Which Is Better for an Attic, Cellulose or Fiberglass Insulation?
For most attics in western Pennsylvania, cellulose is the stronger choice. It delivers more R-value per inch, packs densely enough to slow air moving through the layer, and fills the gaps around wiring and framing that leave cold spots. Fiberglass wins where depth is plentiful, where a past leak makes moisture a concern, or where you want a material that barely settles. Both are blown in loose-fill, and either one beats batt insulation in an existing attic, because loose-fill lands as a continuous blanket with no seams for air to slip through.
That’s the short version. The longer answer depends on your R-value target, the depth you have to work with, and what’s already up there.
What Is the R-Value of Blown-In Insulation?
R-value measures resistance to heat flow, and it’s rated per inch — so the material decides how deep the layer has to be.
| Material | R-Value Per Inch | Best For |
|---|---|---|
| Loose-fill cellulose | R-3.2 to R-3.8 | Attic retrofits, dense-pack walls |
| Loose-fill fiberglass | R-2.2 to R-2.7 | Attic retrofits |
| Fiberglass batt | R-3.1 to R-3.4 | Open joist bays, new walls |
| Mineral wool batt | R-3.7 to R-4.3 | Walls, sound control |
| Closed-cell spray foam | R-6.0 to R-7.0 | Rim joists, cathedral ceilings |
Most of our service area sits in climate zone 5, where ENERGY STAR recommends R-49 to R-60 in the attic. Reaching R-49 means roughly 14 inches of cellulose or 18 to 20 inches of loose-fill fiberglass. A fiberglass batt rated R-38 is about 12 inches thick — and only delivers R-38 where it sits flat and full-width against the framing.
That’s the point homeowners miss: rated R-value and installed R-value are different numbers. A batt compressed under a duct, cut short around a brace, or curled over a wire delivers a fraction of what the label says.
How Cellulose and Fiberglass Differ Once They're Installed
Both go in the same way. They behave very differently after that.
Cellulose is made largely of recycled paper treated with borate for fire and pest resistance. It’s denser, delivers more R per inch, and packs tightly enough to slow air movement through the layer — useful in a drafty older home. It settles more over time, which is why installers blow to a settled-depth target.
Fiberglass loose-fill is lighter, doesn’t absorb water as readily, and settles less. It needs more depth for the same R-value, so it’s a better fit where you have plenty of headroom above the joists and want a material that shrugs off an incidental leak.
For a typical attic in Mercer, Erie, or Mahoning County with lots of air leakage, cellulose usually wins. Where moisture history is a concern, fiberglass often makes more sense. Neither is a bad answer; the wrong depth is.
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Blown-In vs Batt Insulation: When Batts Still Make Sense
Batts aren’t obsolete. They’re the right call when:
– The framing is new, square, and on standard 16- or 24-inch centers.
– You’re insulating walls rather than a floor plane.
– You want to keep a decked storage area and need insulation between joists, not over them.
– The area is small enough that mobilizing a blower isn’t worth it.
The best attics we work in have both: batts between the joists from the original build, with blown-in over the top running perpendicular to bury the framing. That’s not a compromise — it’s how you stop thermal bridging through the joists.
How Does Blown-In Compare to Spray Foam?
Spray foam is a different category. Closed-cell foam air seals and insulates in one step and carries the highest R per inch on the list, but it costs considerably more and changes how the assembly works — you’re typically creating an unvented, conditioned attic instead of a vented one.
For a standard vented attic, blown-in insulation over a properly air-sealed ceiling reaches code-level R-value for far less. Foam earns its price in specific places: rim joists, cathedral ceilings with no room for a vent channel, and attics being turned into living space. If someone quotes foam for a plain vented attic, ask what problem it’s solving that depth wouldn’t.
Can You Add Blown-In Insulation Over Existing Batts?
Usually yes, and it’s one of the most cost-effective upgrades available. Three conditions:
- The existing insulation has to be dry and clean. Wet, matted, or mold-stained material comes out first — adding depth on top of a moisture problem just hides it. If your attic smells musty, start with what causes mold in an attic.
- Air seal the ceiling plane first. Insulation doesn’t stop air; it only slows conduction. Hatches, can lights, fan housings, and top plates get sealed before anything is blown.
- Don’t stack a faced batt on top. A second vapor retarder in the middle of the assembly traps moisture. New batts going over old ones should be unfaced.
Why Attic Insulation Is a Roofing Service
We install insulation because we keep finding the same three failures from the roof side. An under-insulated attic melts snow off the roof deck, and that meltwater refreezes at the eaves — which is exactly how ice dams form. An attic that’s also under-ventilated grows condensation on the underside of the decking, then mold, then soft sheathing that has to be replaced during the next roof job. And insulation blown carelessly over the soffit vents blocks the intake air the whole roof depends on, which is why baffles go in at every rafter bay.
Insulation, ventilation, and the roof are one system. Treating them separately is how a home ends up with new shingles over rotted decking.
FAQ'S
Roughly 14 inches of loose-fill cellulose or 18 to 20 inches of loose-fill fiberglass, depending on the specific product’s rated R-value per inch. ENERGY STAR recommends R-49 to R-60 for attics in climate zone 5, which covers most of western Pennsylvania and eastern Ohio. Measure from the top of the ceiling drywall, not from the top of the joists.
Cellulose settles noticeably in the first year or two, which is why it’s installed to a settled-depth target rather than an initial depth. Loose-fill fiberglass settles very little. Neither should drop below its rated R-value if it was installed correctly, but insulation that’s been walked on, stored over, or wetted by a leak will be well under spec.
In an existing attic, generally yes, because it fills irregular spaces and leaves no seams. Batts are competitive in new construction with clean, standard framing, and in walls. The material matters less than the installed depth and whether the ceiling below was air sealed first.
Not on its own. Mold needs moisture, and in an attic that moisture comes from air leaks carrying humid house air upward, blocked ventilation, or a roof leak. Insulation installed over an unsealed ceiling or across the soffit vents can make an existing moisture problem worse by trapping it, which is why sealing and ventilation come first in the sequence.
It will if nobody installs baffles. Loose-fill material naturally drifts toward the eaves, which is the exact spot where intake air has to enter. A baffle in each rafter bay holds that channel open so the insulation can go deep at the perimeter without choking the ventilation.
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