Fiberglass vs Cellulose Insulation: Which Is Better for Your Home?

Fiberglass and cellulose are the two most affordable insulation materials in the US — and for attic and wall applications, they're the two most common choices. They're close enough in price that the decision should be based on performance, not budget. The real differences show up in R-value per inch, settling behavior, air sealing capability, and fire safety.

Quick Answer: For attic floors, blown-in cellulose edges out fiberglass: higher R-per-inch (R-3.2–3.8 vs R-2.2–2.7), better coverage around obstructions, and modest air sealing properties. For wall retrofits, dense-pack cellulose is the clear winner — it's the only practical drill-and-fill option with meaningful air sealing. Fiberglass wins on settling resistance (1–3% vs ~20% for cellulose), non-combustibility (FSI ≤25/SDI ≤50 vs ≤25/≤450), and lower moisture risk. Both are good options — the best choice depends on your specific application.

Table of Contents

• Master Comparison Table
• R-Value Comparison
• Cost Comparison
• Settling
• Air Sealing
• Fire Safety
• Moisture Behavior
• Environmental Impact
• Wall Retrofits: Cellulose Wins
• Choose Fiberglass When / Choose Cellulose When
• Real Project Cost Comparison
• Key Takeaways
• FAQ

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Master Comparison Table

For complete guides on each material: fiberglass insulation and cellulose insulation.

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R-Value Comparison

Cellulose delivers more R-value per inch than blown-in fiberglass — and this translates to meaningfully less depth required in attic applications.

Cellulose achieves R-49 in approximately 14 inches of settled depth. Blown fiberglass needs about 20 inches — 43% more depth for the same R-value. In attics with limited headroom at the eaves, that difference matters.

Fiberglass batts have a higher R-per-inch (R-3.0–3.7) than blown-in fiberglass because the manufacturing process creates uniform fiber alignment. But batts have a different problem — installation quality. A RESNET Grade III batt installation (gaps, compression around wiring, voids at edges) drops real-world performance by 30%+. Blown-in materials avoid this because they fill around obstructions automatically.

Winner: Cellulose — higher R-per-inch than blown fiberglass and more consistent real-world performance than fiberglass batts. The R-value per inch chart ranks every insulation type.

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Cost Comparison

Fiberglass and cellulose are remarkably close in price for blown-in attic work. The cost difference is small enough that it shouldn't drive your decision.

1,000 sq ft Attic — Installed Cost by R-Value Target (2025–2026)

At R-49, the difference between blown cellulose and blown fiberglass is roughly $100–$200 for a 1,000 sq ft attic. That's negligible. Choose on performance characteristics, not price.

Both materials qualify for DIY installation with a blowing machine (~$100/day rental, often free with 20+ bag purchase at Home Depot or Lowe's). DIY drops the cost to $360–$540 for cellulose or $300–$500 for fiberglass at R-49 coverage.

Detailed pricing at blown-in insulation cost and the insulation cost calculator.

Winner: Tie. Fiberglass batts are cheapest overall, but blown-in cellulose and fiberglass are nearly identical. Choose on performance.

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Settling

This is cellulose's most-discussed weakness — and it's real, but manageable.

Blown-in cellulose settles approximately 20% in the first 2–3 years. Install 17–18 inches to end up with 14 inches (R-49) after settling. Manufacturers account for this in their coverage charts — the bags-per-1,000-sq-ft specifications reflect settled depth. If your installer follows the chart and you verify depth before they leave, settling is already handled.

Blown-in fiberglass settles only 1–3%. Glass fibers are springy and hold their loft. The installed depth is essentially the final depth — no over-fill needed.

Dense-pack cellulose in walls barely settles. At 3.0–3.5 lb/ft³, the material is packed tightly enough that further compression is minimal (2–5%).

Our perspective: Settling is a valid concern, not a dealbreaker. Over-install cellulose per the manufacturer's chart and it hits its target R-value after settling. If the thought of 20% settling bothers you, blown fiberglass eliminates it. But we've insulated hundreds of attics with cellulose and never had a performance complaint from a properly installed job.

Winner: Fiberglass — 1–3% settling vs 20% is a clear advantage in long-term depth retention.

Pro Tip: Measure cellulose depth in 6–8 locations across the attic before the installer leaves. Installed depth should be 20–25% above the settled target printed on the bag chart. If it's not, have them add more material on the spot — it's trivial to add a few inches while the machine is still set up.

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Air Sealing

Dense-pack cellulose provides something fiberglass simply cannot: meaningful air sealing.

Building Science Corporation research shows dense-pack cellulose at 3.0–3.5 lb/ft³ reduces wall air leakage by 30–40% compared to fiberglass batts. The densely packed fibers resist air movement through the cavity — not a true air barrier by code definition, but a significant practical improvement.

Loose-fill cellulose in attics (at 1.5 lb/ft³) provides minimal air sealing. Blown-in fiberglass (at comparable attic densities) provides none. For attic applications, air sealing the floor before insulating is essential regardless of which material you use — the DOE estimates this step alone saves 15–25% on heating and cooling.

Winner: Cellulose — especially dense-pack in walls, where the air sealing advantage is most meaningful.

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Fire Safety

Both materials carry Class A fire ratings, but the underlying performance is different.

Fiberglass is non-combustible. It's spun glass — it melts at 1,300–1,500°F but does not ignite or sustain flame. Flame Spread Index ≤25, Smoke Development Index ≤50. This performance is inherent to the material — no treatment needed.

Cellulose is treated-combustible. It's recycled newspaper — highly flammable without treatment. Borate fire retardants (boric acid + ammonium sulfate) give it a Class A rating with FSI ≤25. But the Smoke Development Index is ≤450 — significantly higher than fiberglass (≤50). In a fire, cellulose produces substantially more smoke.

The borate treatment is effective and permanent under normal dry conditions. The concern: prolonged moisture exposure can leach borates, reducing fire resistance. For fire-critical applications (garage walls, fire-rated assemblies), fiberglass or mineral wool (FSI 0, SDI 0) are safer choices.

Winner: Fiberglass — inherently non-combustible with lower smoke production. For maximum fire safety, mineral wool is better than both.

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Moisture Behavior

Fiberglass doesn't absorb water chemically, but it traps moisture in the air spaces between fibers. When wet, R-value drops significantly — a soaked fiberglass batt may perform at R-1 or less. The good news: once dried, fiberglass recovers its R-value. It won't support mold growth itself (inorganic glass), but trapped moisture promotes mold on adjacent organic materials (wood framing, paper-faced drywall).

Cellulose is hygroscopic — it actively absorbs and releases moisture from the air. In moderate amounts, this can be an advantage: cellulose acts as a moisture buffer in wall assemblies, absorbing excess humidity during damp periods and releasing it when conditions dry out. The risk: prolonged saturation causes cellulose to clump, sag, and potentially promote mold.

Neither material is suitable for perpetually wet locations (crawl spaces, below-grade). For those, closed-cell spray foam or mineral wool are better choices.

Winner: Slight edge to fiberglass — more predictable moisture recovery and lower risk of long-term moisture damage. But neither handles persistent moisture well.

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Environmental Impact

Cellulose wins here convincingly.

Cellulose has approximately 2× the recycled content and requires a fraction of the energy to manufacture. It's biodegradable at end of life. If environmental impact is a priority, cellulose is the better choice. Energy Star's insulation guidance lists both materials as recommended options.

Winner: Cellulose — highest recycled content of any insulation at 80–85%.

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Wall Retrofits: Cellulose Wins

For insulating existing walls without removing drywall, dense-pack cellulose is the standard — and fiberglass isn't a realistic competitor.

Dense-pack cellulose is blown through 2–3 inch holes at 3.0–3.5 lb/ft³ density, filling every gap in the cavity including around wiring, plumbing, and fire stops. It provides R-12 to R-13 in a 2×4 wall plus 30–40% air leakage reduction. Cost: $1.50–$3.00/sq ft installed.

Blown-in fiberglass in wall cavities exists (products like Johns Manville Spider and Knauf JetSpray), but they're less commonly available and don't provide the same air-sealing benefit at standard blown-in densities. Dense-pack fiberglass is a niche product; dense-pack cellulose is the industry default.

For wall retrofit details, our guide on insulating walls without removing drywall covers the complete process.

Winner: Cellulose — dense-pack is the gold standard for wall retrofits, with no practical fiberglass equivalent at scale.

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Choose Fiberglass When / Choose Cellulose When

Choose Fiberglass When:

• Non-combustibility is required — fiberglass won't burn; cellulose relies on treatment
• Minimal settling is important — 1–3% vs 20% for loose-fill cellulose
• Wet-prone areas — fiberglass recovers R-value more predictably after wetting
• New construction walls with skilled installers — fiberglass batts at Grade I installation are cost-effective and perform well
• Budget is extremely tight — fiberglass batts at $0.30–$1.50/sq ft are the cheapest option

Choose Cellulose When:

• Attic blow-in — better R-per-inch (less depth needed), superior coverage around obstructions
• Wall retrofit (drill-and-fill) — dense-pack cellulose is the only practical option with air sealing benefits
• Environmental priority — 80–85% recycled content, lowest manufacturing energy
• Air sealing matters — dense-pack provides 30–40% air leakage reduction in walls
• Irregular cavities with many obstructions — cellulose conforms better than batts

Pro Tip: You don't have to choose just one. We commonly use cellulose in the attic (better coverage) and fiberglass batts in new-construction walls (cost-effective in open cavities with good installers). Different applications, different strengths — use the right material where it matters.

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Real Project Cost Comparison

Scenario: 1,500 sq ft ranch in climate zone 5 with an uninsulated 1,200 sq ft attic and uninsulated 2×4 walls (~1,000 sq ft of exterior wall area).

Note that only cellulose provides a practical wall retrofit option. If walls are already insulated, the comparison is attic-only — and the cost difference between blown cellulose and blown fiberglass is under $200 for a 1,200 sq ft attic.

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Key Takeaways

• For attic blow-in, cellulose has a slight performance edge: R-3.2–3.8/inch vs R-2.2–2.7 (blown fiberglass), better coverage around obstructions, and modest air sealing.
• For wall retrofits, dense-pack cellulose is the clear winner — the only practical drill-and-fill material with meaningful air sealing (30–40% reduction in wall leakage).
• Fiberglass settles 1–3% vs cellulose's ~20% in attics. Both are manageable — cellulose requires 20–25% over-fill.
• Fiberglass is inherently non-combustible (SDI ≤50); cellulose relies on borate treatment (SDI ≤450).
• Cellulose has 80–85% recycled content — the highest of any insulation.
• Cost difference for blown-in attic work is minimal ($100–$200 for a 1,000 sq ft attic). Choose on performance.
• Neither material provides true air sealing in attic applications — always air seal the attic floor first.

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FAQ

Which is better for attic insulation — fiberglass or cellulose?

For blown-in attic floors, cellulose has a slight edge: higher R-per-inch means less depth (14" vs 20" for R-49), it conforms better around wiring and plumbing, and it's comparable in cost. Blown fiberglass settles less (1–3% vs 20%) and is non-combustible. Both are solid choices — the performance difference is smaller than the benefit of doing the project at all. If you want the most R-value in the least depth with the best coverage, choose cellulose. If minimal settling and non-combustibility matter more, choose fiberglass.

Does cellulose insulation really settle 20%?

Yes — loose-fill cellulose in open attics settles approximately 20% in the first 2–3 years. This is a well-documented, predictable property that manufacturers account for in their coverage charts. Install to 20–25% above the settled depth target (17–18 inches for a settled R-49 of 14 inches). Dense-pack cellulose in walls settles much less (2–5%) because the higher density resists compression. Settling is manageable, not a dealbreaker.

Can I blow cellulose over existing fiberglass?

Absolutely — and it's one of the most common attic upgrades. R-values are additive. Blow cellulose directly over existing fiberglass. If the old fiberglass has a kraft vapor retarder facing, slash it (don't remove the old insulation, just cut the facing) so moisture can pass through. Verify the existing insulation is dry and free of mold or pest damage first. The R-value chart shows target values by climate zone.

Is fiberglass or cellulose safer?

Fiberglass is inherently non-combustible but causes skin, eye, and respiratory irritation during installation (wear an N95 mask, gloves, and long sleeves). Cellulose is borate-treated and safe in normal conditions, but it's technically combustible — the borate treatment is what prevents ignition. Both materials are considered safe for residential use when properly installed. For handling comfort during installation, cellulose is less irritating to skin than fiberglass. For fire safety, fiberglass has a better inherent profile. Complete safety info at is fiberglass safe? and our fire safety guide.