Measure the attic floor, pick your climate zone and say what is up there now. You get the R-value to add, how deep to blow it and how many bags to buy, from the makers’ own coverage charts.
Attic insulation
Add R-60: 32 bagsof Owens Corning AttiCat, blown 20½″ deep over 1,000 sq ft · ENERGY STAR’s level for zone 4
Quick answer: ENERGY STAR says to add R60 to an empty attic in climate zones 4 to 8, R49 in zones 2 and 3 and R30 in zone 1 (R49, R38 and R25 over 3–4 inches already there). For 1,000 sq ft at R-60 that is 32 bags of Owens Corning AttiCat blown 20½″ deep, or 109 bags of Greenfiber cellulose blown 18⅜″ deep.
How to use the attic insulation calculator
- Measure the attic floor: its length and width, the same as the house’s footprint under it.
- Pick your climate zone (1 to 8, from the Department of Energy’s climate zone maps or your building department).
- Say what is up there now: nothing, 3–4 inches, or an R-value you know from its label.
- Pick fiberglass or cellulose and read the bags to buy and how deep to blow them.
The rules behind the numbers
How much: ENERGY STAR’s R-values
ENERGY STAR’s “Recommended Home Insulation R–Values” table, based on the 2021 International Energy Conservation Code, gives the R-value to add to an attic by climate zone: one column “If your attic is uninsulated”, one “If you already have 3–4 inches of insulation”. With another R-value already there, the calculator takes the empty attic’s level and subtracts what you have.
How many bags and how deep: the makers’ charts
Every bag of blown-in insulation prints a coverage chart. The calculator uses Owens Corning’s for AttiCat blown-in fiberglass (27.5 lb bags) and Greenfiber’s for its blow-in cellulose (19.05 lb bags): for each R-value, the bags per 1,000 sq ft and the thickness. Both say to install “both the minimum number of bags per 1,000 square feet and the minimum installed thickness”. An R-value between two rows takes the next row up.
Keep the vents open
The building code: insulation “shall not block the free flow of air”, with at least 1″ between it and the roof sheathing at the vent (IRC R806.3).
ENERGY STAR attic R-values by climate zone
| Climate zone | Empty attic: add | 3–4″ there: add |
|---|---|---|
| 1 | R30 | R25 |
| 2 and 3 | R49 | R38 |
| 4 to 8 | R60 | R49 |
Bags and depth per 1,000 sq ft
| R-value | AttiCat bags | AttiCat depth | Greenfiber bags | Greenfiber: blow / settled |
|---|---|---|---|---|
| R-30 | 14.6 | 10¾″ | 46.1 | 9⅝″ / 8⅝″ |
| R-38 | 19 | 13½″ | 61.1 | 12″ / 10¾″ |
| R-49 | 25 | 17″ | 83.5 | 15¼″ / 13⅝″ |
| R-60 | 31.5 | 20½″ | 108.1 | 18⅜″ / 16½″ |
Four worked examples
An empty 40 × 25 ft attic in zone 4. Add R-60: 32 bags of AttiCat, blown 20½″ deep.
The same attic in zone 5, with 3–4″ already there, in cellulose. Add R-49: 84 bags of Greenfiber, blown 15¼″ deep to settle at 13⅝″.
A 30 × 20 ft attic in zone 3 with R-19 in it. Zone 3’s level is R-49, so add R-30: 9 bags of AttiCat, 10¾″ deep.
Zone 1, 3–4″ already there. ENERGY STAR adds R25; AttiCat’s chart has no R-25 row, so R-26: 13 bags, 9½″ deep.
Common attic insulation mistakes
- Blowing to a depth and stopping. The makers want both the bags and the thickness: fewer bags at the same depth is fluffed, lower R-value.
- Blowing cellulose to its settled depth. Blow it to the chart’s installed thickness; it settles to the other number.
- Burying the soffit vents. Leave the air path at the eaves open (IRC R806.3).
- Comparing bags between products. At R-60, 1,000 sq ft takes 31.5 bags of AttiCat and 108.1 of Greenfiber cellulose.
Venting the attic too? The free attic ventilation calculator gives the vent area the code asks for. Turning the attic into a room? The free 2D planner lays it out to scale.
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Attic insulation questions
How much attic insulation do I need?
ENERGY STAR: add R60 to an empty attic in zones 4 to 8, R49 in zones 2 and 3, R30 in zone 1. Over 3–4 inches already there, add R49, R38 and R25.
What R-value is recommended for an attic?
R60 in climate zones 4 to 8, R49 in zones 2 and 3 and R30 in zone 1, the amounts ENERGY STAR adds to an uninsulated attic, from the 2021 IECC.
How many bags of blown-in insulation do I need?
The attic’s area in thousands of square feet × the bag’s chart: at R-49, 25 bags of AttiCat or 83.5 of Greenfiber cellulose per 1,000 sq ft, so a 1,200 sq ft attic takes 30 or 101 bags.
How thick is R-38 insulation?
13½″ of AttiCat blown-in fiberglass; Greenfiber cellulose is blown 12″ deep and settles to 10¾″.
How thick is R-60 insulation?
20½″ of AttiCat; Greenfiber cellulose is blown 18⅜″ deep and settles to 16½″.
Does blown-in insulation settle?
Cellulose does: Greenfiber’s chart blows R-60 at 18.37″ and rates it at 16.53″ settled, about a tenth less. AttiCat’s chart gives one minimum thickness.
What is the R-value per inch of blown-in insulation?
From the makers’ R-60 rows: about R-2.9 an inch for AttiCat fiberglass (60 ÷ 20.5″) and R-3.6 an inch for Greenfiber cellulose once settled (60 ÷ 16.53″).