Blown-In Insulation Calculator
Blown-in insulation is the easiest way to top up an under-insulated attic, but coverage math is confusing: bags are rated in cubic feet, R-value depends on depth, and the blower machine's coverage chart rules the job.
Enter your attic's square footage and target depth. The calculator estimates bags, cost, and the approximate R-value you'll achieve (cellulose at roughly R-3.7 per inch, fiberglass blown at R-2.2–3.2 per inch).
14 in ≈ R-49 cellulose · R-38 ≈ 10–11 in cellulose
- Insulation volume
- 1,167 cu ft
- Bags needed
- 28 bags
- Approximate R-value
- ≈ R-39
- Material cost (est.)
- $420 – $700
Estimate based on cellulose (≈R-3.7/in, ~27 bags per 1,000 sq ft at R-38). Your product's bag label and blower coverage chart override all estimates — manufacturers state exact bags per square foot at each depth.
How This Calculator Works
Bags = (Area ÷ 1,000) × ~27 bags × (Depth ÷ 14 in) | R ≈ Depth × 2.8–3.7 per inch- Determine target depth: existing insulation + added depth must reach your R-goal (R-38 minimum in most US attics, R-49–R-60 recommended in cold climates).
- Estimate bags: a common cellulose product blows about 20–35 bags per 1,000 sq ft at R-38 (≈10–14 in settled).
- Scale bags by depth relative to the 14-inch benchmark.
- Estimate the R-value achieved: cellulose ≈ R-3.7 per inch, blown fiberglass ≈ R-2.2–3.2 per inch.
- Depth rulers: place a yardstick every ~300 sq ft so you can verify depth while blowing.
Example: A 1,000 sq ft attic topped to 14 in of cellulose: about 28 bags, ≈R-49 after settling, roughly $225–$550 in material.
Frequently Asked Questions
How many inches of blown-in insulation equals R-38?
For cellulose (R-3.7/inch), R-38 takes about 10–11 inches settled; for blown fiberglass (R-2.2–3.2/inch), expect 12–16 inches. Always use the settled depth from the bag's coverage chart — loose-blown depth fluffs up 15–20% and settles.
How much does blown-in insulation cost in 2026?
Bags of cellulose run roughly $15–$25 each (a 25 lb Greenfiber bag is about $16–$20 at big-box stores; premium lines reach ~$37). DIY material for a 1,000 sq ft attic at R-49 costs about $420–$700, with the blower machine usually free with a minimum bag purchase. Professional installation adds roughly $1–$2.50 per sq ft.
Can I blow insulation over existing insulation?
Yes — it's one of the best ROI projects in a home. Check that existing insulation is dry and unfaced (don't cover a vapor barrier with new fill), air-seal penetrations first, and keep insulation off recessed light fixtures and soffit vents using baffles.
How deep should attic insulation be for my climate?
US DOE guidance: R-38–R-49 for warm climates (zones 1–3), R-49–R-60 for most of the country, and R-60 for cold zone 7–8. In cellulose terms that's roughly 11–16 inches. Existing depth counts — measure and add what's missing.
Cellulose or blown fiberglass — which is better?
Cellulose (ground paper, borate-treated) settles denser: better R-per-inch, better air sealing, but heavier on ceilings. Blown fiberglass is lighter and itchier but slightly lower R per inch. Both cost similar per achieved R; pick on price and availability locally.
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Last updated: 2026-10-26