Blown-In Insulation Coverage Calculator
Estimate bags needed, depth, and cost for attic blown-in insulation projects.
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How to Use This Blown-In Insulation Calculator
Enter your attic or floor area in square feet, choose your insulation type (fiberglass, cellulose, or mineral wool), and drag the R-value slider to your target. The calculator auto-fills the typical bag coverage rate for your chosen product. Add an existing R-value if you're topping up rather than starting fresh, set a waste factor (10% is standard), and hit Calculate Coverage.
You'll get the exact number of bags to buy, the required settled depth, total estimated cost, and a visual comparison of common R-value targets.
Why This Matters
Heating and cooling costs account for roughly 50–70% of the average US home energy bill. The Department of Energy estimates that proper attic insulation alone can cut those costs by 10–50%. For a typical 1,500 sq ft attic, the difference between R-19 and R-49 can mean $300–$600 per year in energy savings — with payback on materials in as little as 2–4 years.
Blown-in insulation is ideal for attics because it conforms to irregular spaces, covers joists, and achieves a consistent depth without gaps. Contractors and experienced DIYers both use bag count as the primary planning metric. Buying too few bags means a second trip to the store mid-job; buying too many wastes money. A 1,200 sq ft attic targeting R-38 with cellulose, for instance, typically requires around 32–38 bags depending on bag size and settled depth — a $960–$1,140 material cost that's easy to miscalculate without a proper tool.
How It's Calculated
Step 1 — Net R-Value Needed: Net R = Target R − Existing R
Step 2 — Required Depth: Depth (inches) = Net R ÷ R-value per inch (varies by material)
- Fiberglass blown: ~2.2–2.7 per inch (we use 2.5)
- Cellulose blown: ~3.2–3.8 per inch (we use 3.5)
- Mineral wool blown: ~3.0–3.3 per inch (we use 3.1)
Step 3 — Base Bags: Base Bags = Area ÷ Coverage per Bag (adjusted for required depth vs. bag reference depth)
Step 4 — With Waste: Final Bags = Base Bags × (1 + Waste% ÷ 100), rounded up to nearest whole bag.
Bag coverage figures listed on manufacturer packaging assume a specific settled depth (usually 3.5"–4" for standard R-value). This calculator adjusts coverage proportionally for your target depth.
Tips & Common Mistakes
- Always round UP: It's better to have one extra bag than to run short with half the attic done. Most home improvement stores accept unopened bag returns.
- Account for existing insulation: Measure what's already there with a ruler before ordering. Every inch of existing R-value matters — topping up from R-19 to R-38 needs far fewer bags than starting from zero.
- Use depth markers: Install insulation depth rulers (free at many stores) before blowing to ensure consistent coverage across the attic.
- Don't block soffit vents: Maintain ventilation channels at the eaves — use baffles before blowing. Blocking vents causes moisture damage that defeats the purpose of insulating.
- Cellulose settles ~20%: Fresh cellulose is fluffy; it compresses over time. Your calculator already accounts for this via the settled R-per-inch figure, but buy the bags per the tool output, not your own visual estimate.
Frequently Asked Questions
What R-value do I need for my attic?
The DOE recommends R-38 to R-60 for most US attics, depending on your climate zone. Cold climates (zones 5–8, like the Midwest and Northeast) need R-49 to R-60; warmer zones (1–4, like the South and West) typically need R-38 to R-49. Check the DOE's Zone Map or your local energy code for the exact requirement.
How does cellulose compare to fiberglass blown-in?
Cellulose has a higher R-per-inch (~3.5 vs ~2.5 for fiberglass), so you need less depth to achieve the same R-value. It's made from recycled paper and is generally more eco-friendly. Fiberglass is moisture-resistant and doesn't settle as much. Cellulose tends to be cheaper per bag but may require more bags per square foot depending on the target R-value.
Can I blow in insulation myself (DIY)?
Yes — most home improvement stores (Home Depot, Lowe's) rent blowing machines for free when you purchase a minimum number of bags. A typical 1,200 sq ft attic can be completed in 4–6 hours with two people. Wear a respirator, safety glasses, and disposable coveralls, and seal all attic bypasses (gaps around pipes and wires) before starting.
Why use a waste factor?
Blown-in insulation doesn't go in perfectly — some material lands on joists rather than cavities, some escapes during setup, and uneven surfaces require a little extra. A 10% waste factor is standard for smooth attic jobs; increase to 15% for heavily obstructed attics with lots of mechanical equipment, ducts, or irregular framing.