Cellulose vs fiberglass insulation
R-value per inch, settling, moisture and weight — how loose-fill cellulose and fibreglass differ, and how to size the bags for whichever you choose.
Side by side
| Cellulose (loose-fill) | Fibreglass (loose-fill) | |
|---|---|---|
| R-value per inch | ~R-3.5 | ~R-2.5 |
| Air resistance | Higher (denser) | Lower |
| Weight | Heavier | Lighter |
| Settling | More (planned for on the bag) | Less |
| Moisture | Absorbs; dries slowly | Does not absorb |
| Made from | Recycled paper, borate-treated | Spun glass fibre |
How to choose
Shallow attic or you want the best air resistance: cellulose — it reaches the R-value in less depth and resists air movement better. Weight is a concern, or the space can get damp: fibreglass — lighter on the ceiling below and unaffected by moisture. For most attic floors either is a sound choice; the install quality and total depth matter more than the brand on the bag.
On the "cellulose insulation calculator"
There is nothing special to calculate for cellulose specifically — the maths is the same area-to-bags method as any loose-fill. What changes is the coverage per bag at your R-value, which is printed on the cellulose you buy (and differs from fibreglass). Put the attic area and that coverage into the insulation calculator and it gives the bag count. See also blown-in vs batt and how to find your attic R-value.
Questions
Is cellulose or fiberglass insulation better?
Both work well; they trade off differently. Cellulose packs denser, resists air movement better and has a higher R-value per inch (~R-3.5 vs ~R-2.5 for loose-fill fibreglass), so it needs less depth for the same R-value. Fibreglass is lighter, does not absorb water, and does not settle as much. For an attic floor where depth is not a constraint, either is fine; cellulose edges ahead on air-sealing, fibreglass on weight and moisture.
Which has the higher R-value per inch?
Cellulose, at about R-3.5 per inch, versus about R-2.5 per inch for loose-fill fibreglass. So cellulose reaches a given R-value in less depth — useful in a shallow attic. Batts differ again; always use the coverage chart on your actual product.
Does cellulose settle more than fiberglass?
Yes. Cellulose settles more over time, which is why installers blow it above the target depth and the bag coverage already accounts for settled thickness. Fibreglass settles less. Either way, follow the bag’s stated coverage, which plans for settling.
How many bags of each will I need?
It comes from the coverage printed on the bag at your target R-value, which differs between the two. Enter your attic area and that coverage in the insulation calculator for whichever product you buy.