The attic is where most homes lose the largest share of their heat, and blown-in cellulose is one of the most effective ways to close that gap. The material starts as recycled newspaper, gets treated with fire-retardant borates, and is blown into place with a hose that fills every gap around framing, wires, and pipes. Homeowners comparing blown-in insulation options usually end up weighing cellulose against fiberglass and spray foam.
This article covers what cellulose is made of, why air sealing has to happen first, how the material is installed, how it performs against fiberglass on the numbers that matter, and how much of it your attic actually needs.
What Cellulose Insulation Is Made Of
Cellulose insulation is shredded recycled newspaper, mostly newsprint and other low-grade paper, processed into a fluffy fiber and treated with fire retardants. The treatment chemicals, typically sodium borate, boric acid, and ammonium sulfate, serve two purposes: they stop the paper from burning, and they make the material unappealing to insects, rodents, and mold.
The Borate Treatment
Boron compounds are the key ingredient. They are the same salts used in pest control and wood preservatives, and they are what let a pile of shredded newspaper sit in an attic for decades without feeding anything. The treatment is bonded into the fiber during manufacturing, so it does not wash out under normal conditions.
Recycled Content
Cellulose typically contains 80 percent or more recycled fiber. That recycled content is the main environmental argument for the material, and it is a strong one: the insulation in your attic is paper that would otherwise have gone to a landfill. The full comparison of fiberglass, cellulose, spray foam, and rigid foam insulation shows where each material wins and where it gives ground.
Air Sealing Comes First
Insulation slows conduction, but it does almost nothing for air movement. A 10-mile-per-hour wind blowing through a crack in the top plate carries heat out of the house whether the attic has R-38 or R-60. Air leakage accounts for 25 to 40 percent of the heat loss in a typical home, which is why the air-sealing work comes before the insulation truck arrives.
Where the Leaks Live
- Top plates where interior walls meet the attic
- Plumbing vents and electrical penetrations
- The attic hatch and pull-down stairs
- Recessed light fixtures that are not IC-rated
- Chimney and flue chases
- Knee wall doors and the floor behind knee walls
Knee walls are a classic failure point in older homes. The short wall between the attic floor and the roof deck hides a cavity that connects the house to the attic, and air sealing attic knee walls is one of the highest-value fixes in a retrofit. Seal the wall’s top plate, the floor behind it, and the door, then insulate the cavity.
Why Sealing Beats Adding Depth
Adding insulation over an unsealed attic is like putting a thicker blanket on a window that is open. The blanket helps at the edges, but the air flow still short-circuits the whole system. Sealing the holes first means the insulation actually sees the temperature difference it is rated for.
Details That Decide Performance
Three details separate a good attic insulation job from a bad one: ventilation, fixture clearance, and insulation stops. Roof ventilation needs a clear path from the soffit vents up to the ridge, so baffles at the eaves keep the insulation from plugging the air channel. Recessed lights need to be IC-rated, meaning insulation contact is allowed, or they need a box built around them. Flues and chimneys need a clearance gap, typically 2 to 3 inches, that no insulation touches.
Ventilation Baffles
A baffle is a plastic or foam channel that holds the insulation back at the eave while keeping the vent path open. Without one, blown cellulose buries the soffit vent and the roof deck above it stays hot and damp, which is the recipe for ice dams in winter and condensation in summer.
Insulation Stops
Insulation stops are the physical barriers that keep loose fill where it belongs: at the eaves, around hatches, and along the edges of walkways. The wall sheathing as an insulation stop is a detail worth stealing: when the top of a wall is sheathed, the sheathing itself can form the stop that keeps attic insulation from falling into the wall cavity.
How Cellulose Is Installed
Blown-in cellulose is installed with a machine, not by hand. A truck-mounted blower feeds the material through a hopper and pushes it through a flexible hose into the attic, where an installer aims the hose into every bay and cavity. Loose-fill attic work uses a light, fluffy application, while wall cavities use dense-pack, where the material is blown in under pressure until the cavity is full.
Settling and Coverage
Loose-fill cellulose settles after installation, typically 10 to 15 percent. Installers compensate by blowing deeper than the target: 15 inches of fresh material settles to about 14 inches, which at R-3.5 per inch works out to roughly R-49. Depth markers on the trusses or a marked stick tell the installer when a bay has enough material.
The Blow-In Sequence
- Seal all air leaks and install baffles at the eaves
- Mark the target depth on the trusses before the crew starts
- Protect the attic hatch area and any walkways with boards
- Blow the material from the far end of the attic toward the hatch
- Cover every bay to the marked depth, including the eaves behind the baffles
- Check fixture clearance and re-level the material in high spots
- Install the hatch cover with its own insulation and weatherstripping
The job is dusty. The fine fibers get into clothing and lungs, so installers wear respirators and eye protection, and homeowners should stay out of the attic until the dust settles. The complete cellulose insulation installation process, including dense-pack wall work, is covered in the dedicated guide at build-construct.
Cellulose vs. Fiberglass: What the Numbers Say
Cellulose and fiberglass are the two most common loose-fill choices, and they perform differently on almost every metric. Cellulose is denser, which makes it a better air blocker and a better sound absorber, but it settles more and absorbs more moisture. Fiberglass is cheaper per bag, lighter, and faster to blow, but it leaves more air paths and does nothing for sound.
| Metric | Cellulose | Fiberglass loose-fill | Fiberglass batts |
|---|---|---|---|
| R-value per inch | 3.5 | 2.5 to 3.0 | 3.2 |
| Air infiltration resistance | Good, dense fiber | Poor to fair | Poor at edges |
| Recycled content | 80 percent or more | Low | Low |
| Sound absorption | Good | Fair | Fair |
| Settling | 10-15 percent | 5-10 percent | None |
| Moisture behavior | Absorbs, treated with borates | Wicks, dries slowly | Holds water between fibers |
The most visible advantage shows up at the framing. A batt ceiling leaves voids above each truss bottom chord, because the batt is cut to the joist spacing and cannot wrap the chord. Blown cellulose surrounds every framing member and utility, which is one of the main benefits of blown-in cellulose that contractors cite when homeowners ask for a recommendation.
Filling the Voids
In a truss roof, the bottom chords sit below the insulation plane. A batt laid between the chords leaves a gap above every chord, and each gap is a straight path for heat to short-circuit the insulation. Loose fill pours over the chords and fills the space completely.
How Much Insulation Your Attic Needs
Building codes in most of the country call for R-38 in new attics, which is about 11 inches of settled cellulose. R-49, roughly 14 inches, is the common upgrade target and the level many energy programs rebate. Going from R-38 to R-49 adds about 30 percent more material for a modest cost, and it is usually the cheapest energy upgrade available per dollar spent.
More is not automatically better. The placement rules matter: too much insulation can be a problem when it buries wiring, covers non-IC fixtures, or blocks the vent path, and the same over-insulation logic applies in walls and roofs where the assembly needs to breathe. Understanding proper insulation placement in roofs and walls prevents the mistakes that turn a good idea into a moisture problem.
The attic is one side of the thermal envelope, and it is the side that pays back fastest. The foundation is the other side, and slab insulation strategies, from perimeter to full under-slab systems, close the loop on the house as a whole. Attics and slabs are the two places where insulation stops the biggest leaks, and blown cellulose is the highest-impact upgrade most attics can receive.