Blown-In Insulation Calculator
Estimate insulation bags, inches added, and cost
Floor or attic area to be insulated
Target total R-value (e.g., R-38 for attics)
Current insulation depth (0 if none)
Inches Added
—
in
Insulation Bags
—
bags
Estimated Cost
—
USD
Disclaimer: Results are estimates based on standard formulas. Not a substitute for professional advice from a licensed contractor, engineer, or architect. Verify all calculations before making purchasing or construction decisions.
How to Use This Calculator
Blown-in insulation depth is driven by your target R-value. Subtracting existing depth prevents overfilling and keeps your material budget tight.
- Measure the attic or floor area in square feet
- Set your desired total R-value (R-38 to R-60 is common for attics)
- Enter the existing insulation depth (0 if starting fresh)
- Use a blowing machine for even coverage and consistent density
Expert Insights
Smart analysis for your Insulation project
R-Value to Depth
Fiberglass blown-in delivers about R-3.2 per inch. AI converts your target R-value into the exact inches of new material to add.
Bag Yield
A standard bag covers ~8 sq ft per inch of depth. The calculator sizes your order based on area and the inches needed.
Air Sealing First
Seal attic penetrations and recessed lights before blowing insulation. Air leaks cut effective R-value by up to 30% regardless of depth.
Calculation Formula
Inches Needed = Desired R-Value / 3.2 - Existing Depth
Bags = ceil(Area x Inches Needed / 8)
Cost = ceil(Bags x $9)
Blown-In Insulation Comparison
| Material Type | R per Inch | Cost per Bag | Settling | Best Use |
|---|---|---|---|---|
| Fiberglass (Loose-Fill) | 3.2 | $8-$10 | Low | Attics |
| Cellulose (Loose-Fill) | 3.5 | $9-$12 | Medium | Attics, walls |
| Mineral Wool | 3.3 | $12-$16 | Very Low | Fire-rated assemblies |
| Cellulose (Dense-Pack) | 3.8 | $11-$15 | Low | Walls, retrofits |
Authority References
Our calculations are based on guidelines and standards from these authoritative sources:
- North American Insulation Manufacturers Association (NAIMA) — Blown-In Insulation Guidelines. Learn more
- U.S. Department of Energy (DOE) — Recommended R-Values by Climate Zone. Learn more
- International Energy Conservation Code (IECC) — Insulation Requirements Table. Learn more
Frequently Asked Questions
How many inches of blown-in insulation do I need for R-38?
For fiberglass blown-in insulation at roughly R-3.2 per inch, R-38 requires about 12 inches. Cellulose at R-3.5 per inch needs about 11 inches for the same R-value.
How many square feet does a bag of blown-in insulation cover?
A typical bag of blown-in fiberglass or cellulose covers about 40-50 sq ft at 1 inch depth. Coverage varies by product density, so check the manufacturer's coverage chart.
Can I blow insulation over existing insulation?
Yes, as long as the existing insulation is dry and free of mold. Measure the current depth and R-value, then add only enough new material to reach your target R-value.