill cavities |
| Approximate R-value | About R-3 to R-3.7 per inch, depending on product and installed density |
| Fire performance | Treated with fire retardants; it is not fireproof |
| Main advantage | Fills gaps and irregular spaces while using substantial recycled content |
| Main limitation | Must be protected from persistent moisture and installed with proper clearances |
How Is Cellulose Insulation Made?
Cellulose insulation is made by shredding and milling recovered paper into small fibers. Manufacturers then treat the fibers with chemicals that improve fire resistance and help protect against fungal growth and pests. Common treatments include boric acid, borax and ammonium sulfate.
The recycled content varies by product. A U.S. Department of Energy insulation guide describes cellulose products as containing at least 70% recycled paper. EPA purchasing guidance has also recommended high post-consumer recycled-paper content for cellulose insulation. Check the product documentation if recycled content is important to your project.
How Is Cellulose Insulation Installed?
Installers use three main cellulose installation methods: loose-fill, dense-pack and damp-spray.
Loose-Fill Cellulose
Loose-fill cellulose is blown across an unfinished attic floor with pneumatic equipment. The fibers form a blanket around joists, wiring and other obstructions.
This method is suitable for:
- Unfinished attic floors
- Irregularly shaped areas
- Hard-to-reach spaces
- Adding insulation over existing attic insulation
ENERGY STAR identifies cellulose as a suitable material for unfinished attic floors and enclosed existing wall cavities.
Dense-Pack Cellulose
Dense-pack cellulose is blown under pressure into enclosed wall or roof cavities. The higher installed density helps limit gaps and settling compared with lightly installed loose-fill material.
Dense-pack cellulose can fill existing walls during a retrofit through small access holes. It is not, however, a complete air-sealing system. Building Science Corporation notes that dense-packed assemblies are not automatically airtight. Cracks, penetrations and other air leaks still need to be sealed separately.
Damp-Spray Cellulose
Damp-spray cellulose is applied to open wall cavities with a small amount of moisture or adhesive. The binder helps the material adhere to the cavity and resist settling before the wall is closed.
The wall assembly must dry properly before drywall or other finishes are installed.
How Well Does Cellulose Insulation Work?
Cellulose insulation slows heat movement by trapping air between its fibers. Its thermal performance is commonly about R-3 to R-3.7 per inch, although the exact value depends on the product, installation method and installed density.
Twelve inches of cellulose could provide roughly R-36 to R-44 before accounting for framing, compression, gaps and other parts of the building assembly. The final result depends on the entire wall or attic system, not insulation depth alone.
Cellulose can also improve acoustic comfort because its relatively dense fiber structure helps reduce sound transmission through walls and floors.
What Are the Advantages of Cellulose Insulation?
Cellulose insulation has several practical benefits, especially in retrofit work.
High Recycled Content
Cellulose commonly uses recovered paper that might otherwise enter the waste stream. Its environmental impact also depends on manufacturing, transport, installation, building performance and end-of-life options. EPA identifies recycled content, R-value, embodied energy and disposal options as factors to consider when comparing insulation products.
Good Coverage in Irregular Spaces
Blown cellulose can surround wiring, pipes and framing irregularities. This makes it useful in older homes where wall cavities are difficult to fit with standard batts.
Useful for Retrofit Work
Contractors can often install dense-pack cellulose through small access holes in existing walls. This can avoid removing all interior or exterior wall finishes.
Sound Control
Cellulose can reduce sound transmission between rooms when installed at sufficient density, particularly in wall and floor cavities.
Suitable for Many Attic Floors
Loose-fill cellulose is commonly used on attic floors. Installers can distribute it across large areas and build up the depth needed for the target R-value.
What Are the Disadvantages and Risks?
Cellulose insulation must be kept dry, installed with the required clearances and distributed at the correct depth and density.
Cellulose Is Not Waterproof
Cellulose fibers can absorb moisture. Persistent leaks, condensation or high humidity can reduce thermal performance and contribute to damage or mold in surrounding building materials. Correct moisture problems before installing cellulose.
Cellulose insulation is not a substitute for:
- Roof repairs
- Flashing
- Vapor-control design
- Proper attic ventilation where required
- Air sealing
- Drainage and weatherproofing
Fire-Retardant Treatment Does Not Make It Fireproof
Cellulose is treated to improve fire resistance, but it remains a combustible fiber product. Install it according to the product instructions and keep it away from chimneys, flues, recessed lights and other heat sources. Required clearances and protective measures still apply.
Installation Produces Airborne Dust
Blowing cellulose produces airborne fibers and dust. Professional installers should use appropriate respiratory and eye protection. Occupants should stay out of the work area until installation and cleanup are complete.
Poor Installation Can Reduce Performance
Insufficient depth, uneven coverage, compression and gaps around obstructions can reduce the insulation's actual performance. For attic work, the installer should document the installed depth and target R-value rather than report only the number of bags used.
Cellulose vs. Fiberglass Insulation
| Difference | Cellulose | Fiberglass |
|---|---|---|
| Material | Recycled paper fiber | Glass fibers |
| Common forms | Blown-in, dense-pack and damp-spray | Batts and blown-in |
| Irregular cavities | Usually fills them well when properly installed | Can leave gaps if batts are poorly fitted |
| Typical R-value | About R-3 to R-3.7 per inch | Commonly about R-3 to R-3.5 per inch |
| Moisture concern | Can absorb water and lose performance when wet | Also requires protection from bulk water |
| Recycled content | Often high | Varies by product |
| Best use | Retrofit walls, attic floors and irregular spaces | Standard framed cavities and accessible areas |
The better material depends on the building assembly and installation quality. Air sealing, moisture control and complete coverage usually matter more than choosing cellulose or fiberglass based only on the material name.
Is Cellulose Insulation a Good Choice?
Cellulose insulation is a good choice for attic upgrades, existing wall retrofits, irregular cavities and projects where recycled content and sound control matter. It is especially practical when a contractor can dense-pack existing walls or blow insulation across an accessible attic floor.
Choose another insulation system when the project requires:
- Very high R-value in limited thickness
- Continuous exterior insulation
- Direct exposure to water
- Specialized high-temperature performance
- A complete air barrier from the insulation alone
Before installation, confirm the target R-value, air-sealing plan, moisture conditions, electrical and flue clearances, and the installer's method for verifying coverage.