Fiberglass vs cellulose vs mineral wool (R/inch, cost, use)
For batts and loose-fill, three materials cover most jobs: fiberglass, cellulose and mineral wool. They hit R at different depths and different prices — and each has a spot where it clearly wins.
Same target R, different depth
Depth for a target R is target R ÷ R-per-inch, so the higher-R/inch material needs less thickness. For an R-49 attic over 1,000 ft²:
| Material | R/inch | Depth for R-49 |
|---|---|---|
| Fiberglass batt | ~3.2 | ~15.3 in |
| Cellulose | ~3.5 | ~14.0 in |
| Mineral wool | ~3.1 | ~15.8 in |
Close on depth — the real differences are cost, fire/water behavior and air-tightness. Compare depth and your own priced columns in the fiberglass vs cellulose vs mineral-wool tool, and check R/inch in the R/inch reference.
Cost: on your own numbers
cost = area × your $/ft² per material. Labeled installed bands as a sanity check: fiberglass ~$0.80–$2.40/ft², cellulose ~$1.00–$2.60, mineral wool ~$1.40–$4.00. Fiberglass is usually cheapest, mineral wool the priciest. Enter your quoted price — the site keeps no price list.
Where each one wins
- Fiberglass — cheapest, lightest, everywhere-available. Batts for open framing, blown for attics. Weak spots: it air-passes (loses performance in wind-washed, leaky attics) and irritates skin/lungs on install.
- Cellulose — recycled paper, dense, resists air movement better than fiberglass, and treated for fire and pests. Great for dense-pack retrofits and attics. Weak spots: heavier (check ceiling load), can settle if not stabilized, absorbs water if it gets wet.
- Mineral wool (Rockwool) — fire-resistant (melts well above 1,000°F), water-shedding, dimensionally stable, and the best of the three for sound. Batts hold their shape and friction-fit cleanly. Weak spot: costs more.
Beyond R/inch: the properties that decide it
| Property | Fiberglass | Cellulose | Mineral wool |
|---|---|---|---|
| Cost | Lowest | Low–mid | Highest |
| Fire resistance | Non-combustible | Treated | Best |
| Water behavior | Dries | Absorbs | Sheds |
| Air resistance | Poor (batts) | Better (dense) | Fair |
| Sound | Fair | Good | Best |
The thing R/inch doesn't tell you
All three are air-permeable — none is an air barrier. A rated R-49 in a leaky attic underperforms because air moves through and around it. Air-seal first, install to full loft without gaps, and the material choice comes down to budget, fire/water exposure and whether you care about sound. For an interior sound wall specifically, mineral wool is worth the premium — count it in the acoustic insulation calculator.
What to weigh before you buy
- Budget vs. fire/water/sound needs for the specific location.
- Open framing (batts) vs. closed cavity or attic (loose-fill / dense-pack).
- Ceiling load if you're piling deep cellulose.
- That you'll air-seal regardless of material.
Worked example: the same attic, priced three ways
Take that 1,000 ft² attic to R-49. Blown fiberglass at ~$1.40/ft² installed ≈ $1,400; cellulose at ~$1.70/ft² ≈ $1,700 (denser, more material, a little more labor); mineral-wool batts at ~$2.60/ft² ≈ $2,600. All three hit R-49; the spread is real money, and it usually decides the attic in favor of loose-fill fiberglass or cellulose — you rarely pay for mineral wool on an open attic floor. Where mineral wool earns its keep is a fire-rated assembly or an interior sound wall, not a hidden attic. Put your own quoted prices in the comparison tool.
Weight, settling and the ceiling below
Density cuts both ways. Cellulose is roughly 2–3× the weight of blown fiberglass for the same job, so on a marginal or older ceiling — especially deep piles going for R-49–R-60 — check that the drywall and framing can carry it, and watch loose-fill depth over a 1/2" ceiling on 24" centers. Fiberglass barely settles; un-stabilized cellulose can settle ~10–20% (blow to the installed thickness to land at target); mineral-wool batts don't settle at all. If a ceiling has had past water stains, find and fix the leak before you bury it — wet cellulose mats down and loses R permanently, wet fiberglass dries but sheds performance while damp, and mineral wool sheds the water.
Health, handling and irritation
Field comfort is a real selection factor. Fiberglass itches and sheds respirable fibers — long sleeves, gloves and an N95 minimum. Cellulose is dusty going in (borate treatment); a respirator and eye protection, and expect a haze in the attic for the blow. Mineral wool cuts cleanly with a serrated blade and sheds far less, which is one reason installers like it in occupied retrofits. All three are non-food, non-structural — the choice is budget, fire/water exposure, sound, weight and how much you value a clean install.
Quick numbers to leave with
- Depth for R-49: fiberglass batt ~15.3", cellulose ~14.0", mineral wool ~15.8".
- Bands: fiberglass ~$0.80–$2.40, cellulose ~$1.00–$2.60, mineral wool ~$1.40–$4.00/ft².
- Cellulose denser, resists air movement, heavier — check ceiling load and keep it dry.
- Mineral wool best for fire and sound; worth the premium on rated or sound walls, not a hidden attic.
- None is an air barrier — air-seal separately regardless of material.
None of the three is wrong — match the material to the job, install it clean, and seal the air leaks that no R-value fixes.