Blown-In Insulation: Fiberglass vs. Cellulose

Overview

Blown-in insulation is a common way to improve attic performance and, in some cases, dense-pack wall cavities in existing homes. For homeowners, the most common comparison is fiberglass versus cellulose. Both products can work well. Both can underperform when installed poorly. The material matters, but installation quality and air sealing matter just as much.

Fiberglass blown-in insulation is made from spun glass fibers. Cellulose is typically made from recycled paper treated for fire resistance and pest resistance. On paper, they may look like close substitutes. In practice, they differ in density, settling behavior, air resistance, moisture response, dust, and how they interact with old houses.

Homeowners should be cautious about broad claims. One contractor may say cellulose is always better because it is denser. Another may say fiberglass is always cleaner and safer. Neither statement is reliable without discussing the assembly, the climate, and the installation method.

Key Concepts

Loose-Fill vs. Dense-Pack

Loose-fill is commonly used in open attics. Dense-pack is used in enclosed cavities and requires different equipment and technique.

Installed Depth Matters

Performance depends on final settled depth and proper coverage, not just how many bags are delivered.

Air Sealing Comes First

Blown-in insulation should not be used as a substitute for sealing major air leaks.

Core Content

Fiberglass Blown-In Insulation

Blown fiberglass is lightweight and noncombustible. In attics, it can provide good thermal performance when installed to the correct depth and coverage. Because it is light, it is often easier to level across large open attic areas. It can also be less affected by small amounts of incidental moisture than some other products, although wet insulation of any type is a problem.

The weakness of fiberglass loose-fill is that it is not especially good at resisting air movement on its own. If the attic floor has open bypasses, warm indoor air can still move through or around the insulation layer. This is why homeowners sometimes add fiberglass and see less improvement than expected.

Cellulose Blown-In Insulation

Cellulose is heavier and denser than loose-fill fiberglass. That density can help reduce convective looping and can make cellulose perform well in older attics with irregular framing. Many installers also like dense-pack cellulose for wall cavities because it can fill voids more thoroughly when done correctly.

Cellulose can settle if it is underinstalled, and improper density in walls can lead to voids over time. It also holds more moisture than fiberglass, which can be a drawback if the assembly has roof leaks, chronic humidity issues, or poor drying potential.

Performance Differences in Real Houses

In a well-sealed attic, both materials can deliver meaningful energy savings. In a leaky attic, neither material will perform as promised until the major air pathways are sealed. Cellulose often gets credit for better real-world performance in imperfect conditions because its density can slow airflow better than light fiberglass. But that does not eliminate the need for proper air sealing.

Fiberglass can be a reasonable choice where moisture concerns, noncombustibility, cost, or installer familiarity weigh heavily. Cellulose can be a strong choice where better cavity fill and airflow resistance are priorities. The product decision should follow diagnosis, not marketing preference.

Installation Risks Homeowners Should Watch

The most common failures are not about brand. They are about depth, coverage, obstruction, and prep work. Walkboards, low roof edges, mechanical equipment, soffit vents, bath fan terminations, and attic storage areas all affect installation quality. If the crew buries vents, leaves bypasses open, or installs too little material at the eaves, the job can disappoint quickly.

Homeowners should ask how final depth will be marked, whether rulers will be left in place, how ventilation pathways will be protected, and whether existing insulation will be cleaned up or leveled first.

Cost and Value Questions

Fiberglass is often cheaper, but low price is not always low total cost if the result does not solve the problem. Cellulose may cost more per job depending on region and scope, but the better value is the product that fits the house and is installed correctly. If a contractor gives a very low quote without specifying target depth, coverage area, and prep work, the homeowner should expect shortcuts.

Special Cases in Existing Homes

Older homes with knob-and-tube wiring, active roof leaks, mold, vermiculite, or recessed lighting safety issues need investigation before insulation is added. These are not minor details. They can affect code compliance, fire safety, and whether the project should proceed at all.

State-Specific Notes

Climate affects material choice. Cold states often place more emphasis on attic depth, air sealing, and controlling warm moist air from below. Humid regions may care more about roof assembly drying potential and mechanical moisture loads. Fire-prone states or local jurisdictions may also have specific concerns about ember intrusion at vents, attic vent baffles, and exposed ignition risks around recessed fixtures or old wiring. Some state energy programs offer incentives, but the inspection requirements and approved installation standards vary.

Key Takeaways

Fiberglass and cellulose can both work well when the house is prepared properly.

Air sealing is usually more important than the brand of blown-in insulation.

Cellulose is denser and often handles irregular cavities better, while fiberglass is lighter and often less expensive.

Homeowners should compare installed depth, prep work, and quality control, not just price per bag.