Insulation Calculator: Batt, Roll & Blown-In R-Value
Calculate batt rolls, blown-in bags, and target R-values for insulation.
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How to Use the Insulation Calculator: Batt, Roll & Blown-In R-Value
- 1Select application area (attic, exterior walls, interior walls, crawlspace).
- 2Enter length and width to find total square footage.
- 3Choose framing spacing (16-inch or 24-inch on-center) and target R-value.
- 4Review required batt rolls or blown-in cellulose bags.
Formula & How It Works
Insulation Material Calculation
Rolls = Net Framing Area / Roll Coverage. Blown Bags = Area / Sqft_per_Bag_at_Rvalue.
Worked Examples
Attic Top-Up to R-38
Input: 1,000 sq ft attic, blown-in fiberglass
Explanation: 1,000 sq ft / 38.5 sq ft per bag at R-38 = 26 bags.
2x4 Wall Framing R-13
Input: 400 sq ft wall, 16" OC batts (40 sq ft/pkg)
Explanation: 400 / 40 = 10 batt packages.
Basement Rim Joist
Input: 150 sq ft, R-19 rolls
Explanation: 150 / 50 sq ft per roll = 3 rolls.
Frequently Asked Questions
What does R-value mean?▼
R-value measures thermal resistance; higher R-values indicate greater insulating power and energy savings.
What R-value is recommended for an attic?▼
The US Department of Energy recommends R-38 to R-49 in southern climates and R-49 to R-60 in northern climates.
How many bags of blown-in insulation do I need for 1000 sq ft?▼
For an R-38 rating, about 25 to 30 bags of blown-in cellulose or fiberglass are typically required.
Can I install new insulation over old insulation?▼
Yes, as long as the old insulation is dry, mold-free, and unfaced (or has the vapor barrier removed).
What is the difference between 16" and 24" batts?▼
Batts are manufactured 15 inches wide for 16" on-center stud bays, and 23 inches wide for 24" on-center framing.
Is this calculation private?▼
Yes, all building measurements are computed locally in your web browser.
Quick Reference & Conversion Table
| Climate Zone | Attic Recommendation | Wall Recommendation | Floor Recommendation |
|---|---|---|---|
| Zone 1-2 (Hot / South) | R-30 to R-49 | R-13 to R-15 | R-13 |
| Zone 3-4 (Moderate) | R-38 to R-60 | R-13 to R-15 | R-19 |
| Zone 5-7 (Cold / North) | R-49 to R-60 | R-20 to R-21 | R-25 to R-30 |
⚠️ Common Pitfalls to Avoid
- •Installing compressed fiberglass into cavities (compressing insulation lowers its effective R-value).
- •Blocking attic soffit vents with blown-in insulation without installing baffles.
💡 Practical Real-World Uses
- •Upgrading attic insulation to meet Department of Energy recommendations and tax credits.
- •Insulating finished basement framing cavities.
- •Soundproofing home theater interior walls with mineral wool batts.
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