Fiberglass vs. Cellulose Insulation: Head-to-Head Comparison

Fiberglass and cellulose are the two dominant loose-fill insulation materials for attics. Both are machine-blown and both perform well when installed to the right depth, but they behave differently once they are in place. The honest answer to "which is better" is that it depends on your attic conditions, moisture exposure, and budget — not on a single winner.

All cost statements below are editable planning assumptions, not verified national averages. Use them to frame a decision, then confirm with local quotes.

The table summarizes the practical differences as planning guidance:

Note how cost cuts both ways: fiberglass can be cheaper per bag yet need more bags, while cellulose can cost more per bag but reach the target in fewer. Compare total material for your R-target, not the per-bag price.

Your target R-value is not a universal number — it is set by your climate zone and local code, so confirm it with your building department. Once you know it, the material's R-per-inch sets the depth: the lower R-per-inch of fiberglass means more installed inches than cellulose for the same target.

Follow the bag coverage chart either way. With cellulose, remember it settles — coverage charts give a higher initial blow depth so the settled depth still meets the target. With fiberglass, depth is more stable but you still must blow to the charted minimum and avoid compressing it.

• Tight depth / want fewer inches: cellulose, for its higher R per inch.
• Humid attic or any moisture risk: lean fiberglass, since cellulose is more prone to clumping and mold if it gets wet repeatedly.
• Air leakage and small gaps: dense cellulose fills nooks tightly, though neither loose fill replaces a proper air-sealing pass.
• Lowest embodied environmental impact: cellulose, with its high recycled content.
• Lightweight on a marginal ceiling, or pest/fire simplicity: fiberglass.

In many attics either material works; the deciding factor is usually moisture exposure and how much depth you have to work with.

The material choice does not change the safety gates. Regardless of fiberglass or cellulose, stop and bring in a qualified professional if you find:

• Pre-1978 homes: possible asbestos or lead in older materials needs assessment before you disturb anything.
• Knob-and-tube wiring: burying this older wiring in loose fill is a fire hazard — have an electrician evaluate or replace it first.
• Moisture or mold: fix the water source before insulating, and favor fiberglass only after the area is dry; neither material belongs over an active leak.

Insulation rarely needs a permit, but the hazard check comes before any material decision.

• Picking by per-bag price instead of total bags needed for your R-target.
• Blowing cellulose to the final settled depth and ending up below target after it settles.
• Using cellulose in a chronically damp attic, where it can clump and lose performance.
• Assuming either loose fill air-seals the attic — it does not replace sealing penetrations.
• Treating R-per-inch and settling figures as exact; they are approximate planning values that vary by product.

Both materials are proven; the limitation is that neither performs to its rated R if it is compressed, under-blown, or sitting over air leaks.