By The Roofing Brief Team. Last reviewed: October 2026.
Cellulose vs fiberglass attic insulation comes down to depth versus weight: blown cellulose reaches R-49 at 13.3 inches settled but loads the ceiling with 1.77 lb per sq ft, while blown fiberglass needs 17 to 18.25 inches for R-49 and weighs 0.688 to 0.727 lb per sq ft, per the Greenfiber SANCTUARY, Owens Corning AttiCat and CertainTeed InsulSafe SP coverage charts. Material cost is close: about $1.13 per sq ft for cellulose and $1.01 for fiberglass at R-49 on October 2026 shelf prices. Pick cellulose for a shallow, dry attic over a ceiling rated for the load; pick fiberglass over 1/2-inch drywall on 24-inch framing, in an attic with a moisture history, or where nobody wants to manage a settling allowance.
Cellulose vs fiberglass attic insulation at a glance
Both are loose blown-in materials rated for open attic floors. Cellulose is recycled paper with fire retardant (85% recycled on Greenfiber’s SANCTUARY page); fiberglass is spun glass fiber that Owens Corning lists as non-combustible under ASTM E136. The table uses blown-in values from named coverage charts, since that is how most attic floors get insulated.
| Metric | Blown cellulose | Blown fiberglass | Source |
|---|---|---|---|
| R-value per inch | R-3.7 (SANCTUARY); R-3.68 at R-49 settled | R-2.88 (AttiCat, R-49 at 17 in); R-2.68 (InsulSafe SP, R-49 at 18.25 in) | Greenfiber, Owens Corning and CertainTeed charts |
| Inches to reach R-49 | 14.3 in installed, 13.3 in settled | 17 in (AttiCat); 18.25 in (InsulSafe SP) | Same charts |
| Bags per 1,000 sq ft at R-49 | 70.8 bags of 25 lb | 25 bags of 27.5 lb (AttiCat); 23.5 bags (InsulSafe SP) | Same charts |
| Material cost at R-49 (October 2026) | About $1.13 per sq ft (70.8 x $15.97 at Lowe’s) | About $1.01 per sq ft (25 x $40.34 at Home Depot) | Retail listings; Roofing Brief arithmetic |
| Installed cost at R-49 | $1,100 to $1,900 per 1,000 sq ft in HomeGuide’s blown-in R-value table | HomeGuide, Nov 5, 2025 | |
| Settling | Settled is 90% of installed | None listed (AttiCat: settled equals installed) | Greenfiber; Owens Corning |
| Weight on ceiling at R-49 | 1.77 lb per sq ft (about 1,770 lb per 1,000 sq ft) | 0.688 lb per sq ft (AttiCat); 0.727 (InsulSafe SP) | Same charts |
| Fire behavior | Class 1/A fire resistance (Greenfiber) | Non-combustible per ASTM E136 (AttiCat) | Maker pages |
| Moisture | Absorbs water; dry out or replace after a leak | Does not absorb moisture per ASTM C1104; does not support mold per ASTM C1338 (AttiCat) | Owens Corning AttiCat data sheet |
| Recycled content | 85% (SANCTUARY) | Varies by brand | Greenfiber |
Sources: Greenfiber SANCTUARY coverage chart and SANCTUARY product page; Owens Corning AttiCat data sheet, Pub. No. 10011273-L (May 2026); CertainTeed InsulSafe SP chart (10/25); Home Depot and Lowe’s prices checked October 2026; HomeGuide blown-in insulation cost guide. Per-inch, weight and cost figures are Roofing Brief arithmetic from those documents.
Which to choose: decision table
| If your attic has | Choose | Threshold and source |
|---|---|---|
| 1/2-inch gypsum wallboard ceiling on framing 24 in on center | Fiberglass, or cellulose no deeper than R-44 | GA-216 Table 4 caps overlaid insulation at 1.6 psf; cellulose is 1.77 psf at R-49 and 1.59 psf at R-44 (Greenfiber chart) |
| 5/8-inch gypsum or 1/2-inch sag-resistant ceiling board | Either, up to R-60 | GA-216 cap 2.2 psf; cellulose R-60 is 2.16 psf, AttiCat R-60 is 0.866 psf |
| Under 15 inches of clear depth above the ceiling | Cellulose | R-49 at 14.3 in installed vs 17 to 18.25 in for blown fiberglass |
| A roof leak in the last few years, or condensation stains | Fiberglass, after the leak is fixed | AttiCat does not absorb moisture (ASTM C1104); cellulose absorbs water and can mat |
| Winter design temperature of about -20 F or colder | Cellulose, or fiberglass blown to the maker’s cold-climate table | CertainTeed adds 1-1/2 in of InsulSafe SP at R-49 for -20 to -23 F |
| Climate Zone 2 or 3 (2021 IECC R-49) | Either; compare bids | Material differs by about $120 per 1,000 sq ft at R-49 |
| Climate Zone 4 to 8 (2021 IECC R-60) | Check ceiling first, then either | Cellulose 17.3 in and 2.16 psf; AttiCat 20.5 in and 0.866 psf |
Which has the higher R-value?
Cellulose has the higher R-value per inch: Greenfiber lists SANCTUARY at R-3.7 per inch, and its chart reaches R-49 at 13.3 inches settled (R-3.68 per inch). Open-attic blown fiberglass runs R-2.88 per inch on the AttiCat chart and R-2.68 on the InsulSafe SP chart at R-49. That gap of 3.7 to 4.95 inches at R-49 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. For that comparison, see our guide to blown-in insulation vs batts.
The per-inch number is only half the story. Low-density loose-fill fiberglass can lose effective R-value in very cold weather because air convects through it: in 1992 laboratory tests, Oak Ridge National Laboratory researchers Kenneth Wilkes and Phillip Childs found one loose-fill fiberglass product about 35 to 50 percent lower in R-value at temperature differences of 70 to 76 degrees Fahrenheit than at small differences. Current makers publish the effect: CertainTeed’s Minnesota sheet puts InsulSafe SP’s cold-temperature R-value loss at winter design conditions at 2% at R-49 in Minneapolis and 18% at R-49 in Bemidji, and adds 1/2 to 3-1/2 extra inches at R-49 to hold the label R-value. Cellulose packs denser and slows airflow, so convective loss is less of a concern. 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. HomeGuide’s blown-in guide (November 5, 2025) puts R-49 at $1,100 to $1,900 per 1,000 square feet installed, or $1.10 to $1.90 per square foot, and lists blown cellulose overall at $0.60 to $4.20 per square foot depending on type. In October 2026 retail listings, R-49 material ran about $1.01 per square foot for Owens Corning AttiCat fiberglass ($40.34 per bag at Home Depot, 25 bags per 1,000 sq ft) and about $1.13 for Greenfiber SANCTUARY cellulose ($15.97 per bag at Lowe’s, 70.8 bags per 1,000 sq ft), a difference of about $120 on a 1,000 square foot attic.
Cellulose needs 70.8 bags per 1,000 square feet at R-49 against 25 for AttiCat, so a DIY job means nearly three times as many bags through the machine, at Greenfiber’s stated 2 to 3 minutes per bag. Where cost separates is dense-pack cellulose for walls, which HomeGuide prices at $1.60 to $4.20 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
Greenfiber states that its SANCTUARY R-values are based on settled thickness, which is 90% of installed thickness, so an installer must blow it deeper than the finished target to end at the right R-value: 14.3 inches installed to settle at 13.3 inches for R-49. Owens Corning’s AttiCat chart lists the same figure for minimum and settled thickness, so what goes down is what the chart counts. A cellulose attic blown only to the settled figure of 13.3 inches will settle to about 12 inches, close to the chart’s R-44 depth of 12.0 inches settled.
Greenfiber’s chart requires the installer to meet both the minimum bags per 1,000 square feet (70.8 at R-49) and the minimum installed thickness (14.3 inches), and warns that job conditions can move coverage by as much as 10%. Ask for the empty bag count and the chart before you sign off, and check that depth rulers show the installed figure, not the settled one. Fiberglass avoids the settling step, though CertainTeed’s chart still requires its own minimum of 23.5 bags per 1,000 square feet at R-49.
Air sealing, fire safety, and moisture
Neither loose-fill material is an air barrier on its own, but they differ at the edges. Greenfiber says SANCTUARY forms “a dense barrier that reduces air infiltration”; loose fiberglass at about 0.5 lb per cubic foot (0.688 lb per sq ft over 17 inches on the AttiCat chart) lets air pass more freely, which is why CertainTeed publishes a cold-climate depth table. 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, and keep insulation 3 inches from non-IC recessed lights as the InsulSafe SP sheet requires. Our attic air sealing guide covers what to seal and in what order.
On fire, Owens Corning lists AttiCat as non-combustible under ASTM E136. Greenfiber rates SANCTUARY Class 1/A with fire retardants it calls permanent. On moisture, AttiCat does not absorb moisture under ASTM C1104 and does not support mold under ASTM C1338, so it recovers 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?
The 2021 IECC sets R-49 for ceilings in Climate Zones 2 and 3 and R-60 in Zones 4 through 8 (Delaware DNREC summary of 2021 IECC changes), and the two materials reach those values at very different depths. On Greenfiber’s SANCTUARY chart, cellulose is blown to 14.3 inches for R-49 and 17.3 inches for R-60 (settling to 13.3 and 16.1 inches). Blown fiberglass needs 17 inches (AttiCat) to 18.25 inches (InsulSafe SP) for R-49 and 20.5 to 21.75 inches for R-60. Confirm your zone requirement before ordering, because it drives the bag count. For bag counts and bag weight by R-value on the most common fiberglass product, see our AttiCat bag and weight comparison.
| Target | SANCTUARY cellulose: installed / settled, bags, lb per sq ft | AttiCat fiberglass: depth, bags, lb per sq ft | InsulSafe SP fiberglass: depth, bags, lb per sq ft | Source |
|---|---|---|---|---|
| R-38 | 11.2 / 10.4 in, 52.6 bags, 1.32 | 13.5 in, 19 bags, 0.523 | 14.5 in, 17.9 bags, 0.556 | Maker coverage charts |
| R-49 | 14.3 / 13.3 in, 70.8 bags, 1.77 | 17 in, 25 bags, 0.688 | 18.25 in, 23.5 bags, 0.727 | Maker coverage charts |
| R-60 | 17.3 / 16.1 in, 86.0 bags, 2.16 | 20.5 in, 31.5 bags, 0.866 | 21.75 in, 28.9 bags, 0.897 | Maker coverage charts |
- Find your climate zone requirement (the 2021 IECC sets R-49 in zones 2 and 3 and R-60 in zones 4 through 8; the ENERGY STAR retrofit table uses the same targets for an uninsulated attic).
- Read the minimum thickness and minimum bags per 1,000 square feet for that R-value off the bag’s coverage chart, rather than dividing by a per-inch figure.
- For cellulose, blow to the chart’s initial installed thickness 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; Greenfiber says SANCTUARY “can be easily installed over existing insulation.” The new layer adds R-value without removing the old material, and cellulose fills the gaps in loose fiberglass. The caution is weight: topping 13.5 inches of R-38 AttiCat (0.523 lb per sq ft) up to roughly R-60 with about R-22 of cellulose (0.66 lb per sq ft on the Greenfiber chart) totals about 1.18 lb per sq ft, while the same top-up in fiberglass stays under 1 lb.
The Gypsum Association’s GA-216 (2010), Table 4 caps overlaid unsupported insulation at 1.6 psf for single-layer 1/2-inch gypsum wallboard on ceiling framing 24 inches on center, and at 2.2 psf for 5/8-inch board or 1/2-inch ceiling board. Add the pounds per square foot of the old and new layers from the bag charts and stay under your board’s limit. 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 tight, 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 R-49 in 14.3 inches instead of 17 to 18.25, better cold-weather performance, and 85% recycled content, and where the ceiling is rated for 1.77 psf or more. Choose fiberglass for a moisture-prone attic, a 1/2-inch drywall ceiling on 24-inch framing (1.6 psf GA-216 cap), or a fast production job where 25 bags per 1,000 sq ft beats 70.8.
- Pick cellulose if: depth is under 15 inches, winter design temperatures fall to about -20 F, you want fewer inches to code, or recycled content matters.
- Pick fiberglass if: the attic has had moisture, the ceiling is 1/2-inch wallboard on 24-inch framing, or you want an install with no settling allowance on the AttiCat chart.
- 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, and the roofing and insulation learning hub covers the fundamentals.
Frequently asked questions
Which is better for attic insulation, cellulose or fiberglass?
For a dry, well-sealed attic floor over a ceiling rated for the load, cellulose edges out fiberglass: R-3.7 per inch on Greenfiber’s SANCTUARY versus R-2.68 to R-2.88 for blown fiberglass, so R-49 takes 14.3 inches instead of 17 to 18.25. Fiberglass wins in moisture-prone attics and over 1/2-inch drywall on 24-inch framing, where cellulose at R-49 (1.77 psf) exceeds the 1.6 psf GA-216 limit.
Does cellulose insulation really settle 20 percent?
Not on current manufacturer data. Greenfiber states that SANCTUARY’s settled thickness is 90% of installed thickness, about a 10% drop (14.3 inches installed to 13.3 settled at R-49), which is why installers blow it deeper than the finished target. Owens Corning’s AttiCat fiberglass chart lists the same installed and settled thickness. Ask your installer to show a coverage chart listing both before the job.
Can I blow cellulose over existing fiberglass insulation?
Yes. Greenfiber says SANCTUARY can be installed over existing insulation. Add the weight of both layers from the bag charts (cellulose is 1.77 psf at R-49, AttiCat 0.688) and keep the total under your ceiling’s GA-216 limit: 1.6 psf for 1/2-inch wallboard on 24-inch framing, 2.2 psf for 5/8-inch or ceiling board. Remove the old layer first if it is wet, moldy, or matted.
Is fiberglass or cellulose safer?
Both meet code when installed per the bag chart. AttiCat fiberglass is non-combustible under ASTM E136 but can irritate skin and lungs, so wear a respirator and cover skin during install. SANCTUARY cellulose carries a Class 1/A fire rating from fire retardants. In an attic with recurring moisture, fiberglass is the safer long-term choice because it does not absorb moisture (ASTM C1104), while cellulose can mat.
How many inches of cellulose or fiberglass do I need for R-49?
You need about 14.3 inches of blown cellulose (settling to 13.3 inches) or 17 to 18.25 inches of blown fiberglass to reach R-49, per current Greenfiber, Owens Corning and CertainTeed coverage charts. Blow cellulose to the chart’s installed depth so the settled depth still meets R-49. Confirm whether your climate zone requires R-49 or R-60 before ordering material.