The three most common cavity insulation materials in residential construction are fiberglass, mineral wool, and cellulose. Each has been used for decades, each has loyal advocates, and each performs admirably when installed correctly. The trouble is that marketing tends to flatten the differences, leaving homeowners to choose on price alone. A closer look reveals that these materials behave differently in fire, moisture, sound, and settling, and those differences should shape your decision.
Fiberglass: The Familiar Default
Fiberglass is made by melting sand and recycled glass and spinning it into fine fibers, which are then bound into batts or left loose for blowing. It is inexpensive, widely available, and lightweight, which is why it has been the default choice in North American homes for generations. Its thermal resistance sits around R-3.1 to R-3.4 per inch for standard batts, with higher-density and encapsulated versions reaching slightly higher.
The material’s greatest strength is also its weakness. Because it relies on trapped air within a loose web of fibers, fiberglass is sensitive to air movement. When air can wash through it, convective looping erodes its real performance, particularly in cold weather. Fiberglass installed in a leaky wall can disappoint, while the same product in an airtight, carefully filled cavity performs close to its rating. It does not absorb much water itself, but it provides no air-sealing benefit and offers little resistance to the spread of fire beyond being non-combustible.
Mineral Wool: Density and Fire Resistance
Mineral wool, sometimes called rock wool or stone wool, is spun from molten basalt rock and steel slag. The result is a denser, stiffer batt that holds its shape and resists slumping. Its R-value runs around R-3.7 to R-4.2 per inch, modestly higher than standard fiberglass, and its density gives it advantages that go well beyond thermal resistance.
The standout property of mineral wool is fire resistance. It can withstand temperatures well above a thousand degrees Celsius without melting, which makes it a favorite for fire-rated assemblies, around chimneys, and in multifamily party walls. The same density that resists fire also makes it an excellent sound absorber, dampening both airborne noise and the drumming of impact sounds. Mineral wool is hydrophobic, shedding liquid water while still allowing vapor to pass, so a wall that gets briefly wet can dry rather than trapping moisture. The trade-offs are higher cost and a heavier, slightly more labor-intensive batt to cut and fit.
Cellulose: Recycled and Dense-Packed
Cellulose is made primarily from recycled newspaper and cardboard, treated with borate compounds for fire and pest resistance. It is installed either as a loose fill in attics or, more impressively, dense-packed into wall cavities where it fills every void and resists settling. Its R-value sits around R-3.5 to R-3.8 per inch, comparable to the others, but its real advantage emerges from how thoroughly it fills space.
Dense-packed cellulose is remarkably good at reducing air movement within a cavity because the tightly packed fibers slow convective looping in a way that a fluffy batt cannot. This gives it strong real-world performance, especially in retrofits where it can be blown into existing closed walls through small holes. The borate treatment also makes it inhospitable to insects and rodents and contributes meaningful fire resistance. Its main concerns are moisture management, since cellulose can hold water and must be kept dry, and the dust generated during installation, which calls for proper masking and ventilation.
Comparing the Materials Side by Side
When you line up the three honestly, no single product wins on every metric. The right choice depends on which properties matter most for your specific wall, climate, and budget.
- For lowest upfront cost and easy availability, fiberglass is hard to beat, provided the cavity is air-sealed and the batts are installed without gaps or compression.
- For fire resistance, sound control, and moisture resilience, mineral wool justifies its premium, especially in shared walls and high-risk areas.
- For retrofits, air-tightening, and recycled content, dense-packed cellulose offers excellent real-world performance and an environmental story many homeowners value.
Installation Quality Outweighs Material Choice
It is tempting to obsess over which material is theoretically best, but decades of field experience point to a humbler truth: installation quality matters more than the material itself. A premium mineral wool batt jammed in with voids and gaps will lose to a cheap fiberglass batt that is cut precisely, fitted around obstructions, and paired with a well-sealed air barrier. The grading systems used by energy raters reflect this, penalizing even high-R products when gaps, compression, and misalignment appear.
This is why dense-pack cellulose often outperforms its rated R-value in practice, while batts of any kind frequently underperform theirs. The act of completely filling a cavity, leaving no channel for air to circulate, captures performance that the laboratory number cannot promise on its own.
Making the Decision
Start by identifying your priorities. If you are building new walls in a mild climate on a tight budget and you trust your air-sealing details, fiberglass remains a sensible, economical pick. If you are concerned about fire spread, want a quieter home, or are building in an area prone to wetting, mineral wool earns its place. If you are upgrading an older home without tearing open the walls, or you want the air-tightening bonus of a dense fill, cellulose is often the smartest path. Whatever you choose, budget for a careful installation, because that single factor determines whether your walls live up to the promise on the package.