Fiberglass vs Cellulose Blown in Attic Insulation
The wind off Lake Michigan hit the siding hard this morning, pushing the temperature down to thirty-one degrees by sunrise. I was standing on a ladder near the eave of a commercial retail build in Naperville, watching my guy prep for a retrofit job where we needed to upgrade the thermal envelope before winter locked in. You see guys arguing over insulation every time I walk onto a site, but seeing it from the ground level changes how you think about R-value and moisture control. In 2026, contractors aren’t just throwing material into cavities anymore; they are calculating exact loads against IRC Table N1102.1.3 requirements to avoid callbacks when the freeze-thaw cycle kicks in hard. If you are standing on a ladder today deciding between fiberglass and cellulose for an attic project, here is what I’ve learned running crews through ten thousand square feet of rough framing over fifteen years.
Cellulose insulation relies on recycled paper treated with borates to resist fire and pests, making it denser than fiberglass by design. While I do not have a specific model number for every cellulose brand available in 2026, the industry standard density ranges from 3.5 to 4.5 lbs per cubic foot, significantly heavier than the Owens Corning fiberglass at 0.7 lbs/sq ft. This density allows cellulose to fill gaps between rafters more effectively and bridge small air leaks that fiberglass often misses in complex roof decks.
The settling rate is the main concern for contractors managing long-term performance. Cellulose settles over time, typically losing about half its volume within the first year if not installed with enough excess material. To counter this, you must aim for an R-value higher than the code minimum initially to account for that natural compression. In Chicago weather patterns, where temperatures swing from 40°F highs down to single digits overnight, cellulose’s thermal mass helps moderate temperature fluctuations better than loose fiberglass fibers.
One major advantage in professional applications is the ability to use a blower machine to reach tight spaces without disturbing existing wiring or plumbing lines. You will see this on renovation sites where we cannot cut into the finished ceiling below. The downside remains cost; cellulose typically costs more per square foot upfront and requires specialized equipment that not every general contractor owns in their fleet.
Bottom line: Cellulose wins on air sealing and gap filling, but you pay for it with higher initial material costs and potential settling over time.
