Cellulose vs Fiberglass Insulation: Cost & R-Value

⚖️ Cellulose vs. Fiberglass at a Glance

Prices last reviewed July 2026. Installed insulation cost swings with attic access, how much material a crew actually blows, and regional labor — treat these as wide relative ranges, not quotes. For a size-specific estimate, use the insulation cost guide.

💵 Cost: Roughly a Wash at Equal R

Insulation is only comparable at the same thermal performance, and once you hold R-value constant, the installed cost of blown cellulose and blown fiberglass overlaps almost entirely — roughly $1–$3 per square foot installed for a straightforward attic top-up, with the high end reserved for hard-access, deep-fill, or old-insulation-removal jobs. Fiberglass often has a slightly lower material price; cellulose needs fewer inches. Divide either one by the R it delivers and cost-per-R is essentially a tie. The number that actually moves your bill is installation quality, not the label on the bag.

If you're doing the attic yourself, blown-in is the one genuinely DIY-friendly insulation job: big-box stores rent the blower free with a bag minimum, and cellulose materials for a ~1,000 sq ft attic to R-49 run about $300–$500. Dense-pack walls and spray foam are not DIY — they need a high-pressure rig or two-part chemistry and PPE.

📏 R-Value & Thickness

Cellulose delivers about R-3.2 to R-3.8 per settled inch versus roughly R-2.5 per inch for blown fiberglass, so cellulose hits a target in less depth — about 14 inches for an R-49 attic against 19–20 inches of loose-fill fiberglass. That matters when you're depth-constrained (shallow attics, over a finished ceiling) or want to keep material off the eaves. In a 2×4 wall cavity the reverse quirk applies: high-density fiberglass batts squeeze R-15 into 3.5 inches, while cellulose dense-packs to about R-13 in the same cavity — so for open new-construction walls, batts can edge ahead on paper.

🧯 Three Claims That Are Out of Date

More of this comparison is polluted by stale "facts" than almost any other. Three to retire:

• "Cellulose is a fire hazard." False as stated. Borate-treated cellulose is Class A (ASTM E84, flame spread ≤25) and must pass the federal CPSC smoldering-combustion and critical-radiant-flux tests (16 CFR 1209) before it can be sold. It is organic, so it's combustible — but flame-retardant treated — while fiberglass is non-combustible glass. Both carry Class A ratings.
• "Fiberglass causes cancer." Out of date. The IARC reclassified insulation glass wool from a possible carcinogen to Group 3, "not classifiable" in 2001, and the U.S. National Toxicology Program delisted biosoluble glass wool in 2011 — the cancer warning labels came off. It's a mechanical irritant (itch/dust), not a classified carcinogen.
• "Fiberglass loses half its R-value in the cold." Historically real, largely engineered out. A 1992 Oak Ridge National Lab study found convective looping cut the R of some 1990s cube-cut loose-fill fiberglass by 35–50% at large temperature differences — but it explicitly found no significant convection in batts or in cellulose. Manufacturers re-engineered the nodule size, and modern loose-fill fiberglass holds its rating in the cold. Don't apply a 30-year-old result to today's product.

📉 Settling and the R You Actually Get

Loose-fill cellulose settles roughly 15–20% over the first few years; fiberglass barely settles. This sounds like a strike against cellulose, but the FTC coverage charts (built on ASTM C739) are already written on a settled-thickness basis — blow to the chart depth and you meet the labeled R after it settles. Dense-pack cellulose at ≥3.5 lb/ft³ barely moves at all.

💧 Moisture & 🔇 Sound

• Moisture. Cellulose is hygroscopic — it absorbs moisture and temporarily loses R, but borate-treated cellulose resists mold and pests and recovers when it dries, provided you fix the leak. Sustained wetting clumps it and costs loft. Fiberglass doesn't absorb water and dries readily, but it loses R while wet and won't stop mold from growing on dust/binder. Neither is a fix for a moisture problem — solve the water first.
• Sound. Cellulose wins on mass. A dense-pack 2×4 wall lands around STC 44–46 versus roughly STC 38–40 for R-13 fiberglass batts — a real, audible difference for party walls, home offices, and media rooms.

📋 Code: R-Targets, Clearances & the WUI Myth

Building code is material-neutral — you comply by the R-value you achieve, not the product you pick. Under the 2021 IECC ceiling/attic targets: R-30 in the hottest zones (0–1), R-49 in zones 2–3, and R-60 in zones 4–8 (the 2021 code raised zone 4 from R-49 to R-60). A raised-heel-truss exception lets full-height R-49 over 100% of the ceiling satisfy an R-60 requirement.

• Heat-source clearance (IRC R302.14 / NEC 410.116). Combustible cellulose must stay 3 inches from non-IC recessed lights and other heat-producing devices; IC-rated (Type IC) fixtures may contact insulation. Never bury active knob-and-tube wiring in either material.
• Depth markers & certificate (IRC N1101.10.1.1). Loose-fill jobs need thickness markers at least every 300 sq ft, and the installer certificate must list settled thickness, settled R-value, density, coverage, and bag count.
• Vapor retarder (IRC R702.7). A Class I or II interior retarder is required on frame walls in colder zones (5–8, Marine 4); neither insulation is itself a vapor retarder.

🎯 Which Should You Use?

• Retrofitting closed existing walls → cellulose (dense-pack fills around wires and irregular cavities and cuts air infiltration).
• Depth-constrained attic or the best R-per-inch → cellulose (~14 in to R-49 vs ~19–20 in).
• Sound control (party walls, offices, media rooms) → cellulose (more mass).
• Green / embodied-carbon priority → cellulose (~82–85% recycled).
• Open new-construction walls with a skilled crew → fiberglass batts (fast, cheap, predictable to Grade I).
• Lowest material cost or a low-mess DIY weekend → fiberglass.
• Damp-prone spaces → fiberglass leans better as a non-absorbent material — but fix the moisture source first, regardless of material.
• Either way → air-seal the attic first and blow to the full bag count. That decision outweighs cellulose-vs-fiberglass.