After eight years of crawling through attics across the country with Miller Attics, I’ve heard just about every question imaginable when it comes to choosing between attic insulation materials. The debate around attic insulation fiberglass vs cellulose never seems to get old, and honestly, I understand why homeowners struggle with this decision. Both materials have their champions and critics, and depending on who you ask online, you’ll get completely different answers. Let me share what I’ve actually seen work in real homes, not just what looks good on paper.
The Cold Hard Truth About Performance
When homeowners start researching cellulose vs fiberglass attic insulation, they usually come across R-value comparisons first. Yes, cellulose does have a higher R-value per inch—typically around 3.2 to 3.8 compared to fiberglass at 2.2 to 2.7. However, what surprised me early in my career was learning that these numbers don’t tell the whole story. I remember one particularly cold winter when we started getting calls from homeowners who’d installed fiberglass insulation just a year or two earlier. They were shocked that their heating bills hadn’t dropped as much as expected.
The issue? Fiberglass doesn’t handle air movement well. When temperatures drop below twenty degrees, I’ve seen fiberglass lose up to forty percent of its insulating capacity. This happens because air can pass right through those glass fibers, creating convection loops that carry heat away. Meanwhile, cellulose is much denser, packing into every crack and corner. It essentially stops air movement dead in its tracks, which is why I often recommend it for clients in colder climates. That said, this same density means cellulose weighs significantly more, something we need to consider when evaluating older homes with questionable ceiling joists.
The Moisture Question Everyone Asks
Without fail, the biggest concern I hear about attic insulation cellulose vs fiberglass revolves around moisture. Homeowners read horror stories online about cellulose absorbing water like a sponge, and honestly, those concerns aren’t completely unfounded. I’ve pulled out wet cellulose from attics where it absorbed two or three times its weight in water from a roof leak. Once cellulose gets soaked, it can hide plumbing problems and create perfect conditions for mold growth. The material dries slowly, sometimes taking weeks, and during that time it settles and loses insulating value.
Fiberglass handles moisture differently. When it gets wet, water passes through the fibers rather than being absorbed. This means leaks become obvious quickly—you’ll see water stains on your ceiling instead of the insulation hiding the problem for months. However, wet fiberglass has its own issues. The material can compress and lose shape, and if it stays damp, you’re still looking at potential mold growth. The key difference is detection time. With fiberglass, you know about problems fast. With cellulose, you might not realize there’s an issue until significant damage has occurred.
Installation Realities That Matter
The internet loves to debate cellulose vs fiberglass attic insulation from a theoretical standpoint, but installation makes a massive practical difference. Blown-in cellulose creates an enormous dust cloud that settles throughout your entire house if you’re not careful. I’ve had customers complain about finding dust on their furniture days after installation, even with proper containment procedures. The material also requires more skill to install correctly since you need to achieve the right density. Too loose and it settles too much over time. Too dense and you’re wasting material and money.
Fiberglass, particularly the newer blown-in products, installs cleaner and faster. It’s lighter to handle, which means my crews can work longer without getting exhausted. The material doesn’t settle as much over time, maintaining its loft better than cellulose. However, achieving complete coverage with fiberglass requires attention to detail because gaps and voids dramatically reduce performance. I’ve inspected plenty of fiberglass installations where I could see right through to the drywall below because the installer rushed the job or the material shifted.
Cost Considerations Based on Real Projects
When homeowners ask me about pricing for attic insulation fiberglass vs cellulose, I tell them the materials themselves cost roughly the same—somewhere between seventy cents and a dollar fifty per square foot depending on the depth needed. What changes the final bill is labor time and material weight. Cellulose usually takes longer to install properly because of the density requirements and cleanup involved. Fiberglass can go in faster, but you need more depth to achieve the same R-value, which sometimes means more bags of material.
I’ve noticed regional price variations too. In some areas, cellulose costs less because it’s made from recycled newspaper, and there’s good local supply. Other regions see fiberglass priced lower due to manufacturing presence. Beyond the immediate installation cost, consider longevity. Cellulose typically maintains its R-value better over decades if it stays dry, while fiberglass can compress in high-traffic attics or lose effectiveness in extreme temperature swings. For most homeowners, the initial cost difference between the two materials ends up being just a few hundred dollars on an average home, making performance and climate factors more important than pure price.
Climate Makes the Decision Easier
Living through hundreds of installations taught me that geography often determines which material performs better. In cold northern climates where temperature differences are extreme, cellulose wins hands down. The density prevents convection loops, and that matters when it’s negative ten degrees outside. I worked on homes in Michigan where homeowners saw immediate energy bill reductions after switching from fiberglass to cellulose. The difference was dramatic enough that several called to thank us.
However, in humid southern climates or areas prone to roof leaks, fiberglass makes more sense despite its lower R-value per inch. The ability to dry quickly and not hide moisture problems outweighs the performance advantage of cellulose. I’ve also noticed that homes with chronic ventilation issues do better with fiberglass because it doesn’t hold moisture the way cellulose does. One customer in Florida learned this the hard way after insisting on cellulose despite my recommendations. A small roof leak went undetected for months, resulting in serious structural damage that could have been caught much earlier with fiberglass.
What I Actually Recommend
After eight years and countless installations, I don’t believe there’s one perfect answer to the attic insulation fiberglass vs cellulose question. Instead, I ask homeowners about their specific situation. Do you live somewhere with harsh winters? Cellulose probably makes sense. Is your roof older or do you have past leak issues? Fiberglass offers better peace of mind. Are you sensitive to dust or have respiratory concerns? Modern fiberglass products are less irritating than cellulose during and after installation.
I also consider the home’s age and condition. Older homes with questionable structural integrity might not handle the weight of dense-packed cellulose well. Newer homes with proper ventilation and vapor barriers can take advantage of cellulose’s superior R-value without moisture concerns. The honest truth is that proper air sealing matters more than which insulation you choose. I’ve seen poorly sealed attics with cellulose underperform compared to well-sealed attics with fiberglass. Fix the air leaks first, then either material will serve you well.
The bottom line from my years crawling through attics? Both cellulose and fiberglass work when installed correctly in the right situation. Focus less on winning the debate and more on matching the material to your climate, home condition, and specific needs. That’s what actually keeps your energy bills down and your family comfortable year-round.
