Attic Insulation Calculator
This free attic insulation calculator tells you how much insulation your attic needs to reach the recommended R-value — for either blown-in loose-fill or batts. Enter the attic floor area, set the target by IECC climate zone (or type an R-value), and it returns the whole-bag count with a waste allowance already added, using each product's FTC-mandated coverage chart or bag label.
Attics are their own job, so the calculator handles what a generic insulation estimate misses: it subtracts any existing insulation so you only top up the difference, reports the settled depth and the deeper blow-to depth for loose-fill separately, and sizes the eave baffles you need to stop insulation from choking the soffit vents. It also flags when a deep cellulose fill would overload a thin ceiling.
Everything is grounded in the ENERGY STAR / 2021 IECC recommended attic R-values (R-30 to R-60 by zone), the FTC R-value Rule (16 CFR 460), ASTM C739/C764/C665, IRC R806 attic ventilation, and USG ceiling dead-load guidance. Quantities only — no pricing, no signup.
Attic Insulation Calculator
Find how much insulation your attic needs to hit the code R-value — bag count for blown-in or batts, the depth to blow to, and the eave vent baffles to keep the soffits clear. Free, no signup.
How you're insulating
Blown-in loose-fill is the attic default; batts drop between the joists.
Attic area
The net floor area you're covering — roughly your home's footprint.
Target R-value
How much insulation you want — by climate zone, or a value you enter.
Not sure of your zone? Use the climate zone calculator to look it up by ZIP. Targets follow the ENERGY STAR / 2021 IECC recommendation; local code may be lower.
Existing insulation
If there's already insulation up there, we subtract its R-value so you only add what's missing.
Material & waste
Fiberglass is lighter and settles less; cellulose reaches the R-value in less depth.
Coverage comes from the selected product's FTC bag chart. Bags always round up. 5–10% overblow covers job variation.
Eave ventilation baffles
Baffles keep insulation out of the soffit vents — one per rafter bay along each vented eave.
Measure the eave runs that have soffit vents (usually the two long sides). Rough guide for a square attic: about 2 × √(area) ≈ 63 ft.
Attic depth chart — Owens Corning ProPink L77
Bags per 1,000 sq ft and the depth to blow at each attic R-value, straight from the manufacturer's FTC coverage chart (16 CFR 460.12). Change the product above to see its chart.
| R-value | Coverage (sq ft/bag) | Bags per 1,000 sq ft | Settled depth | Blow to |
|---|---|---|---|---|
| R-13 | 184.6 | 5.4 | 4.8" | 4.8" |
| R-19 | 125 | 8 | 7" | 7" |
| R-30 | 77 | 13 | 11.1" | 11.1" |
| R-38 | 59.9 | 16.7 | 14.1" | 14.1" |
| R-49 | 45 | 22.2 | 18.1" | 18.1" |
| R-60 | 35.8 | 28 | 22.2" | 22.2" |
Coverage is "maximum net" (framing excluded). Cellulose is rated at settled thickness, so its blow-to depth is deeper than the settled depth. Charts are periodically revised by the manufacturer.
Why the material changes the depth
An attic's R-value is a depth problem. The bag count comes off the coverage chart, but how deep you build depends on the material's R-per-inch. Every column below is the same R-value — the higher a material's R-per-inch, the less depth it takes to get there. That's why blown fiberglass sits deeper than blown cellulose for the same attic R, and why an R-60 attic can be surprisingly thick.
Calculation Formulas
Only add the insulation the attic is missing. Existing R is estimated from the current settled depth times the material's R per inch.
Example:
Target R-49 minus 3" of old fiberglass (3 × 2.7 ≈ R-8) = R-41 to add.
The FTC coverage chart gives the minimum bags per 1,000 sq ft of attic at each R-value. Scale by area, add an overblow allowance, and round up.
Example:
1,000 sq ft at R-49 with Owens Corning L77 = 22.2 bags/1,000 × 1.10 ≈ 25 bags.
Attic batts are bought by the bag rated at a fixed R-value and coverage. Take net attic area, add waste, and divide by the square feet per bag printed on the label.
Example:
1,000 sq ft at 10% waste ÷ 40 sq ft/bag ≈ 28 bags.
The finished, settled thickness that delivers the R-value. Fiberglass loose-fill is ~2.5–3.0 R/inch settled; cellulose ~3.6–3.8.
Example:
R-49 ÷ 2.7 R/inch (fiberglass) ≈ 18.1" settled.
Cellulose is rated at its settled thickness, so you blow deeper and let it settle. Fiberglass barely settles, so blow-to and settled depth are the same.
Example:
13.3" settled ÷ (1 − 0.10) ≈ 14.8" to blow (Greenfiber, ~10% settling).
One baffle per rafter bay along each vented eave keeps loose-fill and batts out of the soffit vents so the attic keeps breathing.
Example:
60 ft of eave (720") ÷ 24" o.c. = 30 baffles.
The insulation's weight on the ceiling drywall, in lb per sq ft. Deep cellulose can exceed the drywall sag limit at high attic R-values.
Example:
Cellulose R-60 ≈ 90 bags/1,000 × 23 lb ÷ 1,000 ≈ 2.1 lb/sq ft — over the 1.3 psf limit for ½" drywall @24" o.c.
Standard Constants
| Constant | Value | Description |
|---|---|---|
| Attic target R (2021 IECC) | R-30 / R-49 / R-60 | Recommended bare-attic R by zone: R-30 (Zone 1), R-49 (Zones 2–3), R-60 (Zones 4–8). Attics carry the highest R-value of any assembly in the house. |
| Fiberglass R/inch (settled) | ≈ 2.5–3.0 | Owens Corning L77 ~2.7, JM Climate Pro ~2.8, CertainTeed InsulSafe SP ~2.6 — chart-derived settled values. |
| Cellulose R/inch (settled) | ≈ 3.6–3.8 | Greenfiber Sanctuary ~3.65, Nu-Wool 3.8. Cellulose reaches the target in less depth — useful under a low roof deck. |
| Cellulose settling | ≈ 10–14% | Cellulose charts are on a settled basis (Greenfiber ~10%, Nu-Wool ~14%). Blow deeper; the bag count already compensates. |
| Attic ventilation ratio | 1/150 or 1/300 | IRC R806: 1 sq ft of net free vent area per 150 sq ft of attic (or 1/300 with a balanced ridge/soffit split and vapor control). Baffles preserve the soffit half. |
| Ceiling dead-load limit | 1.3 psf (½" @24" o.c.) | USG sag limit for unsupported insulation over ½" gypsum on 24" o.c. framing; 2.2 psf for 5/8" @24", 2.4 psf for ½" @16". |
| Recessed-fixture clearance | 3" unless IC-rated | Keep insulation 3" clear of non-IC-rated recessed lights (CPSC 16 CFR 1404); IC-rated fixtures may be covered. |
| Overblow / waste | 5–15% | 5–10% overblow for loose-fill; up to 15% for cut-up batt attics with lots of framing and obstructions. Round up to whole bags. |
Note: All calculations include appropriate waste factors based on project complexity and material type. Results are estimates and should be verified by professionals before purchasing materials.
ENERGY STAR / 2021 IECC — Recommended Attic R-Values(IECC Table R402.1.3)
View StandardThe current recommended attic insulation levels by climate zone that drive this calculator's target R. Attics get the highest R-value of any assembly because most heat loss is through the ceiling.
Key Requirements:
- •Uninsulated attic target: R-30 (Zone 1), R-49 (Zones 2–3), R-60 (Zones 4–8)
- •Raised-heel-truss eave substitution: R-38 full-depth over the top plate can satisfy R-49 (§R402.2.1)
- •Many jurisdictions still enforce 2015/2018 IECC (R-38/R-49) — verify with the local AHJ
- •Compliance uses settled, not blown, thickness
FTC R-Value Rule — 16 CFR Part 460(16 CFR 460.12)
View StandardThe federal rule that requires every bag of loose-fill (and every batt package) to carry a coverage chart. This is why the per-bag square footage and bags-per-1,000 numbers this calculator uses are standardized.
Key Requirements:
- •Loose-fill packages must list minimum settled thickness, initial installed thickness, max net coverage, and bags per 1,000 sq ft at each R-value
- •Batt packages must list R-value and coverage per bag
- •R-values tested at 75°F mean temperature (§460.5)
- •Installer must leave a receipt stating coverage, thickness, and R-value (§460.17)
ASTM C739 / C764 / C665 — Attic Insulation Materials(ASTM C739, C764, C665)
View StandardC764 specifies mineral-fiber (fiberglass) loose-fill, C739 specifies cellulosic loose-fill, and C665 specifies mineral-fiber blanket (batt). These are the material standards behind the products this calculator estimates.
Key Requirements:
- •Loose-fill thermal resistance measured on settled material (C687 with C518)
- •Cellulose carries a borate/ammonium-sulfate fire-retardant treatment (Class A / Class 1)
- •Unfaced batts (C665 Type I) are correct for a vented attic — no vapor retarder needed
- •Coverage charts developed per ASTM C1015 / C1630
IRC R806 — Attic Ventilation(IRC R806)
View StandardThe code that requires a vented attic to keep a net free vent area, and the reason eave baffles matter: they stop the new insulation from choking the soffit vents that make the ratio work.
Key Requirements:
- •1 sq ft of net free ventilating area per 150 sq ft of attic (1/150)
- •1/300 permitted with 40–50% of the vent area high (ridge) and the balance low (soffit)
- •Baffles required at the eaves to maintain the air path over the insulation
- •Insulation must not block soffit intake vents
USG / IRC R702.3.5 — Ceiling Dead Load(IRC R702.3.5)
View StandardThe structural limit on how much insulation weight a gypsum ceiling can carry without sagging — the reason heavy cellulose at high attic R can require a thicker ceiling board.
Key Requirements:
- •≤ 1.3 lb/sq ft over ½" gypsum on 24" o.c. framing (USG)
- •≤ 2.2 lb/sq ft over 5/8" gypsum on 24" o.c.; ≤ 2.4 lb/sq ft over ½" on 16" o.c.
- •IRC requires 5/8" (or ½" sag-resistant) ceiling board at 24" o.c. where it supports insulation
- •Cellulose at R-49/R-60 can exceed the ½"/24" limit — use heavier board or fiberglass
Standards Disclaimer: Standards and codes are subject to periodic updates. Always verify current requirements with local building authorities and professional engineers before beginning construction. Links provided are for reference only.
Target R by Climate Zone
Colder attics need more
The recommended attic R-value climbs with climate zone: R-30 in the far south, R-49 in the mid-south, and R-60 across the northern half of the country. This calculator sets the target from the ENERGY STAR / 2021 IECC table.
Regional Examples:
Blown-In vs. Batts in the Attic
Coverage vs. cut-in
Blown-in loose-fill flows over the top of the joists and around obstructions for a seamless blanket — the attic default. Batts drop between the joists and suit small, walkable, obstruction-free attics, but leave gaps where they're cut around framing.
Regional Examples:
Code Lag — 2018 vs 2021 IECC
Enforced minimum may be lower
The 2018 IECC required only R-38 (Zones 2–3) and R-49 (Zones 4–8). Many jurisdictions still enforce the 2015 or 2018 edition, so the legal minimum is often below the current recommendation.
Regional Examples:
Settling & Blow Depth
Blow cellulose deeper
Fiberglass barely settles, so the depth you blow is the depth you keep. Cellulose is rated at its settled thickness and settles ~10–14%, so you blow deeper and let it drop to the target.
Regional Examples:
Ventilation & Fixture Clearance
Prep before you insulate
Insulation will drift into soffit vents and bury recessed lights if the attic isn't prepped. Install eave baffles and keep insulation clear of non-IC-rated fixtures — this calculator sizes the baffles from your eave length.
Regional Examples:
Before You Build
- •Contact your local building department for specific requirements
- •Verify frost line depths, wind zones, and seismic requirements for your area
- •Check if permits are required and schedule required inspections
- •Consult with a local contractor familiar with local codes
Plan disposal before you start
Smaller jobs still produce more debris than a few trash bags can hold. Check what's allowed in a dumpster and which disposal option fits the scope.
See disposal options →
Related Calculators
Blown-In Insulation Calculator
How many bags of blown-in insulation do you need? Free calculator for fiberglass or cellulose attic R-value: bag count, settled depth, and blow-to depth.
Batt & Fiberglass Insulation Calculator
Free batt insulation calculator — how many bags of fiberglass or mineral wool you need by wall or attic, with the IECC R-value for your climate zone.
Attic Ventilation Calculator
Calculate attic ventilation net free area, soffit intake and ridge exhaust vent counts, and the 1/150 vs 1/300 IRC R806 ratio. Free, no signup.
How to Use This Calculator
- Choose your method: blown-in loose-fill (the attic default) or batts between the joists.
- Enter your attic floor area in square feet — roughly your home's footprint (or the area you're insulating).
- Set your target R-value: pick your IECC climate zone for the recommended level, or enter an R-value directly. Not sure of your zone? Use the climate zone calculator to look it up by ZIP.
- For blown-in, subtract any existing insulation (depth and type) and pick the product; for batts, enter the coverage printed on the bag.
- Add your eave length and rafter spacing to size the ventilation baffles, then click Calculate.
- Read the bag count, the blow-to depth (blown-in), and the baffle count — plus the coverage chart below for depth at every attic R-value.
Why the Attic Gets the Most Insulation in the House
The attic carries the highest R-value of any assembly in a house — R-49 to R-60 across most of the US — because heat rises and the ceiling is the biggest escape path. That's why "how much insulation for my attic" is a different question from insulating a wall or floor. Two things make the attic estimate its own calculation. First, depth: reaching R-60 with blown fiberglass takes about 20–22 inches of loose-fill, and the calculator reports both the settled depth (what delivers the R-value) and the deeper blow-to depth for cellulose, which settles 10–14%. Second, ventilation: a vented attic needs its soffit-to-ridge airflow to keep working, so you install a baffle in each rafter bay at the eaves before you add insulation — otherwise the new insulation buries the intake vents and traps moisture. This calculator sizes both the insulation and the baffles, subtracts insulation you already have so you only top up the gap, and warns you if a heavy cellulose fill would exceed what your ceiling drywall can hold.
Frequently Asked Questions
How much insulation do I need for my attic?
It depends on your climate zone and how much you already have. The ENERGY STAR / 2021 IECC recommendation is R-30 in the far south (Zone 1), R-49 in the mid-south (Zones 2–3), and R-60 across the northern half of the US (Zones 4–8). For a bag count: divide your attic area by the coverage on the bag at your target R and round up. Worked example: 1,000 sq ft at R-49 with Owens Corning L77 blown fiberglass covers 45 sq ft/bag, so ~23 bags before waste (~25 with 10% overblow). This calculator sets the target from your zone, subtracts any existing insulation, and returns the bag count for blown-in or batts.
What R-value should attic insulation be — R-38, R-49, or R-60?
The current 2021 IECC / ENERGY STAR recommendation is R-60 for most of the US (climate zones 4–8), R-49 for zones 2–3, and R-30 for zone 1. Older code editions still enforced in many areas required only R-38 (zones 2–3) or R-49 (zones 4–8), so your legal minimum may be lower than the recommendation. R-38 is now considered a minimum retrofit rather than a target. Set your climate zone in the calculator to see the recommended level, and verify the enforced edition with your local building department.
How deep should attic insulation be for R-49 or R-60?
Depth depends on the material. Blown fiberglass is about 2.5–3.0 R per inch settled, so R-49 is roughly 16.5–18.5" and R-60 is about 20–22". Blown cellulose is denser at ~3.6–3.8 R per inch, so R-49 is about 13–13.5" and R-60 about 16". Batts reach the R-value at their labeled thickness (an R-49 attic batt is ~16"). Important for loose-fill: cellulose is rated at its settled depth and settles 10–14%, so you blow it deeper and let it drop; fiberglass barely settles, so blow-to and settled depth are the same.
Should I use blown-in or batt insulation in my attic?
Blown-in loose-fill is the attic default: it flows over the top of the joists and around wiring, chimneys, and framing for a seamless blanket, and it's fast to reach R-49/R-60. Batts suit small, walkable, obstruction-free attics you can reach by hand, but every cut around framing leaves a gap that cuts real-world R-value. For a deep target over an open attic, blown-in usually wins on both coverage and labor; for a tiny attic, batts between the joists are workable. This calculator estimates either — pick the method at the top.
Do I need baffles when I add attic insulation?
Yes, in a vented attic. Baffles (rafter/soffit vent chutes) keep the new insulation from drifting into the soffit vents and blocking the intake air that IRC R806 requires — without them you trap moisture and invite ice dams. Install one baffle per rafter bay along each vented eave. This calculator sizes them from your eave length and rafter spacing: 60 ft of eave at 24" on-center is 30 baffles. Also keep insulation 3" clear of any non-IC-rated recessed lights (CPSC 16 CFR 1404).
Can I add attic insulation over what's already there?
Yes — new blown-in loose-fill or unfaced batts go right over old, dry insulation, as long as it isn't compressed, wet, or moldy (and remove any existing vapor facing if you add over batts). Estimate the R-value you already have (existing depth × the material's R per inch), subtract it from your target, and only add the difference. The blown-in mode of this calculator does that subtraction for you when you enter the existing depth and type, so you don't over-buy.
Can attic insulation be too heavy for the ceiling?
Yes — it's an overlooked risk with deep cellulose. USG limits unsupported insulation to about 1.3 lb/sq ft over ½" drywall on 24" o.c. framing. Cellulose at R-60 runs about 2.1–2.3 lb/sq ft, which exceeds that limit and can sag the drywall; blown fiberglass at R-60 is only ~0.9 lb/sq ft and is well under. If you're blowing deep cellulose, use 5/8" ceiling board, 16" o.c. framing, sag-resistant board, or switch to fiberglass. The calculator flags this automatically when cellulose crosses the limit.