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How to Calculate Blown-In Insulation
Blown-in insulation bags list a coverage table on the label showing how many square feet one bag covers at different depths (and corresponding R-values). The calculation is simple: Area / Coverage Per Bag.
| Type | R-38 Coverage/Bag | Depth at R-38 | R per Inch |
|---|---|---|---|
| Cellulose | ~40 sq ft | ~10.5 in | 3.6–3.8 |
| Fiberglass blown-in | ~50 sq ft | ~12 in | 2.2–2.7 |
| Mineral / Rock wool | ~45 sq ft | ~10 in | 3.0–3.3 |
Frequently Asked Questions
Divide total square footage by the coverage per bag at your R-value target listed on the bag. A 25-lb bag of cellulose covers about 40 sq ft at R-38. For a 1,200 sq ft attic: 1,200 / 40 = 30 bags.
The DOE recommends R-38 to R-60 for attics depending on climate zone. Zone 1-2 (South): R-30 to R-49. Zone 3-4 (Central): R-38 to R-60. Zone 5-7 (North): R-49 to R-60. Check the DOE ZIP code lookup tool.
Yes. You only need to add enough to reach your target R-value. Enter your current R-value in the calculator above — it will calculate only the additional depth needed.
Cellulose (recycled paper) has higher density (3.6 R/inch) and is better at blocking air infiltration. Fiberglass (2.2–2.7 R/inch) is lighter, moisture-resistant, and will not settle as much over time. Cellulose is more eco-friendly.
At R-49, cellulose needs about 13–14 inches, fiberglass needs about 18–20 inches. At R-38: cellulose ~10.5 in, fiberglass ~12–15 in. Measure your current insulation depth to calculate what you already have.
Yes. Most home improvement stores rent blower machines for free or low cost when you buy a minimum number of insulation bags. Professionals typically charge $0.50–$2.00 per sq ft installed.