Cellulose vs fiberglass attic insulation comes down to one trade-off: cellulose packs a higher R-value per inch and slows air movement, while blown fiberglass weighs less and barely settles. For a typical attic floor, blown cellulose is the slightly stronger pick because it hits code depth in fewer inches and conforms around wiring, but fiberglass wins when ceiling weight, moisture recovery, or a lower price per bag matters more. The right answer depends on your climate zone, what is already up there, and whether you are topping up or starting bare.
Cellulose vs fiberglass attic insulation at a glance
Both are loose blown-in materials rated for open attic floors, but they behave differently once installed. Cellulose is ground recycled paper treated with borate; fiberglass is spun glass fiber. The table below compares the two on the metrics that decide an attic job, using blown-in values since that is how most attic floors get insulated.
| Metric | Blown cellulose | Blown fiberglass |
|---|---|---|
| R-value per inch | R-3.2 to R-3.8 | R-2.2 to R-2.7 |
| Inches to reach R-49 | ~14 in | ~20 in |
| Installed cost (per sq ft) | $0.60 to $2.30 | $0.50 to $2.00 |
| Settling | ~20% (loose-fill, first 2 to 3 years) | 1% to 3% |
| Air resistance | Slows airflow, denser | Minimal, air passes through |
| Fire behavior | Borate-treated, Class A per ASTM E84 | Non-combustible glass |
| Recycled content | 80% to 85% | 40% to 60% |
| Weight on ceiling | Heavier (~1.5 to 2.0 lb/cu ft) | Lighter (~0.5 to 1.0 lb/cu ft) |
| Typical lifespan | 15 to 30 years | 15 to 30 years |
Which has the higher R-value?
Cellulose has the higher R-value per inch, at roughly R-3.2 to R-3.8 compared with R-2.2 to R-2.7 for blown fiberglass. That gap means cellulose reaches the same thermal performance in less depth, which matters in a shallow attic or one crowded with ductwork. Fiberglass batts, a separate product, land higher at R-3.0 to R-3.7 per inch, but batts are not what most people compare against blown cellulose on an attic floor.
The per-inch number is only half the story. Fiberglass loses some effective R-value in very cold weather because air can convect through low-density loose fill, an effect measured below roughly 20 degrees Fahrenheit. Cellulose resists that convective loss because it packs denser and blocks airflow, so its rated R-value holds up better on the coldest nights. See the insulation R-value chart for per-inch figures across every common material.
How the installed cost compares
Installed cost is close enough that price rarely decides the job. Blown fiberglass runs about $0.50 to $2.00 per square foot installed; loose-fill cellulose runs about $0.60 to $2.30. On a 1,000 square foot attic brought to R-49, the material difference often lands within $100 to $200, which is small next to the labor and access cost that both share.
Cellulose needs more bags by weight to hit a target R-value, but each bag is cheap, so the totals stay near each other. Where cost separates is dense-pack cellulose for walls, at $1.50 to $3.00 per square foot, but that is a wall retrofit product, not an attic-floor decision. For a full attic budget, see how the numbers break down in the cost to insulate an attic guide.
Settling: why cellulose loses depth and fiberglass does not
Loose-fill cellulose settles roughly 20% over its first two to three years, so an installer must blow it deeper than the finished target to end at the right R-value. Blown fiberglass settles only 1% to 3%, so what goes down is close to what stays. This is the single biggest install detail people miss, because a cellulose attic blown to a finished 14 inches will drop toward 11 or 12 inches and lose R-value if it was not overfilled.
Reputable cellulose installers account for settling by blowing to a coverage chart that lists installed thickness, settled thickness, and bags per 1,000 square feet. Ask to see that chart and confirm the settled depth, not just the day-one depth. Fiberglass avoids this planning step, which is one reason production crews favor it for speed.
Air sealing, fire safety, and moisture
Neither loose-fill material is an air barrier on its own, but they differ at the edges. Dense-pack cellulose in a wall cavity can cut air leakage 30% to 40%; loose cellulose on an attic floor slows airflow modestly, while loose fiberglass lets air pass through almost freely. Insulation of either type only performs if the attic is air sealed first, so seal top plates, penetrations, and the access hatch before blowing anything. Our attic air sealing guide covers what to seal and in what order.
On fire, fiberglass is non-combustible spun glass. Cellulose is treated with borate to reach Class A flame spread per ASTM E84, and that treatment also deters pests, but it relies on the chemical staying in the fiber. On moisture, fiberglass sheds water and recovers its R-value once it dries; cellulose absorbs moisture, can mat down, and may lose performance if it gets repeatedly wet from a roof leak. Fix any leak before insulating, and lean toward fiberglass in an attic with a history of moisture.
How many inches of each do you need for R-49 or R-60?
Most 2021 IRC and IECC climate zones require R-49 to R-60 on an attic floor, and the two materials reach that at very different depths. At roughly R-3.5 per inch, cellulose needs about 14 inches for R-49 and about 17 inches for R-60. At roughly R-2.5 per inch, blown fiberglass needs about 20 inches for R-49 and about 24 inches for R-60. Confirm your zone requirement before ordering, because it drives the bag count.
- Find your climate zone requirement (commonly R-49 in zones 4 to 5, R-60 in zones 6 to 8).
- Divide the target R by the material’s per-inch value: R-49 divided by 3.5 gives about 14 inches of cellulose.
- For cellulose, add the overfill your installer’s coverage chart specifies so the settled depth still meets code.
- Check that the finished depth clears the top of the ceiling joists, or install rulers to verify.
For the full depth-by-material breakdown, see the R-49 insulation thickness reference.
Can you blow cellulose over existing fiberglass?
Yes, you can blow cellulose on top of existing fiberglass, and it is a common way to top up an under-insulated attic. The new layer adds R-value without removing the old material, and cellulose fills the gaps in loose fiberglass reasonably well. The main caution is weight: cellulose is heavier than fiberglass, so a deep new layer adds load to the ceiling drywall below.
Standard half-inch drywall on framing 24 inches on center is rated for a limited insulation load, so keep total attic weight reasonable and avoid piling many inches of the heavier material over old batts. If the existing fiberglass is wet, moldy, or matted, remove it first rather than burying the problem. When the goal is only to add depth cheaply and the ceiling load is fine, topping fiberglass over fiberglass avoids the weight question entirely.
When to choose cellulose and when to choose fiberglass
Choose cellulose for a standard attic floor where you want the most R-value per inch, better cold-weather performance, and higher recycled content, and where the ceiling can carry the weight. Choose fiberglass for a moisture-prone attic, a ceiling with tight weight limits, or a fast production job where minimal settling simplifies the depth math. Both are proven for 15 to 30 years when kept dry.
- Pick cellulose if: depth is limited, winters are cold, you want fewer inches to code, or recycled content matters.
- Pick fiberglass if: the attic has had moisture, ceiling weight is a concern, or you want the lowest-settling, fastest install.
- Either works if: the attic is dry, air sealed, and deep enough for the required depth.
Read the single-material deep dives for install detail: cellulose attic insulation and fiberglass attic insulation. If you are weighing other materials too, the best insulation for an attic guide compares these against spray foam and mineral wool. For the fundamentals, start at the roofing and insulation learning hub.
Frequently asked questions
Which is better for attic insulation, cellulose or fiberglass?
For a dry, well-sealed attic floor, cellulose edges out fiberglass because it delivers a higher R-value per inch (R-3.2 to R-3.8 versus R-2.2 to R-2.7), reaches code depth in fewer inches, and holds performance better in cold weather. Fiberglass wins in moisture-prone attics or where ceiling weight is limited. Both last 15 to 30 years when kept dry.
Does cellulose insulation really settle 20 percent?
Loose-fill cellulose settles roughly 20% over its first two to three years, which is why installers blow it deeper than the finished target so the settled depth still meets the R-value goal. Blown fiberglass settles only 1% to 3%. Ask your installer to show a coverage chart listing both installed and settled thickness before the job.
Can I blow cellulose over existing fiberglass insulation?
Yes. Adding cellulose on top of existing fiberglass is a common way to boost R-value without removing the old layer. The main limit is weight, since cellulose is heavier and adds load to the ceiling drywall. Remove the old layer first if it is wet, moldy, or matted, and keep total depth reasonable for the ceiling structure.
Is fiberglass or cellulose safer?
Both are safe when installed correctly. Fiberglass is non-combustible spun glass but can irritate skin and lungs, so wear a respirator and cover skin during install. Cellulose is treated with borate to reach Class A flame spread and to resist pests. In an attic with recurring moisture, fiberglass is the safer long-term choice because it recovers R-value after drying, while cellulose can mat.
How many inches of cellulose or fiberglass do I need for R-49?
You need about 14 inches of blown cellulose or about 20 inches of blown fiberglass to reach R-49, based on roughly R-3.5 and R-2.5 per inch respectively. Add overfill for cellulose so the settled depth still meets R-49. Confirm whether your climate zone requires R-49 or R-60 before ordering material.
Reviewed by The Roofing Brief Team. Last reviewed July 2026.