Cellulose vs Fiberglass Blown-In Insulation: Which One Belongs in Your Attic?
If you are getting quotes on attic insulation for your Houston or Katy home, the two names you will hear most often are blown in fiberglass and blown in cellulose. Both are loose fill materials installed with a blowing machine. Both can fill an attic floor to any target R-value. And both have been used in millions of homes across the country.
But they are made from completely different materials, they behave differently in Houston's heat and humidity, and the one you choose affects how long your insulation lasts, how well it holds its rated performance, and how much maintenance it needs over the years.
This guide compares cellulose vs fiberglass blown in insulation across every factor that matters: R-value, settling, moisture, fire safety, sound, cost, lifespan, and environmental impact. The goal is to help you make a confident decision based on your attic, your climate, and your priorities.
Quick Summary: Cellulose vs Fiberglass
If you want the short version before the deep dive:
Choose blown in fiberglass if you want insulation that resists moisture, holds its R-value over decades with minimal settling, costs less to install, and lasts 40 to 80 years without replacement. It is the stronger choice for Houston area attics where humidity is constant and long-term durability is the priority.
Choose blown in cellulose if your primary concern is maximizing recycled content and environmental sustainability, or if you need slightly better air-blocking density in an enclosed wall cavity. Cellulose also has built-in pest resistance from its borate treatment.
For most Houston homeowners insulating an open attic, blown in fiberglass delivers the best combination of performance, durability, and value over the full life of the home.
What Is Cellulose Insulation?
Cellulose insulation is made primarily from shredded recycled newspaper and cardboard, treated with borate compounds for fire resistance and pest deterrence. It has a gray, dense, fibrous appearance and is almost always installed in blown-in form.
In attics, cellulose is blown as loose fill across the attic floor, similar to fiberglass. In walls, it can also be dense-packed into enclosed cavities through small drilled holes, which is one of its strongest applications. Dense-pack cellulose fills wall cavities tightly enough to resist air movement through the wall, making it a solid retrofit option for older homes with empty wall spaces.
What Is Fiberglass Insulation?
Fiberglass insulation is made from spun glass fibers, manufactured from sand and recycled glass. It comes in two primary forms: pre-cut batts (the pink or yellow rolls you see at home improvement stores) and loose fill blown-in material.
Blown in fiberglass is a different product from batt fiberglass. Where batts must be cut to fit and leave gaps at every irregularity, blown in fiberglass flows freely into every space in the attic, conforming to framing, pipes, and wiring without seams. For attic applications, blown-in is the superior installation method regardless of which material you choose.
How Blown-In Insulation Is Installed
Both cellulose and fiberglass blown-in insulation are installed using the same basic method. A commercial blowing machine feeds the loose fill material through a flexible hose into the attic. The installer distributes the material in systematic passes across the attic floor, building up to a target depth that corresponds to the desired R-value.
The key differences in installation are weight and behavior. Cellulose is significantly heavier than fiberglass at the same R-value, which affects ceiling load in older homes. Cellulose also produces much heavier dust during installation. Fiberglass is lighter but causes skin and eye irritation during the install process, requiring full protective equipment for the installer.
Both materials are used in attic floors and wall cavities, though cellulose is more commonly dense-packed into walls while fiberglass is the dominant choice for open attic applications.
R-Value and Energy Efficiency
R-value measures how well insulation resists heat flow. Higher is better.
| Material | R-Value Per Inch | Depth for R-38 | Depth for R-49 |
|---|---|---|---|
| Blown in Fiberglass | R-2.2 to R-2.7 | ~15 inches | ~20 inches |
| Blown in Cellulose | R-3.2 to R-3.8 | ~11 inches | ~14 inches |
Cellulose has a higher R-value per inch, which means it reaches the same insulation rating in less depth. This matters in attics with limited vertical clearance.
However, R-value on install day is only part of the story. What matters for long-term energy efficiency is how well the insulation holds that R-value over years and decades. That is where settling and moisture behavior become critical, and where fiberglass has a meaningful advantage in humid climates like Houston's.
For reference, Houston falls in IECC Climate Zone 2. The Department of Energy recommends R-30 to R-60 for attics in this zone. Texas building code targets R-38 minimum for new construction. Most homes built before 2000 are well below that.
Air Sealing and Insulation Performance
Insulation alone does not stop air movement. Air sealing is the process of closing gaps and penetrations in your attic floor, around pipes, wiring, recessed lights, and duct boots, so conditioned air does not leak up into the attic and hot attic air does not leak down into your living space.
Cellulose has a slight natural advantage here. Its denser structure slows air movement through the material itself better than fiberglass, which is why cellulose is sometimes described as having better "air-blocking" properties.
That said, neither material is a substitute for proper air sealing. The performance difference between a fiberglass attic with good air sealing and a cellulose attic without it is far greater than the difference between the two materials themselves. At Affordable Attic Insulation, Jason seals attic penetrations before any insulation is blown in, which is the step that makes the biggest difference in real-world energy savings regardless of which material is used.
Settling Behavior
This is one of the most important practical differences between cellulose vs fiberglass blown in insulation.
Blown in fiberglass settles approximately 3 to 5 percent over its lifetime. An installation that starts at 15 inches will typically settle to about 14 inches over 20 to 30 years. That is a minimal R-value loss.
Blown in cellulose settles 15 to 20 percent in a typical open attic application. An installation that starts at 11 inches can drop to 9 inches or less within a few years as the heavier material compacts under its own weight. That drops an R-38 installation to approximately R-30 or below.
Dense-pack cellulose in enclosed wall cavities settles much less because the cavity constrains the material. But in an open attic, where gravity works freely against a loose pile of shredded paper, settling is a real and well-documented issue. It is the primary reason cellulose attic installations require more frequent top-ups than fiberglass.
Moisture, Mold, and Durability
Cellulose is hygroscopic. It absorbs and holds moisture from the air. In a humid climate like Houston's, where attic humidity is elevated for much of the year, this creates several risks. Wet cellulose loses R-value, compresses faster, can develop musty odors, and if moisture lingers long enough, becomes a substrate for mold growth despite its borate treatment.
Fiberglass is inorganic. It does not absorb water. If it gets wet from a roof leak, it dries out and returns to full performance once the water source is repaired. It does not support mold growth because mold needs an organic food source, which glass fibers do not provide.
In any climate, proper attic ventilation is essential for insulation longevity. Soffit vents, ridge vents, and correctly installed baffles keep air moving through the attic and prevent moisture from accumulating. But in a Gulf Coast climate where humidity is a constant factor, the inherent moisture resistance of fiberglass provides an additional margin of safety that cellulose does not.
Fire Safety
Fiberglass is non-combustible. It is made from glass. It does not ignite, does not spread flame, and does not require any chemical treatment to achieve its fire rating. This is an inherent property of the material itself.
Cellulose is treated paper. Without chemical treatment, it would burn readily. Borate compounds are added during manufacturing to make cellulose fire-resistant, and when properly treated and installed, it meets all applicable fire codes. However, there are documented cases where fire retardant treatments have degraded over time, particularly in humid attic environments, and where cellulose installed too close to non-IC-rated recessed lighting has been involved in attic fires.
Both materials are code-compliant when installed correctly. But fiberglass achieves its fire resistance without depending on chemical additives, which is a distinction worth noting for long-term performance.
Sound Dampening
Both blown in fiberglass and cellulose provide meaningful sound absorption. Of the two, cellulose has a slight edge due to its higher density. Dense-pack cellulose in walls is particularly effective at reducing sound transmission between rooms.
For attic applications specifically, both materials reduce rain noise, wind, and ambient traffic sound from above. The practical difference between the two in an attic floor application is modest. Homeowners who prioritize soundproofing between rooms or floors will see the most benefit from dense-pack cellulose in wall cavities, which is a different application than an open attic floor.
Environmental Impact
This is cellulose's strongest argument.
Cellulose is made from 80 to 85 percent post-consumer recycled content, primarily newspaper. Its manufacturing process requires less energy than fiberglass production. At end of life, cellulose is biodegradable.
Fiberglass uses 40 to 60 percent recycled glass content. Manufacturing requires higher temperatures and more energy. At end of life, fiberglass is not biodegradable, but it can be recycled through specialized facilities.
However, fiberglass lasts roughly twice as long as cellulose (40 to 80 years vs. 20 to 30 years), which means a single fiberglass installation replaces two cellulose lifecycles. When measured by total environmental impact over the life of the home, including manufacturing, transport, installation, and disposal, the gap is narrower than the recycled-content numbers alone suggest.
If minimizing recycled content in your insulation is a top priority, cellulose is the clear choice. If you factor in longevity and replacement cycles, fiberglass competes more closely than most people realize.
Installation Safety
Both materials present workplace hazards that are managed through proper protective equipment.
Cellulose produces heavy, fine dust during installation. Inhalation is the primary concern. Installers use respirators and eye protection throughout the process. The dust settles after installation and does not affect your living space once the work is complete.
Fiberglass causes skin irritation and can irritate the lungs and eyes during installation. Glass fibers are not pleasant to work around. Installers use full PPE including respirators, gloves, long sleeves, and sealed eye protection. Like cellulose, fiberglass does not affect your living space once installed and undisturbed in the attic.
This is one of the reasons professional installation matters. A trained installer with the right equipment handles these hazards safely and efficiently. Your home is unaffected during and after the process.
Pest Resistance
Cellulose has a built-in advantage here. The borate treatment that provides fire resistance also deters insects, particularly ants, roaches, and termites. This makes cellulose moderately effective as a pest deterrent in the attic.
Fiberglass does not attract pests, but it does not actively repel them either. Rodents do not like nesting in fiberglass because the glass fibers are irritating, but they will tunnel through it if they are already in your attic.
Neither material is a substitute for proper pest prevention. Sealing entry points at the eaves, around vents, and at roof penetrations is far more effective than relying on insulation material alone to keep animals out. If pest contamination does occur, fiberglass can sometimes be salvaged with localized treatment, while cellulose typically absorbs droppings and odors deeply enough to require full removal.
Cost, Lifespan, and Return on Investment
| Factor | Blown in Fiberglass | Blown in Cellulose |
|---|---|---|
| Typical installed cost (Houston) | $1.50 to $2.50 per sq ft | $1.75 to $3.00 per sq ft |
| Lifespan | 40 to 80+ years | 20 to 30 years |
| Top-up frequency | Rarely needed | Often needed within 10 to 15 years |
| Payback period | 3 to 6 years | 3 to 6 years |
Both materials pay for themselves in energy savings within a few years. The difference shows up in total cost of ownership. Fiberglass costs less upfront, lasts longer, and rarely needs top-up service. Cellulose may need supplemental material within 10 to 15 years as settling reduces effective depth, and full replacement within 20 to 30 years. Over a 40-year period, fiberglass typically delivers significantly more insulation value per dollar spent.
Choosing the Right Insulation for Your Project
Blown in fiberglass is the stronger choice when:
- You are insulating an open attic floor in a humid climate like Houston
- Long-term durability and minimal maintenance are priorities
- Your attic has any history of moisture issues or marginal ventilation
- You want non-combustible insulation without chemical fire retardants
- Budget is a factor and you want the lowest total cost of ownership
Blown in cellulose is the stronger choice when:
- Maximizing recycled content and environmental sustainability is your top priority
- You are dense-packing enclosed wall cavities in a retrofit project
- You want the built-in pest deterrent from borate treatment
- Your attic has limited vertical clearance and you need maximum R-value per inch
Mixing the two materials in the same attic is generally not recommended. Different settling rates create uneven coverage, and future top-ups become complicated when two materials are layered together.
Why Professional Installation Matters
Blown-in insulation is sometimes marketed as a DIY project, and equipment rental is available at some home improvement stores. The reality is that the quality of the installation determines the performance of the insulation far more than the material itself.
Common DIY mistakes include uneven depth across the attic, blocked soffit vents, insulation piled too high near eaves without baffles, missed air sealing at penetrations, and compressed insulation around walkways. All of these reduce the effective R-value of the installation regardless of what material was used.
A professional installer brings the experience to air-seal penetrations correctly before blowing, distribute material evenly across the full attic floor, install ventilation baffles where needed, verify depth at multiple points against target markers, and avoid the common pitfalls that turn a good material into a poor-performing attic.
If you are getting quotes, ask each contractor whether they air-seal before insulating, how they verify depth, and whether the owner or a subcontractor will do the work.
Post-Installation Checklist
After any blown-in insulation job, whether fiberglass or cellulose, confirm the following before considering the project complete:
- Insulation depth is uniform across the full attic floor, verified at multiple points with a depth gauge
- Soffit vents are not blocked and ventilation baffles are installed at the eaves
- Recessed light fixtures have proper clearance or airtight covers
- All attic floor penetrations (pipes, wires, duct boots) were air-sealed before insulation was applied
- The product type, manufacturer, installed density, and target R-value are documented for your records
Common Myths
"Cellulose always settles badly." Not entirely accurate. In enclosed wall cavities, properly dense-packed cellulose settles very little. The settling problem is real and significant in open attic applications, where the material is not constrained. The distinction between dense-pack walls and loose-fill attics matters.
"Fiberglass loses R-value over time." This is overstated. Fiberglass settles 3 to 5 percent over its lifetime, which is a minor R-value reduction. The material itself does not degrade chemically. Claims that fiberglass loses dramatic R-value over time are typically based on installations that were compressed, contaminated, or improperly installed, not on the material's inherent properties.
"Cellulose is a fire hazard." When properly manufactured and treated with borate compounds, cellulose meets all applicable fire codes. The concern is not with properly treated product but with older installations where retardant may have degraded, or installations placed too close to heat sources like recessed lights. Both materials are safe when installed correctly.
Final Takeaways
Both blown in fiberglass and blown in cellulose are legitimate insulation products with real strengths. The right choice depends on your attic, your climate, and what you prioritize most.
For Houston and Katy homeowners insulating a vented attic in a hot, humid climate, blown in fiberglass is the more durable, more moisture-resistant, lower-maintenance option that holds its R-value reliably for decades. It costs less upfront, lasts longer, and does not require chemical treatment to be fire safe.
Cellulose earns its place in wall cavity retrofits, in projects where environmental sustainability is the top priority, and in dry climates where its moisture sensitivity is less of a concern.
Whichever material you choose, the two things that matter most are proper air sealing before insulation goes in and professional installation that delivers uniform depth to the correct R-value target. Those two factors have more impact on your energy bills than the material itself.
Get a Professional Recommendation for Your Attic
At Affordable Attic Insulation, Jason personally inspects every attic and handles every installation. After more than 20 years insulating homes across the Houston and Katy area, he can tell you in 20 minutes what your attic needs, what condition your current insulation is in, and what it will realistically cost to bring your home up to the right performance level.
Call (346) 205-1864 for a free attic inspection or request a quote online.
Serving Houston, Katy, Sugar Land, Missouri City, Cinco Ranch, Brookshire, Friendswood, Clear Lake, Pearland, Tomball, Richmond, Fulshear, Cypress, Spring, and Bellaire.
