Attic Insulation Calculator (Square Feet & Cost)
Calculate how much insulation your attic need and how much it will cost you.
Table of Contents
How much insulation do I need for a 1,000 sq ft attic?
For a 1,000 sq ft attic in a moderate climate zone (target R-49) topping up old fiberglass at R-19, you’d need about 37 bags of blown-in cellulose (roughly 8.6″ of added depth), or 25 rolls of rock wool batts (about 10″ of added depth). Your exact target R-value depends on your climate zone — southern states need R-38 to R-49, while northern states need R-49 to R-60. Enter your attic size and existing insulation below for exact numbers.
You need
37 bags
1,000 sq ft attic, cellulose, R-19 → R-49
- Additional depth needed8.6 in
- Total R-value after installR-49
- Estimated material cost$666
In this guide
Why “just add insulation” isn’t specific enough
You may have run into this before: you climb into the attic, see a thin, patchy layer of old insulation, and decide it’s time to add more. So you buy a stack of bags or rolls, guessing at the quantity, and either run out halfway through — leaving cold spots exactly where you stopped — or end up with extra material that sits unused because you overbought.
The right amount of insulation depends on three numbers working together: your attic’s square footage, the R-value you’re targeting for your climate, and how much R-value you already have up there. Skip any one of those and the “how many bags” question doesn’t really have an answer. This calculator ties all three together so you order exactly what the job needs.
How much insulation do I actually need?
Every attic insulation estimate comes down to the same three steps.
The 3-step method
- Find your target R-value. This depends entirely on climate zone — warm southern states target R-38 to R-49, while cold northern states target R-49 to R-60, per DOE recommendations.
- Subtract what you already have. If your attic has old fiberglass batts at roughly R-19, and your target is R-49, you only need to add R-30 worth of new material — not start from zero.
- Convert R-value to bags or rolls. Manufacturers print coverage on the package as “covers X sq ft at R-value Y.” Scale that number to your actual target R-value, and you have your shopping list.
The calculator above runs all three steps automatically — enter your attic size, pick your climate zone, and note what R-value is already up there.
A worked example
Let’s size a 1,000 sq ft attic (40×25 ft) in a moderate climate zone (target R-49), with old fiberglass insulation already at R-19, using blown-in cellulose.
Step-by-step
- Additional R needed: 49 − 19 = R-30.
- Cellulose runs about 3.5 R-value per inch, so added depth: 30 ÷ 3.5 ≈ 8.6 inches.
- A typical bag covers about 50 sq ft at R-19. Scaled to R-30 worth of new material: 1,000 ÷ 50 × (30 ÷ 19) ≈ 31.6 bags.
- Add a 15% settling and waste allowance: 31.6 × 1.15 ≈ 37 bags.
Switch to rock wool batts for the same job and the number changes — rock wool runs about 3.0 R-value per inch (slightly less than cellulose), so it needs a bit more depth, but comes in rolls that cover more area per package, landing around 25 rolls for the same attic.
Rock wool or cellulose?
What if you’re not sure which material fits your project better? Both are solid choices, but they suit different situations.
| Factor | Rock wool (mineral wool) | Cellulose (blown-in) |
|---|---|---|
| R-value per inch | ~3.0 | ~3.5 |
| Format | Batts/rolls — no equipment needed | Loose-fill — needs a blower (often free to rent with bulk purchase) |
| Fire resistance | Naturally fire-resistant, very high melting point | Treated with fire retardant, still combustible material at core |
| Best for | DIYers without a blower, irregular framing, fire-prone areas | Filling gaps around obstructions, topping up open attics |
| Settling over time | Minimal — rigid batts hold their shape | Settles ~15-20%, hence the buy-extra allowance |
Topping up existing insulation
What if your attic already has some insulation up there? Good news — you almost never need to remove it first. R-values are cumulative, so a layer of new material on top of old fiberglass, cellulose, or rock wool simply adds to what’s already there (as long as the old insulation is dry and undamaged). This is one of the most cost-effective energy upgrades available, since you’re only paying for the R-value gap, not a full re-install.
The one exception: if the existing insulation is wet, moldy, or has been compressed flat by foot traffic or storage boxes, it’s worth pulling that section before adding more — new insulation on top of a moisture problem just traps the issue underneath.
Ready to get your exact bag or roll count?
Run your attic size in the calculator above →Common mistakes to avoid
- ✕Confusing thickness with R-value. Different materials need different depths to hit the same R-value — always calculate from R-value, not just “how many inches looks about right.”
- ✕Forgetting the existing insulation. Buying a full R-49 worth of material when you already have R-19 up there means paying for insulation you don’t need.
- ✕Skipping the settling allowance for loose-fill. Blown cellulose settles 15-20% after installation — buying the bare minimum bag count leaves you under target within the first year.
- ✕Blocking soffit vents. Piling insulation over the eave vents cuts off attic airflow, which can lead to moisture problems and ice dams in cold climates — always keep baffles in place near the eaves.
Why trust this guide
R-value targets by climate zone follow U.S. Department of Energy recommendations. Coverage math is based on standard manufacturer coverage charts (bags/rolls covering a stated square footage at a reference R-value), scaled proportionally to your target, plus a settling and waste allowance consistent with the FTC R-Value Rule (16 CFR 460) for loose-fill insulation. Always confirm exact coverage on your specific product’s packaging before purchasing, and consult a local contractor for attics with unusual framing, moisture issues, or ventilation concerns.
Frequently Asked Questions
Related Article: Blown-in Attic Insulation Cost Calculator
Related Article: Attic Insulation Energy Savings Calculator
