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Insulation Calculator
Calculate rolls, bags, or spray foam coverage by R-value and climate zone — with code compliance check
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Insulation Calculator
How to Calculate Insulation
Insulation is measured by R-value — its resistance to heat flow. The higher the R-value, the better the thermal performance. The amount of insulation you need depends on your climate zone, the application (attic, wall, crawl space), and the type of insulation you choose.
Batts & Rolls Formula
Rolls Needed = ⌈(Area ÷ Coverage per Roll) × 1.10⌉
Blown Insulation Formula
Bags Needed = ⌈(Area ÷ 1,000) × 21 bags × 1.10⌉
Spray Foam Formula
Inches Required = ⌈Target R-Value ÷ R-Value per Inch⌉
Board-Feet = Area × Inches Required
Rolls Needed = ⌈(Area ÷ Coverage per Roll) × 1.10⌉
Blown Insulation Formula
Bags Needed = ⌈(Area ÷ 1,000) × 21 bags × 1.10⌉
Spray Foam Formula
Inches Required = ⌈Target R-Value ÷ R-Value per Inch⌉
Board-Feet = Area × Inches Required
DOE Recommended R-Values by Climate Zone
| Climate Zone | Attic Floor | Wall Cavity | Crawl Space / Basement |
|---|---|---|---|
| Zone 1–2 (FL, HI, TX Gulf) | R-30 | R-13 | R-13 |
| Zone 3–4 (VA, TN, NM, OR) | R-38 | R-13 to R-15 | R-19 to R-25 |
| Zone 5–6 (IL, CO, PA, WA) | R-49 | R-15 to R-21 | R-25 |
| Zone 7–8 (MN, MT, AK) | R-60 | R-21 | R-25 |
Insulation Type Comparison
| Type | R-Value per Inch | Best For | Material Cost (sq ft) |
|---|---|---|---|
| Fiberglass Batts | R-2.9 to R-3.8 | Walls, attic floors, floors over crawl spaces | $0.30–$0.65 |
| Blown Fiberglass | R-2.2 to R-2.9 | Attics, existing wall cavities | $0.50–$1.00 |
| Spray Foam Open-Cell | R-3.5 to R-3.9 | Interior walls, rim joists, soundproofing | $0.44–$0.65/in |
| Spray Foam Closed-Cell | R-6.0 to R-6.9 | Rooflines, exterior walls, crawl spaces | $1.00–$2.00/in |
Installation Tips
- Seal all air gaps with caulk or spray foam before installing batt insulation — air infiltration reduces effective R-value significantly
- Wear proper PPE (gloves, goggles, respirator) when handling fiberglass batts or blown insulation
- In attics, maintain a 1-inch gap above soffit vents to preserve airflow and prevent moisture buildup
- Faced batts (with kraft paper or foil) should have the facing toward the warm-in-winter side of the assembly
- Spray foam requires temperature conditions above 40°F for proper adhesion and cure — do not apply in freezing conditions
? Frequently Asked Questions
What R-value do I need for my attic?
R-value requirements depend on your climate zone. Zone 1–2 (warm climates like Florida) requires R-30. Zone 3–4 (mixed climates like Virginia) needs R-38. Zone 5–6 (cold climates like Illinois) requires R-49. Zone 7–8 (very cold like Minnesota) requires R-60. Our calculator checks whether your selected insulation meets these code minimums automatically.
How many bags of blown insulation do I need per 1,000 sq ft?
For blown fiberglass at R-38, you'll need approximately 21 bags per 1,000 sq ft. Always check the coverage chart on the bag for your specific brand, as actual coverage varies. Add 10% to your estimate for waste and settling over time.
What is the difference between open-cell and closed-cell spray foam?
Open-cell spray foam expands more and is softer, achieving about R-3.7 per inch, and is typically used for interior walls and soundproofing. Closed-cell spray foam is denser and achieves R-6.5 per inch — nearly double — making it ideal for rooflines, exterior walls, and anywhere moisture resistance is critical. Closed-cell also adds structural rigidity to walls.
Can I install insulation over existing insulation?
Yes, in most cases you can add new insulation on top of existing insulation in an attic. The combined R-values add together. Do not cover existing insulation with faced batts — use unfaced batts or blown-in insulation instead. Check that existing insulation is dry and free of mold before adding more.
How many rolls of R-30 fiberglass batts do I need for 1,200 sq ft?
R-30 fiberglass batts typically cover about 24 sq ft per roll. For 1,200 sq ft, you'd need 1,200 ÷ 24 = 50 rolls, plus 10% waste = 55 rolls. Our calculator handles this arithmetic and flags whether R-30 is sufficient for your climate zone.
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💡 Pro Tips
Always add 10% extra to material estimates to account for waste, cuts, and mistakes.