Blown-In Insulation: Fiberglass vs Cellulose, Costs & DIY Guide

Quick Answer: Blown-in insulation comes in two types: cellulose (R-3.2–3.8 per inch, $0.60–$2.30/sq ft installed) and fiberglass (R-2.2–2.7 per inch loose-fill, $0.50–$2.00/sq ft installed). Cellulose delivers more R-value per inch, flows better around obstructions, and provides partial air sealing. Blown fiberglass settles significantly less (1–3% vs ~20%). Both are excellent for attic floors; dense-pack cellulose is the standard for wall retrofits.

Table of Contents

• Fiberglass vs Cellulose: The Core Decision
• Attic Application
• Wall Application (Dense-Pack)
• Coverage Charts
• Cost Breakdown
• DIY Blown-In: What to Expect
• Settling: The Real Story
• Batts vs Blown-In
• Common Mistakes
• Key Takeaways
• FAQ

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Fiberglass vs Cellulose: The Core Decision

This is the question we get asked most on blown-in jobs. Both materials work — but they have different strengths, and the best choice depends on your specific application.

Our recommendation: For attic floors, cellulose wins on performance — better R-per-inch means less depth needed, and it conforms better around wiring, plumbing, and junction boxes. The 20% settling is real but fully manageable (install 20–25% extra depth). Blown fiberglass wins if minimal settling and non-combustibility are priorities for you.

For wall retrofits, dense-pack cellulose is the clear winner. At 3.0–3.5 lb/ft³ density, it fills cavities completely, provides meaningful air sealing, and resists settling — capabilities that standard blown fiberglass can't match at low density. Dense-pack fiberglass products (Johns Manville Spider, Knauf JetSpray) are an alternative but less commonly available.

The head-to-head comparison at fiberglass vs. cellulose covers every angle in detail.

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Attic Application

Blown-in insulation on the attic floor is the most cost-effective energy upgrade for most homes, and it's the application where blown-in materials excel versus batts.

How it works: A blowing machine (hopper + hose) breaks up bags of insulation and propels the loose fibers through a hose into the attic. The operator directs the hose to build up even coverage across the attic floor, working from the far end back toward the access point.

Key principles for a good attic blown-in job:

1. Air seal the attic floor first. Blown-in insulation does not stop air leakage through gaps and penetrations. Seal top plates, wire penetrations, recessed lights, and duct boots before any insulation goes in. The DOE estimates air sealing alone saves 15–25% on heating and cooling.

2. Install rafter baffles at every soffit vent bay. These cardboard or foam channels maintain a 1–2 inch ventilation gap between the roof deck and the insulation, keeping airflow from soffit to ridge in vented attics.

3. Blow to consistent depth. Use depth markers (rulers or pre-cut sticks pushed into the insulation at intervals) to verify uniform coverage. Depth should be 20–25% above the settled target for cellulose.

4. Don't bury recessed lights or junction boxes. IC-rated recessed lights can be covered with insulation per code. Non-IC-rated fixtures need clearance — install an approved airtight box around them. Junction boxes must remain accessible.

5. Work from the far end back to the access. Don't blow yourself into a corner.

For the full attic guide including material selection, cost data, and R-value targets by zone, our attic insulation guide has the complete picture.

Pro Tip: When blowing cellulose in an attic, break up the material well in the hopper and use a moderate air setting — too much pressure creates "snowdrifts" and uneven coverage, too little leaves the material clumped. The goal is a uniform, fluffy blanket at consistent depth. Take your time. A 1,000 sq ft attic should take 3–5 hours with two people, not 45 minutes.

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Wall Application (Dense-Pack)

Dense-pack blown-in insulation is the standard method for insulating existing wall cavities without removing drywall or siding. It's a fundamentally different process from attic loose-fill — higher pressure, higher density, and professional equipment.

The drill-and-fill process:

1. Drill 2–3 inch holes through exterior siding (or interior drywall/plaster) — one hole per stud bay, typically at the top of the wall
2. Insert a fill tube through the hole, feeding it down to the bottom of the cavity
3. Blow insulation under high pressure, slowly withdrawing the tube as the cavity fills from bottom to top
4. Target density: 3.0–3.5 lb/ft³ for cellulose (firm enough that you can't push a finger into the material)
5. Plug holes with wood or foam plugs, patch, caulk, and repaint

Dense-pack cellulose is the go-to material: R-3.5–3.8 per inch at 3.0–3.5 lb/ft³. In a standard 2×4 wall (3.5" cavity), this delivers R-12 to R-13 with significant air-sealing benefits — Building Science Corporation research shows 30–40% reduction in wall air leakage versus empty cavities.

Dense-pack fiberglass products like Johns Manville Spider and Knauf JetSpray deliver R-3.7–4.3 per inch in wall applications. These are spray-applied fiberglass products that adhere to cavity walls, reducing settling and improving coverage versus traditional loose-fill fiberglass.

This is a professional job. Dense-pack equipment generates higher pressure than consumer rental machines. Achieving proper density throughout the cavity — especially around fire stops, window headers, and other obstructions — requires experience and technique. Under-packed walls settle and leave gaps; over-packed walls bow drywall or push out siding.

Cost: $1.50–$3.00/sq ft for cellulose, $2.00–$4.00/sq ft for injection/spray fiberglass. A 1,500 sq ft home with ~1,000 sq ft of exterior wall runs $1,500–$3,000 for cellulose dense-pack. The full wall retrofit process is covered at insulate walls without removing drywall and wall insulation.

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Coverage Charts

Blown-In Cellulose — Attic Application (Settled Thickness)

Blown-In Fiberglass — Attic Application

Bag counts for fiberglass vary more by manufacturer. Always reference the coverage chart printed on the bag — it specifies bags per 1,000 sq ft at each R-value target for that specific product.

For detailed coverage charts and blowing machine specifications, check manufacturer resources from Owens Corning's blown-in insulation products and GreenFiber cellulose. For a cross-material thickness comparison covering batts, spray foam, and rigid foam alongside blown-in, check the insulation thickness chart.

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Cost Breakdown

Per Square Foot (2025–2026 Installed Costs)

Total Project Costs (1,000 sq ft Attic to R-49)

DIY savings are substantial: $360–$540 in material versus $1,200–$3,500 installed — a 60–80% reduction. Machine rental is free with 20+ bag purchase at most home centers, or ~$100/day otherwise.

For detailed cost data including regional variation, blown-in insulation cost has the full breakdown. Project-specific estimates at the insulation cost calculator.

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DIY Blown-In: What to Expect

Blowing loose-fill insulation into an open attic is the most realistic insulation DIY project. Wall dense-pack is not — leave that to professionals.

What you'll need:

• Blowing machine (free rental with 20+ bags at Home Depot / Lowe's, or ~$100/day)
• Insulation bags (cellulose or fiberglass — count from coverage chart above)
• N95 respirator, safety glasses, headlamp
• Long sleeves, pants, gloves
• Utility knife for opening bags
• Tape measure or ruler for checking depth
• Kneeling boards (plywood scraps to distribute weight on ceiling joists)
• Rafter baffles (if soffit vents are present)

Realistic time estimate: 4–8 hours for a 1,000 sq ft attic with two people (one in attic directing the hose, one at the machine feeding bags). Add 4–6 hours for thorough air sealing beforehand.

When DIY works well: Open, accessible attic with uniform joists, good headroom, standard access (pull-down stairs or scuttle hatch large enough for equipment and materials), no knob-and-tube wiring, no vermiculite.

When to hire a pro: Complex multi-level attics, significant air sealing work needed, existing moisture/mold problems, knob-and-tube wiring present, vermiculite (potential asbestos), very limited access, or if you're simply not comfortable working in an attic.

Step-by-step instructions at how to install blown-in insulation and how to insulate an attic.

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Settling: The Real Story

Settling is cellulose's most discussed limitation — and we think it's often either overblown or underexplained. Here are the facts.

Loose-fill cellulose settles approximately 20% in the first 2–3 years. If you install 17.5 inches, it settles to about 14 inches. This is not a defect — it's a physical property of a lightweight fibrous material under gravity. Every cellulose manufacturer knows this and accounts for it in their coverage charts.

How manufacturers account for it: The bags-per-1,000-sq-ft chart on every bag specifies the settled R-value at the settled depth. If you follow the chart, you're already installing enough material to hit the target after settling. The problem occurs only when installers eyeball depth instead of counting bags and measuring.

Dense-pack cellulose barely settles. At 3.0–3.5 lb/ft³ in enclosed wall cavities, there's minimal room for further compression. Dense-pack settling is typically 2–5% — comparable to blown-in fiberglass.

Blown-in fiberglass settles only 1–3%. The glass fibers are springy and hold their loft better than cellulose fibers. The installed depth is essentially the final depth. This is a genuine advantage in attic applications.

Our take: Settling is a manageable drawback, not a dealbreaker. Install cellulose per the manufacturer's coverage chart, verify depth with a ruler in 6–8 locations before the job is done, and you'll hit your target R-value after settling. If settling is a major concern for you, blown-in fiberglass eliminates it almost entirely. For a broader material comparison, the R-value per inch chart ranks every insulation type.

Pro Tip: Check your attic insulation depth 2–3 years after installation. If cellulose has settled below target, you can top it off with a few additional bags for minimal cost. This is a 30-minute job with a blowing machine — blow 2–3 inches of fresh cellulose over the settled layer to restore full depth.

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Batts vs Blown-In

Both batts and blown-in insulation are available in fiberglass and mineral wool. When does each form factor make sense?

When batts make sense: New construction walls where cavities are open, framing is standard, and a skilled installer is available. Fiberglass and mineral wool batts in new walls are perfectly adequate when installed to RESNET Grade I standards.

When blown-in wins: Attics (always — blown-in provides dramatically better coverage than batts laid between joists), any retrofit application, irregular or obstructed cavities, and anywhere you need to add insulation over existing material.

In our experience, blown-in has taken significant market share from batts in attic applications over the last decade — and for good reason. The complete comparison is at batts vs. blown-in insulation.

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Common Mistakes

1. Not air sealing the attic floor before blowing insulation. Blown-in insulation (at attic loose-fill density) does not stop air leakage. Warm air rises through unsealed penetrations, top plates, and recessed lights regardless of how much insulation sits on top. Air seal first, insulate second. Our air sealing vs. insulation guide covers the priority order.

2. Blocking soffit vents. Without rafter baffles, blown insulation piles against the roof sheathing at the eaves and blocks soffit ventilation. This causes moisture buildup and potential roof damage in vented attics. Install baffles in every rafter bay at the eaves before blowing.

3. DIY dense-pack wall attempts. Consumer rental blowing machines don't generate sufficient pressure for proper dense-pack installation (3.0–3.5 lb/ft³). Under-packed walls settle, leave voids, and lose air-sealing benefit. Wall dense-pack is a professional job requiring commercial equipment and trained technique.

4. Insufficient depth / skipping the coverage chart. "Looks like enough" is not a measurement. Count bags, check the manufacturer's coverage chart, and measure installed depth with a ruler. For cellulose, install 20–25% above the settled depth target. For fiberglass, install to the chart depth — settling is minimal.

5. Covering knob-and-tube wiring with cellulose. Older homes with active knob-and-tube electrical wiring present a fire risk when covered with cellulose insulation (which insulates the wire, causing heat buildup). Many local codes prohibit covering K&T wiring with insulation. Have the wiring evaluated by a licensed electrician before insulating.

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Key Takeaways

• Blown-in insulation comes in two types: cellulose (R-3.2–3.8/inch, settles ~20%) and fiberglass (R-2.2–2.7/inch loose-fill, settles 1–3%). Both deliver excellent value for attic floors.
• Cellulose wins on R-value per inch, coverage quality, and partial air sealing. Fiberglass wins on settling resistance and non-combustibility.
• Dense-pack cellulose (3.0–3.5 lb/ft³) is the gold standard for wall retrofits — fills enclosed cavities without demolition and provides meaningful air sealing.
• A 1,000 sq ft attic to R-49: $1,200–$3,500 professionally installed, or $360–$540 DIY with free machine rental.
• DIY blown-in attic insulation is realistic for open, accessible attics. Wall dense-pack is strictly a professional job.
• Always air seal the attic floor before insulating — the DOE estimates this step alone saves 15–25% on heating and cooling.
• Install rafter baffles at soffit vents before blowing to maintain attic ventilation.
• Follow the manufacturer's coverage chart — count bags, measure depth, and verify 20–25% over-fill for cellulose.

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FAQ

How much does blown-in insulation cost?

Blown-in cellulose runs $0.60–$2.30/sq ft installed (attic) and $1.50–$3.00/sq ft for dense-pack walls. Blown-in fiberglass runs $0.50–$2.00/sq ft for attic applications. A 1,000 sq ft attic to R-49 costs $1,200–$3,500 professionally or $360–$540 DIY. Free blowing machine rental with 20+ bag purchase at most home centers. For detailed pricing, check blown-in insulation cost and the insulation cost calculator.

Which is better: blown-in cellulose or fiberglass?

For attic floors, cellulose has a slight edge — higher R-per-inch (R-3.2–3.8 vs R-2.2–2.7), better conformity around obstructions, and modest air-sealing properties. Blown fiberglass settles significantly less (1–3% vs ~20%) and is non-combustible. For wall retrofits, dense-pack cellulose is the clear winner for its air-sealing capability and cavity-filling performance. Either material is a solid choice for attics — the differences are smaller than the benefit of doing the project at all. Full comparison at fiberglass vs. cellulose.

Can I blow insulation over existing insulation?

Yes — R-values are additive. If you have 6 inches of existing fiberglass (approximately R-19), blowing 9 inches of cellulose on top adds about R-30, bringing you to R-49 total. Remove or slash any existing kraft vapor retarder facing on the old insulation so moisture can pass through. Existing insulation that's wet, moldy, or contaminated should be removed first. The R-value chart shows target values by climate zone.

How long does blown-in insulation last?

The base material lasts 80+ years for both cellulose and fiberglass. Effective performance life is 20–30 years for cellulose (settling + potential moisture issues) and 20–30+ years for blown-in fiberglass (minimal settling but susceptible to wind washing in vented attics). Dense-pack cellulose in walls holds up well for 30+ years when properly installed at correct density. Plan on checking attic depth every 5–10 years and topping off if needed. For comparison, mineral wool and spray foam maintain performance for 50–80+ years.

Is blown-in insulation better than batts?

For attics, yes — blown-in provides dramatically better coverage around wiring, plumbing, junction boxes, and other obstructions that batts must be cut around (and rarely are, perfectly). For new construction walls with open cavities and a skilled installer, batts are perfectly adequate and often cheaper. For retrofits, blown-in is the only practical option. The full comparison is at batts vs. blown-in.