Fiberglass vs Cellulose Insulation
The two most common home insulation materials compared on R-value, cost, installation, fire safety, and environmental impact.
| Factor | Fiberglass | Cellulose |
|---|---|---|
| Cost per Sq Ft | $0.30–$0.80 | $0.60–$1.20 |
| R-Value per Inch | R-2.9 to R-3.8 | R-3.2 to R-3.8 |
| DIY Friendliness | Very easy (roll and cut batts) | Moderate (blowing machine needed) |
| Fire Resistance | Naturally non-combustible | Treated with fire retardant (Class 1) |
| Moisture Resistance | Does not absorb water | Can absorb moisture and sag |
| Sound Dampening | Good | Excellent — denser material |
| Settling Over Time | Minimal settling | Can settle 15–20% over years |
| Environmental Impact | Made from sand/glass (energy intensive) | 85% recycled paper, low energy |
| Pest Resistance | Rodents can nest in batts | Borate treatment deters pests |
| Coverage Quality | Gaps around pipes and wiring | Fills every cavity and gap |
Cost Comparison
Fiberglass batts are the more affordable option for DIYers. For a typical 1,000 sq ft attic, fiberglass batts cost roughly $300–$800 in materials and can be installed in a weekend with no special equipment. Cellulose for the same attic runs $600–$1,200 in materials, plus you'll need to rent a blowing machine ($50–$100 per day, often free with bulk purchases). If hiring a professional, blown-in cellulose costs $1.00–$1.50 per sq ft installed vs $0.80–$1.20 for fiberglass — but cellulose provides better coverage around obstacles, often making it the better value for retrofit applications.
Fiberglass Insulation: Pros & Cons
Pros
- Lowest cost per square foot for DIY
- Easy to install — just unroll and cut batts
- Naturally non-combustible (won't burn)
- Does not absorb moisture or sag when wet
- Available at every home improvement store
- Doesn't settle significantly over time
Cons
- Leaves gaps around pipes, wires, and obstructions
- Loses R-value if compressed or wet
- Can irritate skin, eyes, and lungs during install
- Rodents can nest in fiberglass batts
- Difficult to retrofit into closed wall cavities
- Lower R-value per inch than cellulose
Cellulose Insulation: Pros & Cons
Pros
- Higher R-value per inch than fiberglass
- Fills every gap, crack, and cavity completely
- Made from 85% recycled paper — eco-friendly
- Borate treatment resists pests and mold
- Excellent sound dampening
- Ideal for retrofitting existing walls (blown-in)
Cons
- Requires blowing machine for installation
- Can absorb moisture and lose R-value if wet
- Settles 15–20% over time (need to over-fill)
- Heavier — may stress ceiling drywall in attics
- Dusty installation process
- More expensive than fiberglass batts
Which Should You Choose?
Choose Fiberglass If:
- You're insulating open wall cavities or a new construction attic
- You want the easiest, most affordable DIY option
- Moisture is a concern (basements, crawl spaces)
- You need non-combustible insulation for code compliance
- You want a quick weekend project with no special equipment
Choose Cellulose If:
- You're retrofitting insulation into existing closed walls
- Maximum coverage and air-sealing are priorities
- Sound dampening between rooms matters
- You prefer an environmentally friendly material
- You're insulating an attic with lots of obstacles (pipes, wires, ducts)
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Cellulose has a slightly higher R-value per inch, ranging from R-3.2 to R-3.8, compared to fiberglass batts at R-2.9 to R-3.8. More importantly, cellulose fills cavities more completely, reducing air gaps that lower the effective R-value of fiberglass batts. In real-world performance, cellulose often outperforms fiberglass by 20–30% due to its superior air-sealing ability.
No. Modern cellulose insulation is heavily treated with borate-based fire retardants and meets Class 1 fire resistance standards. In fire tests, cellulose actually slows the spread of flames because the dense material limits oxygen flow. Both fiberglass and cellulose are considered safe when installed to code. Fiberglass won't burn, but the paper facing on fiberglass batts is combustible.
Yes, both types can be DIY projects. Fiberglass batts are the easiest — just measure, cut, and press into wall or attic cavities. Cellulose requires a blowing machine, which you can rent from most home improvement stores (often free with a large cellulose purchase). Wear proper protective gear for either type: long sleeves, gloves, safety glasses, and an N95 mask.
The amount depends on your climate zone and existing insulation. The Department of Energy recommends R-38 to R-60 for attics in most US climates and R-13 to R-21 for walls. For a 1,000 sq ft attic, that's roughly 15–20 bags of blown-in cellulose or 20–30 rolls of fiberglass batts. Check your local building code for specific R-value requirements for your area.
Fiberglass insulation can last 80–100 years if it stays dry and undisturbed. Cellulose can settle 15–20% over time, reducing its effective R-value, so attics may need a top-off after 15–20 years. Both types should be replaced if they get wet, develop mold, or are disturbed by rodents. If your energy bills are rising or rooms feel drafty, it's worth inspecting your insulation.