Fiberglass vs Cellulose Insulation: R-Value, Cost & Which to Choose

The Myth That Skews This Decision

Fiberglass has dominated residential insulation for decades — not because it is the best performer, but because it was the first widely manufactured product and the industry built its installation practices around it. The result is that "fiberglass batts" became the default mental model for insulation, and anything else requires justification.

Cellulose suffers from two related perception problems: it sounds low-tech (recycled paper) and homeowners worry about moisture. Both concerns are overblown. The borate fire retardants that allow cellulose to meet fire codes are the same compounds used to treat structural lumber and to fight termites. The material has a 50-year track record in North American homes. According to the Cellulose Insulation Manufacturers Association (CIMA), cellulose has the highest post-consumer recycled content of any insulation product on the market.

That said, fiberglass is not a bad product — it just has a narrower sweet spot than most homeowners realize. Understanding where each material genuinely wins is how you make a correct decision.

R-Value Per Inch: The Thermal Performance Gap

R-value is the core metric — it measures resistance to heat flow, and higher is better. But the form of the product (batts vs blown) matters enormously, and most comparisons conflate them.

Blown fiberglass — the kind used in attics and dense-pack wall applications — delivers R-2.2 to R-2.7 per inch. This is the number that matters for retrofit and attic work. Fiberglass batts (the pre-cut rolls used in open framing) perform better at R-3.2 to R-3.8 per inch, but batts cannot be used in closed walls or retrofitted attics.

Blown cellulose delivers R-3.5 to R-3.8 per inch regardless of whether it is loose-fill in an attic or dense-packed into a wall cavity. That is roughly 40% more thermal resistance per inch than blown fiberglass. To hit R-49 in an attic, you need about 13–14 inches of cellulose versus 18–19 inches of blown fiberglass.

This depth difference matters in attics with low eave heights or knee walls where the available depth is limited. In a 2x4 stud wall (3.5-inch cavity), dense-pack cellulose delivers R-13 to R-14; dense-pack fiberglass in the same cavity delivers R-8 to R-9. That is a dramatic performance gap in the same framing.

Head-to-Head Comparison

What It Actually Costs: Project-Level Numbers

Material cost is only part of the picture. Per Gordian's RSMeans cost data updated October 2025, blown fiberglass runs approximately $0.72 per square foot for materials alone, before labor. Cellulose material costs run slightly higher at $0.85–$1.10 per square foot. Labor is the equalizer: both materials use the same blowing machines and similar crew hours, so the installed cost gap is narrower than the material-only comparison suggests.

According to HomeGuide's 2026 national cost data:

• 1,000 sq ft attic to R-49, blown fiberglass: $1,200–$2,500 installed
• 1,000 sq ft attic to R-49, blown cellulose: $1,800–$3,000 installed
• Dense-pack cellulose, existing 2x4 walls (per linear foot of wall): $8–$20 installed
• Dense-pack fiberglass, existing walls: $7–$16 installed

The cellulose premium is real — roughly 20–30% more on attic projects. But you get about 40% more R-value per inch. If you are limited by attic depth and need to hit R-49 in a tight space, cellulose's density advantage may eliminate the need for added rigid foam board at the eaves, which can cost $2–$5 per square foot additional. Run the numbers for your specific situation before assuming fiberglass is cheaper.

Air Sealing: The Performance Factor Nobody Talks About

Here is where the decision often flips in cellulose's favor, especially in older homes. Insulation R-value only matters if the air stays in the building. A poorly air-sealed attic with R-49 insulation can perform worse than a properly air-sealed attic with R-30.

Blown fiberglass is a fibrous material with open pathways through the fill — air can move through it. Dense-pack cellulose at 3.5 lbs per cubic foot physically blocks airflow through the cavity. According to research published by Oak Ridge National Laboratory (ORNL), dense-pack cellulose in a wall cavity reduces air infiltration by 50–65% compared to an empty cavity, even without additional air-sealing measures. Fiberglass batt or blown fiberglass provides minimal air resistance.

This is the primary reason dense-pack cellulose is the insulation of choice for energy retrofit contractors working on pre-1980 homes. Those homes have no house wrap, minimal air sealing, and drafty wall cavities. Dense-packing them with cellulose addresses thermal performance and infiltration in a single step, without opening the walls.

If you are doing a new-construction build where air sealing is handled separately with house wrap, spray foam at penetrations, and sealed vapor barrier, the air-sealing advantage of cellulose matters less. In that scenario, fiberglass batts in open wall cavities followed by blown fiberglass in the attic is a reasonable and cost-effective approach.

Moisture Behavior: Why the "Wet Paper" Fear Is Misplaced

Both materials absorb moisture. The key difference is what happens afterward.

Fiberglass absorbs water and holds it in the fibers. R-value drops significantly when fiberglass is wet — a study by the Florida Solar Energy Center (FSEC) found that fiberglass batts at 10% moisture content can lose 30–40% of their rated R-value. Wet fiberglass also provides a growth medium for mold if it does not dry quickly.

Cellulose is more hygroscopic — it absorbs moisture more readily — but it also releases it. Because borates inhibit microbial growth and the cellulose fibers themselves are not nutritious to mold, cellulose can get damp and dry out without permanent damage. The CIMA reports that cellulose maintains most of its R-value even at elevated moisture content, unlike fiberglass. That said, cellulose should never be installed over an active moisture problem. Fix the source first.

In a hot-humid climate (zones 1A, 2A, 3A), neither blown fiberglass nor blown cellulose is ideal in walls without a proper vapor management strategy. In those climates, the preferred approach is a flash-and-batt (1–2 inches of closed-cell spray foam against the exterior sheathing plus fiberglass or mineral wool batts filling the remainder of the cavity), or high-density open-cell spray foam. See our guide to all insulation types for the full picture on spray foam options.

Settling: What the Bags Actually Tell You

The settling behavior of cellulose confuses homeowners. Here is exactly how it works: freshly blown cellulose settles 15–20% over the first 12 months as the fibers compact under their own weight. After that first year, settling essentially stops. Professional installers account for this by blowing to the initial depth listed on the product bag for the target settled R-value — bags include charts showing coverage per bag at various depths.

Practically: if your target is R-49 and cellulose settles 15%, installers blow to a depth that will settle to the correct R-49 thickness. You should see a depth mark on your attic access hatch or ruler stick indicating the minimum installed depth. The DOE recommends installing attic rulers (available from your utility or free from some insulation manufacturers) to make future depth verification easy.

Blown fiberglass settles less than 2% over its lifetime — an advantage in situations where long-term depth consistency is critical, such as in dense-pack wall applications where shrinkage could create convective loops.

Which to Choose by Application

Attic with Limited Depth (Eave Height Under 10 Inches)

Winner: Cellulose. When depth is constrained at the eaves or in a shallow attic, cellulose's superior R-per-inch lets you hit code-minimum performance in less space. Blown fiberglass at 18+ inches may not fit near the eaves even with ventilation baffles, forcing you to use rigid foam board as a supplement.

Open Attic with Plenty of Depth

Winner: Either — fiberglass is cheaper. If depth is not a constraint, blown fiberglass gets you to R-49 or R-60 at 15–25% lower installed cost. Air-seal all penetrations with canned foam or a combination of rigid foam and acoustical caulk before blowing either product.

Existing Closed Walls (Retrofit)

Winner: Dense-pack cellulose. This is the most significant advantage. Dense-pack cellulose can be blown into closed wall cavities through 2-inch holes drilled in the exterior sheathing (behind new siding) or in the interior drywall. The high pressure and density ensure the cavity fills completely with minimal voids. Dense-pack fiberglass also exists but is less common and slightly less air-resistant.

Use our insulation calculator to estimate wall area and material quantities for a dense-pack retrofit. For a typical 1,500 sq ft two-story home, expect 2,000–3,000 sq ft of wall area — a significant project.

New Construction Open Wall Framing

Winner: Fiberglass batts or mineral wool batts. In open framing with house wrap providing the air barrier, fiberglass batts are the fastest and cheapest install at $0.50–$1.50 per sq ft. Mineral wool batts (Rockwool) cost more at $1.00–$2.50/sq ft but provide better R-value, higher fire resistance, and superior sound control. Blown cellulose in open new-construction walls is less common because the netting required to hold it adds labor cost.

Soundproofing Interior Walls

Winner: Dense-pack cellulose or mineral wool. Sound transmission class (STC) for a standard 2x4 wall with fiberglass batts runs about STC 36–38. Dense-pack cellulose in the same assembly pushes STC to 44–46. Mineral wool batts (Rockwool Safe'n'Sound) reach STC 52+ in a double-layer assembly. If sound control is the primary goal, mineral wool is the top performer. See our DIY vs contractor guide for advice on tackling insulation retrofits yourself.

DIY vs. Professional Installation

Both blown fiberglass and blown cellulose are viable DIY projects for attic applications. Big-box stores (Home Depot, Lowe's) offer free blowing machine rentals with a minimum material purchase — typically 20–30 bags. The machine is straightforward; the primary skill is maintaining consistent depth and not disturbing existing recessed lights or attic ventilation pathways. Wear an N95 respirator, eye protection, and Tyvek suit; both materials are irritating to skin and respiratory tract during installation.

Dense-pack wall installation requires a high-pressure machine that is not typically available for rental — professional equipment. DIY dense-pack with an attic rental machine is possible but produces inconsistent density and voids. For wall retrofits, hire a certified installer. The BPI (Building Performance Institute) and RESNET both certify insulation contractors; look for BPI certification if your project is part of a comprehensive energy retrofit.

Professional attic installation labor runs $0.25–$0.75 per square foot on top of material costs. For a 1,500 sq ft attic, that is $375–$1,125 in labor — often worth paying for the setup, cleanup, and guaranteed coverage verification.

Federal Tax Credits and Utility Rebates

Both fiberglass and cellulose qualify for the federal Energy Efficient Home Improvement Credit (Section 25C) at 30% of material costs, capped at $1,200 per year for insulation. This credit applies to your primary residence and covers the cost of the insulation materials themselves — not labor. To qualify, the insulation must meet or exceed IECC standards for your climate zone.

Beyond the federal credit, many utilities offer additional rebates. According to the Database of State Incentives for Renewables and Efficiency (DSIRE), rebates for attic insulation upgrades range from $0.10 to $0.35 per square foot of coverage. Some utilities offer flat rebates of $200–$600 for whole-attic insulation upgrades that bring the assembly to or above R-49. Call your utility's energy efficiency department or check their website before starting the project — some programs require pre-approval.

Frequently Asked Questions