Fiberglass vs cellulose vs mineral wool calculator
Three common insulations, one target R — but they do not stack the same. This lays fiberglass, cellulose and mineral wool side by side: the depth each needs to hit your R, and the cost from the prices you type in.
Calculator
To reach R-49 you need about 15.3" fiberglass, 14.0" cellulose or 15.8" mineral wool. Cellulose is denser with less air movement, mineral wool is fire/water resistant, fiberglass is cheapest — a labeled note, not a verdict. Enter your own $/ft² for the cost columns.
Formula
depth = target_R ÷ R_per_inch · cost = area × $/ft²
Same target R, three R/inch values → three depths. The cheaper R/inch material simply takes more inches to get there, so the choice is depth (does it fit?) against installed cost (what does your quote say?), not a headline R alone.
Worked example
Take an attic to R-49 over 1,000 ft². Fiberglass batt at 3.2 R/inch needs 49 ÷ 3.2 = 15.3 inches; cellulose at 3.5 needs 49 ÷ 3.5 = 14.0 inches; mineral wool at 3.1 needs 49 ÷ 3.1 = 15.8 inches. Barely two inches separate them — so on an open attic the decision usually comes down to the cost columns and the practical notes below, not the depth.
Which one, from the field
The tool gives you depth and cost; here is how a crew actually picks:
- Fiberglass is cheapest and everywhere, in batts or blown. Its weak spot is install quality — gaps, compression and air movement gut the rated R fast. Great value if it is fitted tight and full-depth.
- Cellulose is denser, packs into odd cavities and slows air movement, so it often performs closer to its rating in a real attic. It is heavier and settles a little, so blow it a touch deep. Recycled paper, treated for fire.
- Mineral wool (Rockwool) costs the most per foot but is fire and water resistant and the best acoustic performer of the three — the reason it shows up on interior and party walls. See the acoustic calculator for sound work.
Measure first: confirm the cavity or joist depth can actually hold the inches the tool reports — a 2×6 wall (5.5") will not take 15 inches of anything. For a wall, work to the cavity R with the thickness calculator; for an attic, depth is free, so chase the cheapest fitted R. These are labeled trade-offs, not a verdict — get itemized quotes before you commit.
Reference table
| Material | Labeled R per inch | Depth for R-30 | Depth for R-49 |
|---|---|---|---|
| Fiberglass batt | R3.1–R3.4/in | 9.2" | 15.1" |
| Cellulose | R3.2–R3.8/in | 8.6" | 14.0" |
| Mineral wool (Rockwool) | R3.0–R3.3/in | 9.5" | 15.6" |
Depths use the band midpoint. R/inch is a labeled typical — confirm the rated value on your product’s data sheet, and remember cost turns on your local $/ft², not the R alone.
Frequently asked questions
Is cellulose or fiberglass better for an attic?
Both hit the same R at similar depth; cellulose is denser and resists air movement, so it often performs closer to its rating in a real attic, while fiberglass is cheaper and lighter. Run your prices through the cost columns and weigh the field notes — neither is universally "better".
Why does mineral wool cost more?
You pay for what it does: mineral wool is fire and water resistant and the best acoustic insulator of the three, which is why it lands on interior and party walls. For a plain thermal attic job the cheaper materials usually win.
How thick does each need for R-49?
At the labeled typicals: about 15.3 inches of fiberglass batt, 14.0 inches of cellulose or 15.8 inches of mineral wool. Change the R per inch to match your product and the depths update.
Are these installed or material prices?
Whatever you type. Enter installed $/ft² to compare finished jobs, or material-only to compare products — just keep all three columns on the same basis, and get itemized written quotes from licensed, insured contractors before you buy.