Quick answerBoth blown cellulose and blown fiberglass reach R-50. Cellulose is denser and reaches R-50 at about 14 inches. Blown fiberglass is lighter, doesn't absorb water, and needs about 18 inches. We choose based on your attic's moisture, ventilation and ceiling.
Side-by-side comparison
| Blown cellulose | Blown fiberglass | |
|---|---|---|
| Made from | Recycled paper, borate-treated | Spun glass |
| Approx. R-value per inch | R-3.5 to R-3.8 | R-2.5 to R-2.9 |
| Depth for R-50 | ≈ 14 in settled | ≈ 18 in (Owens Corning AttiCat chart) |
| Coverage at R-50 | Varies by brand | ≈ 37 sq ft per AttiCat bag |
| Weight on ceiling | Heavier | Lighter |
| Air movement through the layer | Lower | Higher |
| Moisture | Absorbs water; dries if ventilated | Doesn't absorb water |
| Settling | Settles a little; blown slightly deeper | Minimal |
Values are typical. The manufacturer's coverage chart on the bag governs the actual install.
When we recommend cellulose
- Dry, well-ventilated attics
- Older homes with lots of small air gaps, where a denser layer helps
- Attics with limited headroom, where less depth is needed for R-50
When we recommend fiberglass
- Attics with a history of moisture or condensation (after the cause is fixed)
- Older ceilings where extra weight is a concern
- Homeowners who prefer a non-paper product
What matters more than the material
Air sealing, baffles and even depth across the whole attic make a bigger difference than cellulose versus fiberglass. A perfectly chosen material blown over open air leaks and blocked soffits still performs poorly.
FAQ
Is cellulose or fiberglass cheaper?
Installed prices are similar for most GTA attics, typically $1.20–$1.75 per sq ft. Cellulose needs more bags by weight; fiberglass needs more depth.
Can you blow fiberglass over cellulose or vice versa?
Yes, as long as the existing layer is dry, clean and free of mold and pests.
Which lasts longer?
Both can last for decades if they stay dry and undisturbed. Moisture, pests and foot traffic are what shorten their life.