Calculate how much insulation you need for walls, attics, and floors. Get results in batts, rolls, or bags of blown-in insulation based on your target R-value and area.
Doors to subtract
21 sqft each
Windows to subtract
15 sqft each
Typical for: 2×4 walls
Insulating a 1,200 square foot attic to R-38 with blown-in cellulose? You need approximately 34 bags of insulation. At $10–$14 per bag, that's $340–$476 in materials. Most big box stores rent the blowing machine for free when you buy 20+ bags — making this one of the most cost-effective DIY energy upgrades.
| Area | Zone 1–3 (South) | Zone 4–5 (Middle) | Zone 6–7 (North) |
|---|---|---|---|
| Attic | R-30 to R-49 | R-38 to R-60 | R-49 to R-60 |
| Walls (2×4) | R-13 | R-13 to R-15 | R-13 to R-15 |
| Walls (2×6) | R-19 to R-21 | R-19 to R-21 | R-19 to R-21 |
| Floors | R-13 | R-19 to R-25 | R-25 to R-30 |
It depends on the target R-value and your attic size. For a 1,000 sqft attic at R-38, plan on about 28 bags of cellulose or 25 bags of fiberglass. Check the coverage chart on the bag — it varies by manufacturer.
In southern climates (zones 1–3), R-30 to R-49. In northern climates (zones 6–7), R-49 to R-60. Check the Department of Energy's insulation guide for your specific zip code.
Yes, as long as the old insulation is dry and not moldy. Just lay the new material on top without a vapor barrier — adding a second vapor barrier can trap moisture.
Cellulose is denser, provides slightly better air sealing, and is made from recycled paper. Fiberglass is lighter, doesn't settle as much over time, and resists moisture better. Both work well in attics.
For blown-in cellulose at R-38, about 10–11 inches. For R-49, about 13–14 inches. Fiberglass blown-in needs to be about 25% thicker to hit the same R-value.
Typical Results
💡 If bag count seems very high, check you entered attic floor area, not total home square footage.