How Much Insulation for a 1,500 Sq Ft Attic?
Quick Answer
For a 1,500 sq ft attic targeting R-38 (the most common attic requirement), you need approximately 47 rolls of R-38 fiberglass batts or 45–60 bags of blown-in cellulose.
- R-38 fiberglass batts: ~47 rolls (15" wide × 25 ft)
- R-49 fiberglass batts: ~60 rolls (for cold climates)
- Blown-in cellulose (R-38): ~45–60 bags
- Spray foam (R-38): Not typical for attic floors — use batts or blown-in
Formula
Divide attic square footage by the coverage per roll (listed on packaging). For 16" OC joists, each R-38 roll covers about 32 sq ft.
Step-by-Step Calculation
- 1
Attic area
1,500 sq ft
- 2
R-38 batts (15" × 25 ft roll)
1,500 ÷ 32 sq ft per roll = 46.9 → 47 rolls
- 3
Blown-in cellulose
1,500 sq ft × 0.03–0.04 bags/sq ft = 45–60 bags
- 4
Add 10% for irregular spaces
47 rolls × 1.10 = 52 rolls
Insulation for 1,500 Sq Ft Attic by Type
| Insulation Type | R-Value | Depth | Quantity | Material Cost* |
|---|---|---|---|---|
| Fiberglass Batts | R-30 | ~10" | 40 rolls | $600–$1,000 |
| Fiberglass Batts | R-38 | ~12" | 47 rolls | $750–$1,200 |
| Fiberglass Batts | R-49 | ~16" | 60 rolls | $950–$1,500 |
| Blown-In Cellulose | R-38 | ~10.5" | 50 bags | $500–$800 |
| Blown-In Fiberglass | R-38 | ~14" | 55 bags | $650–$1,000 |
*Materials only. Professional installation adds $0.50–$1.50 per sq ft.
Frequently Asked Questions
What R-value do I need for my attic?
R-38 for climate zones 1–3 (southern US), R-49 for zones 4–5 (mid-US), and R-60 for zones 6–7 (northern US/Canada). Check the DOE climate zone map for your area.
Is blown-in or batt insulation better for attics?
Blown-in is generally better for existing attics — it fills gaps, irregular joist spacing, and around obstacles evenly. Batts are better for new construction with open, accessible joists. Cost is similar for both.
Can I add new insulation over old insulation?
Yes, you can add new unfaced insulation on top of existing insulation to increase R-value. Do not use faced (vapor barrier) insulation on top — only on the bottom layer. Remove old insulation first if it is wet, moldy, or pest-damaged.