Cellulose vs Fiberglass Blown-In Insulation: Which Wins in Illinois?

Cellulose and fiberglass are the two materials every blown-in insulation quote in Lake County offers, and they look similar on paper: both are blown through a hose, both reach R-49, and both take a day to install. The differences are in the details: the bag price, the depth, the settling, and how each one handles moisture. Cellulose costs far less per bag but needs more bags. Fiberglass costs more per bag but does not settle. This guide compares them head to head on the same attic, 1,500 square feet to R-49, so the differences are the materials themselves and nothing else.

Head-to-Head on the Same Attic

Factor	Cellulose	Blown fiberglass
R-value per inch	3.5	2.5
Depth to R-49	14.0 in	19.6 in
Bags (1,500 sq ft, R-49)	50	40
Material cost	$550 to $800	$1,400 to $2,000
Installed range	$1,800 to $3,600 (likely $2,700)	$1,500 to $3,000 (likely $2,250)
Settling	Settles a little in the first year; blow slightly deep	Does not settle
Moisture	Absorbs water if a leak sits on it	Does not absorb water easily
Fire resistance	Treated with borate; fire resistant	Non-combustible glass fiber

The table shows the trade-off clearly. Cellulose wins on material cost by a wide margin: $550 to $800 against $1,400 to $2,000 for the same attic. But the installed cost is closer, $1,800 to $3,600 against $1,500 to $3,000, because cellulose needs 50 bags against 40, and blowing 50 bags takes more time. The likely midpoints are $2,700 for cellulose and $2,250 for fiberglass, a difference of about $450 on a 1,500 square foot attic. That is real money, but it is not the whole story, because the two materials behave differently after the install.

Cellulose: The Value Pick

Cellulose is made from recycled paper, mostly old newsprint, treated with borate for fire and pest resistance. Its big advantage is R-value per inch: 3.5 against 2.5 for fiberglass, which means it reaches R-49 in 14.0 inches instead of 19.6. That matters in a low attic, where the extra 5.6 inches of fiberglass may not fit under the roof deck at the eaves without blocking the soffit vents. It also packs tighter than fiberglass, filling small gaps around wires and pipes more completely, which gives a slightly better real-world result than the R-value alone suggests.

The weakness of cellulose is settling. It compresses a little in the first year, so a crew that blows exactly 14.0 inches will have 13 inches a year later, and the R-value drops with it. A good crew blows it slightly deep to allow for the settling, and the settled depth should be checked after a few months. The second weakness is moisture. Cellulose absorbs water if a roof leak sits on it, and wet cellulose loses its R-value and can grow mold. Any leak must be fixed before the install, and the attic should be checked for leaks afterward. If your attic has had a leak in the past, that is a point in favor of fiberglass.

Fiberglass: The Default Pick

Blown fiberglass is the default material for most insulation jobs in Lake County, and the reason is reliability. It does not settle, so the depth on the day of the install is the depth you keep for decades. It does not absorb water easily, so a small leak does not ruin it the way it ruins cellulose. It is non-combustible, it does not feed mold, and crews have been blowing it for decades, so the install is fast and predictable. The blown-in insulation calculator opens with fiberglass as the default material for exactly that reason.

The weakness of fiberglass is the depth: 19.6 inches to reach R-49 is a lot of material, and in a low attic it may not fit at the eaves. The fix is to use cellulose in the tight bays, or to accept R-38 in those bays and R-49 in the open middle. The second weakness is the bag price: $35 to $50 a bag, against $11 to $16 for cellulose, which makes the material cost $1,400 to $2,000 on a 1,500 square foot attic. But the installed cost is lower than cellulose, $1,500 to $3,000 against $1,800 to $3,600, because the crew blows 40 bags instead of 50 and finishes faster. On a straight price comparison, fiberglass usually wins the installed total, and cellulose wins the material total.

Which One for Your Attic?

Choose cellulose if the attic is low and the extra depth of fiberglass will not fit, if the budget for the material matters more than the installed total, or if the attic has many small gaps that a denser material will fill better. Choose fiberglass if the attic has had moisture trouble, if you want a material that will hold its depth for decades without a second visit, or if the installed total is the number you care about. Either way, the target is the same R-49, and the finished attic performs the same. The material is a means to the target, not the target itself.

Two things matter more than the material choice. The first is air sealing: both materials need the gaps around pipes, wires, lights, and the hatch sealed before the insulation goes in, because insulation does not stop air. The second is the depth check: whichever material you choose, the crew should measure the finished depth in several places before they leave, and you should be there to watch them do it. A quote that names the material, the bag count, and the target R-value in writing is the quote to trust, and the fairness checker in the blown-in insulation calculator will tell you whether the total sits inside the typical installed range for your attic.

Related reading: blown-in insulation calculator, plus our guides on blown-in insulation vs batts and blown-in insulation R-value, and the roofing calculators hub.

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