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.
Pro Tips
- For 1,500 sq ft of attic at R-38, blown-in cellulose is almost always faster and cheaper than fiberglass batts. A two-person crew with a blower machine (free with purchase of 20+ bags at most big-box stores) can insulate this area in 4–5 hours. Batts take 8–10 hours.
- Before adding insulation, seal all air leaks: top plates, plumbing penetrations, recessed light cans, and the attic hatch. Air sealing provides 2–3× the energy savings per dollar compared to adding insulation alone.
- Install attic baffles ($1–$2 each) at every rafter bay along the eaves to maintain soffit ventilation. Insulation that blocks soffit airflow causes ice dams in winter and moisture damage year-round.
- If your attic already has R-19 fiberglass batts, add blown-in cellulose on top to reach R-38. You do not need to remove the existing batts — layering is standard practice and the cellulose fills gaps the batts missed.
Key Cost Factors
- Blown-in cellulose for 1,500 sq ft at R-38 requires 45–60 bags at $11–$15 per bag = $495–$900. Fiberglass batts cost $1.00–$1.50 per sq ft × 1,500 = $1,500–$2,250. Cellulose is 40–60% cheaper in materials and faster to install for attic floor applications.
- Professional attic insulation for 1,500 sq ft runs $1.50–$3.00 per sq ft ($2,250–$4,500 installed). DIY blown-in cellulose with the free blower rental drops this to $500–$900 in materials only — one of the best ROI home improvements with energy savings of $200–$400 per year.
Common Mistakes to Avoid
- Insulating to the wrong R-value. Attic targets vary by climate zone (R-38 to R-60). Using a milder-climate R-value in a cold region wastes energy; check your zone before buying.
- Blocking the soffit vents. Packing insulation into the eaves cuts off attic ventilation, causing moisture buildup and ice dams. Install baffles to keep the airflow channel open.
- Ignoring air sealing first. Blowing insulation over unsealed gaps, wire penetrations, and top plates lets air bypass the insulation. Seal leaks before adding R-value.
- Compressing batts to fit. Stuffing thick batts into a shallow space crushes the air pockets that make them work, reducing the actual R-value well below the label.
Related Project Considerations
- Decide blown-in versus batts. Blown cellulose or fiberglass fills irregular attic framing and around obstructions better than batts, and it is faster over 1,500 sq ft.
- Protect recessed lights. Older non-IC-rated recessed fixtures need clearance from insulation to avoid a fire hazard. Identify and box these out, or upgrade to IC-rated fixtures.
- Seal the attic hatch. An uninsulated attic access is a major heat leak. Add weatherstripping and a rigid insulation cover as part of the project.
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 and Canada). Check the DOE climate zone map for your area — your local code may exceed these minimums. Most existing homes have R-19 or less in the attic, so adding insulation to reach R-38 or higher is one of the most cost-effective energy upgrades available.
Is blown-in or batt insulation better for attics?
Blown-in is generally better for existing attics — it fills gaps around wiring, plumbing, and irregular joist spacing evenly without cutting or fitting. Batts are better for new construction with open, accessible, evenly spaced joists where you can lay them flat without compression. Cost per sq ft is similar for both at the same R-value, but blown-in installation is faster on large attic floors, saving on labor.
Can I add new insulation over old insulation?
Yes, you can add new unfaced insulation on top of existing insulation to increase the total R-value — R-values are additive. Do not use faced (vapor barrier) insulation as the top layer — only on the bottom layer closest to the living space. Remove old insulation first if it is wet, compressed, moldy, or shows signs of pest damage. Adding new insulation over damaged material traps moisture and makes the problem worse.