When homeowners weigh blown-in insulation options, the choice often narrows down to cellulose versus fiberglass. The short answer is that cellulose generally provides higher thermal resistance per inch and superior air sealing due to its dense, fibrous structure, while fiberglass offers better moisture resistance and retains its R-value more consistently over time without settling. This article breaks down the thermal performance of both materials to help determine which suits a specific home and climate best.
Understanding these differences requires looking beyond the numbers on the package. All Foam & Insulation, LLC has installed these materials in various structures for years, and the real-world performance often depends on installation quality and the specific conditions of the building envelope. The following analysis examines how these two insulators behave when installed correctly.
R-Value and Density Differences
R-value measures thermal resistance, or how well a material resists heat flow. Fiberglass typically offers an R-value of roughly R-2.2 to R-2.7 per inch. Cellulose, being denser, usually ranges from R-3.2 to R-3.8 per inch. This higher density means cellulose often slows heat transfer more effectively than fiberglass of the same thickness.
However, R-value is not the only factor. Density plays a major role in how insulation feels and acts. Cellulose is heavy and settles into cavities tightly, while fiberglass is light and fluffy. The weight of cellulose helps it conform around wires and pipes, reducing gaps where heat might escape.
Bonus Tip: When checking bids, look at the “installed thickness” rather than just the bag count. Some installers fluff fiberglass to cover more area, which lowers the R-value. Verify the settled depth to ensure the advertised performance matches reality.
Material Comparison Table
| Feature | Cellulose Blown-In | Fiberglass Blown-In |
|---|---|---|
| R-Value per Inch | ~3.2 to 3.8 | ~2.2 to 2.7 |
| Density (lbs/ft³) | 1.5 to 2.5 | 0.5 to 1.0 |
| Weight | Heavy | Light |
| Air Barrier | Good (reduces air movement) | Poor (air passes through easily) |
| Moisture Resistance | Absorbs moisture; dries slowly | Does not absorb water; dries quickly |
Air Infiltration and Stopping Airflow
Insulation does more than just conduct heat. It also affects how air moves through walls and attics. Fiberglass is porous. Air moves through it relatively easily, much like wind passing through a filter. If air leaks exist around outlets or window frames, fiberglass does little to stop that draft.
Cellulose acts differently. It’s small, and the recycled fibers pack together tightly. This density blocks air movement effectively. According to Energy Star, sealing air leaks is often more important than adding insulation service alone because air movement can account for a significant portion of energy loss. Cellulose helps address both issues simultaneously by filling gaps and resisting airflow through the material.
Bonus Tip: In older homes with uneven framing or lots of voids, cellulose is often the better choice. Its ability to flow into irregular spaces makes it ideal for retrofitting walls where dense-packing techniques are used.
Moisture Management and Long-Term Performance
Moisture can ruin an insulation job. Fiberglass does not absorb water. If a leak occurs, water runs through the fiberglass and hopefully hits the floor, where it can be seen. Once the leak stops, fiberglass dries out quickly and usually returns to its original R-value.
Cellulose is made from recycled paper and includes borates for fire and pest resistance. It does absorb moisture. If it gets wet, it holds the water like a sponge. While the borates help prevent mold growth, wet cellulose takes a long time to dry. If it stays wet too long, it can compress or even rot the surrounding wood framing. In climates with heavy rainfall or high humidity, this distinction matters. The U.S. Department of Energy notes that controlling moisture is essential to maintaining a home’s durability and energy efficiency.
Settling Over Time
One common concern with blown-in insulation is settling. Cellulose settles over time as the material compresses under its own weight. Installers must account for this by adding extra thickness initially so that once it settles, it still meets the required R-value.
Fiberglass settles very little. It maintains its loft for decades without significant compression. This means a fiberglass installation might maintain its stated thermal performance longer without the need for topping up, provided it was installed at the correct density to begin with.
Fire Resistance and Safety
Fire safety is a major point of differentiation. Manufacturers treat cellulose with borate fire retardants. These chemicals make the material resistant to combustion. In a fire, dense-packed cellulose can actually slow the spread of flames through wall cavities.
Fiberglass is made from spun glass and is naturally non-combustible because it is essentially sand. It will not burn. However, the paper or foil facings on some fiberglass batts can burn. In blown-in form, fiberglass is considered very safe from a fire standpoint, though it does not provide the same fire-resistive barrier that dense-packed cellulose does.
Things to Consider Before Making a Decision
Choosing the right material involves considering the whole house, not just the insulation.
Climate and Weather Patterns
Homes in very cold climates benefit from cellulose’s higher R-value per inch, especially in tight attic spaces where depth is limited. Conversely, homes in highly humid or wet climates might favor fiberglass to avoid any risk of moisture retention in the insulation layer.
Existing Structure
Consider what is already in the walls. If a home has existing fiberglass batts that have degraded or were poorly installed, blowing dense-packed cellulose over or around them can significantly improve air sealing. If the goal is simply to add more depth to an attic floor, loose-fill fiberglass is quick and effective.
Pest Concerns
Cellulose contains borates, which also act as a pest deterrent. Roaches and ants generally do not like it. Fiberglass does not offer this benefit, though pests can nest in it if it is loose.
Bonus Tip: If squirrels or rodents have been a problem in the attic, fiberglass might be better. Rodents sometimes burrow into cellulose for nesting, whereas they tend to leave fiberglass alone because it irritates their respiratory systems.
Frequently Asked Questions
Does cellulose insulation rot?
Cellulose is treated with chemicals to resist mold and rot. It only rots if it stays saturated with water for a long period. Small leaks or high humidity usually do not cause rot if the attic is properly ventilated.
Which insulation lasts longer?
Fiberglass does not settle or degrade, so it can last indefinitely. Cellulose settles slightly but then stabilizes. It can last for the life of the building as long as it remains dry.
Can cellulose cause corrosion on pipes or wires?
Modern cellulose is treated to be non-corrosive. Older formulations had issues with acidity, but current standards ensure it is safe to use around copper wiring and metal piping.
Is blown-in insulation good for walls?
Yes, dense-pack cellulose is excellent for walls. It fills the cavity completely and stops air movement. Fiberglass can also be blown into walls but requires a specific mesh or netting system to hold it in place.
Which is better for allergies?
Both materials can trap dust and allergens if not properly sealed. Once installed behind drywall, they should not affect indoor air quality. Cellulose is sometimes preferred because the borates inhibit mold and mildew growth.
Final Thoughts
Both cellulose and fiberglass offer effective ways to improve a home’s energy efficiency. Cellulose stands out for its higher density, superior R-value per inch, and ability to seal air leaks. Fiberglass excels in moisture resistance, stability, and lighter weight. The right choice depends on the specific needs of the home, the local climate, and whether air sealing or moisture management is the primary concern.
Homeowners should evaluate their specific situation, looking at factors like existing insulation, roof leaks, and local weather patterns. A professional assessment can help determine which material will deliver the best long-term comfort and savings.
Ready to Improve Your Home’s Efficiency
Deciding on insulation is easier with expert help. All Foam & Insulation, LLC can assess your property and recommend the best solution for your specific needs. Contact the team at [email protected] or call (541) 826-9600 to schedule a consultation and get a clear picture of your options.
Sources
- Energy Star – Official U.S. government site for energy efficiency information, providing data on R-values and air sealing.
- U.S. Department of Energy – National resource for energy science and technology, offering details on insulation materials and moisture control.
