Blown-In Insulation Calculator
This free blown-in insulation calculator tells you how many bags of loose-fill fiberglass or cellulose you need to reach a target attic R-value — using each product's FTC-mandated coverage chart, the same numbers printed on the bag. Enter your attic floor area, set the target by climate zone (or type an R-value), subtract any existing insulation, and it returns the whole-bag count with an overblow allowance already added.
It also solves the part every DIYer gets wrong: depth. Fiberglass settles almost nothing, 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 — the calculator gives you both the settled depth for your R-value and the deeper blow-to depth, while the bag count stays the same because the chart already accounts for settling.
Everything is grounded in the FTC R-value Rule (16 CFR 460), the manufacturer coverage charts (Owens Corning, Johns Manville, CertainTeed, Greenfiber, Nu-Wool), ASTM C687/C764/C739, and the ENERGY STAR / 2021 IECC recommended attic R-values. It even flags when heavy cellulose at a high R-value would exceed the ceiling drywall's sag limit. Quantities only — no pricing, no signup.
Blown-In Insulation Calculator
Find how many bags of blown-in fiberglass or cellulose you need to hit your target attic R-value, the settled depth, and the deeper depth to actually blow to. Free, no signup.
Attic area
Target R-value
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
Material & waste
Coverage comes from the selected product's FTC bag chart. Bags always round up. 5–10% overblow covers job variation.
Coverage chart — Owens Corning ProPink L77
Bags per 1,000 sq ft and settled depth at each 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.
Calculation Formulas
Only add the insulation that's 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 at each R-value. Scale by area, add an overblow allowance, and round up to whole bags.
Example:
1,000 sq ft at R-49 with Owens Corning L77 = 22.2 bags/1,000 × 1.10 = 25 bags.
The other way the chart states coverage: the maximum net square feet one bag covers at the target R-value. Net excludes framing.
Example:
45.0 sq ft/bag at R-49 (Owens Corning L77) — so ~23 bags for 1,000 sq ft before waste.
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).
The insulation's weight on the ceiling drywall, in lb per sq ft. Cellulose is ~3× heavier than fiberglass and can exceed the drywall sag limit at high R.
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.
Coverage charts give maximum NET coverage (framing already excluded). Work from net ceiling area and subtract the attic hatch, chimney chase, and dropped soffits.
Example:
Joist tops are captured by the chart's net basis; a 16" o.c. framing factor is ~9%.
Standard Constants
| Constant | Value | Description |
|---|---|---|
| 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 a target R in less depth than fiberglass. |
| Fiberglass settling | ≈ 0% (negligible) | Fiberglass loose-fill charts state settled ≈ initial installed thickness; blow-to and settled depth are the same. |
| Cellulose settling | ≈ 10–14% | Cellulose charts are on a settled basis (Greenfiber ~10%, Nu-Wool ~14%). Blow deeper; the bag count already compensates. |
| Bags per 1,000 sq ft @ R-49 | FG ~22–25 · Cellulose ~68–75 | Straight from the FTC charts — cellulose takes far more bags because it's applied at a higher density. |
| 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". |
| Overblow / waste | 5–10% | Actual coverage varies ~10% with job conditions; add a modest overblow allowance and round up to whole bags. |
| Cubic-yard basis | n/a — sold by the bag | Loose-fill is sold and charted by the bag, not by volume; the FTC coverage chart is the single controlling data source. |
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.
FTC R-Value Rule — 16 CFR Part 460(16 CFR 460.12)
View StandardThe federal rule that requires every bag of loose-fill insulation to carry a coverage chart. This is why the per-bag square footage and bags-per-1,000 numbers this calculator uses are standardized and legally defensible.
Key Requirements:
- •Loose-fill packages must list minimum settled thickness, initial installed thickness, max net coverage, bags per 1,000 sq ft, and minimum weight per sq ft at R-13/19/22/30/38/49
- •R-values tested at 75°F mean temperature with a 50°F temperature difference (§460.5)
- •Manufacturers commonly add R-60 voluntarily
- •Installer must leave a receipt stating coverage, thickness, and R-value (§460.17)
ASTM C687 / C764 — Fiberglass Loose-Fill(ASTM C687, C764)
View StandardC687 is the method for measuring the thermal resistance of loose-fill insulation; C764 specifies mineral-fiber (fiberglass) loose-fill. Fiberglass blowing wools cite C764 Type I (pneumatic).
Key Requirements:
- •Thermal resistance measured on settled material (C687 with C518 apparatus)
- •Coverage charts developed per ASTM C1630
- •Installed thickness verified per ASTM C1374
- •Type I (pneumatic/blown) application for attic loose-fill
ASTM C739 / CPSC 16 CFR 1209 & 1404 — Cellulose(ASTM C739; 16 CFR 1209/1404)
View StandardC739 specifies cellulosic loose-fill; the CPSC rules add mandatory fire-safety, corrosiveness, and labeling requirements specific to cellulose.
Key Requirements:
- •Borate/ammonium-sulfate fire-retardant treatment; Class A / Class 1 flame spread
- •Settled density, smoldering combustion, and critical radiant flux limits (16 CFR 1209)
- •Fire-hazard labeling and clearance from recessed fixtures and flues (16 CFR 1404)
- •Must be installed dry — cellulose absorbs and redistributes moisture
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. Blown-in compliance is measured at the manufacturer's settled R-value.
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
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 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 zones 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:
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:
Fiberglass vs. Cellulose
Depth vs. weight
Cellulose reaches the target in less depth but weighs about three times more per square foot, which matters for the ceiling drywall. Fiberglass is lighter and non-combustible but needs more depth.
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 blow
Loose-fill will drift into soffit vents and bury recessed lights if the attic isn't prepped. Install baffles at the eaves and keep insulation clear of non-IC-rated fixtures.
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 →
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How to Use This Calculator
- 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.
- If there's already insulation up there, enter its depth and type — the calculator subtracts its R-value so you only add what's missing.
- Choose fiberglass or cellulose and the specific product (its coverage chart drives the math), then set an overblow allowance (5–10%).
- Click Calculate to get the bag count, the settled depth, and the deeper depth to blow to.
- Check the coverage chart below the result to see bags-per-1,000-sq-ft and depth at every R-value for your chosen product.
Why the Bag Count and the Depth Are Two Different Answers
A blown-in job has two numbers that people constantly mix up: how many bags to buy, and how deep to blow. The bag count comes straight off the FTC coverage chart (bags per 1,000 square feet at your R-value) and does NOT change with settling — the chart already builds it in. The depth is where fiberglass and cellulose split. Fiberglass loose-fill settles negligibly, so its installed and settled depth are the same. Cellulose is rated at its settled thickness and settles about 10–14%, so you have to blow it 10–14% deeper than the target and let it drop. That's why this calculator reports a "settled depth" (what delivers the R-value) and a "blow-to depth" (what you aim the hose at) separately — and why buying by depth instead of by the chart is how people end up short. Cellulose also reaches a given R-value in less depth than fiberglass because it's denser, but that density is also why a deep cellulose fill can overload a thin ceiling — a trade-off the calculator flags.
Frequently Asked Questions
How many bags of blown-in insulation do I need?
Divide your attic area by the coverage on the bag's FTC chart at your target R-value, then round up. Worked example: 1,000 sq ft at R-49 with Owens Corning L77 fiberglass covers 45 sq ft/bag (22.2 bags per 1,000 sq ft), so ~23 bags before waste, or ~25 with a 10% overblow allowance. Cellulose at the same R-49 takes far more — Greenfiber Sanctuary is 68.5 bags per 1,000 sq ft — because it's applied at a higher density. This calculator uses the exact coverage chart for the product you pick.
How deep do I blow insulation for R-49 or R-60?
Depth depends on the material's R-per-inch. Fiberglass loose-fill is about 2.5–3.0 R per inch settled, so R-49 is roughly 16.75–18.5", and R-60 is about 20–22". Cellulose is denser at ~3.6–3.8 R per inch, so R-49 is about 13–13.5" settled and R-60 about 16". Important: cellulose is rated at its SETTLED depth and settles 10–14%, so you blow it 10–14% deeper and let it drop. Fiberglass barely settles, so blow-to and settled depth are the same.
Does blown-in insulation settle, and does that change how many bags I need?
Fiberglass loose-fill settles negligibly ("won't settle" per the manufacturers), so its installed and settled thickness are effectively the same. Cellulose settles about 10–14% — Greenfiber ~10%, Nu-Wool ~14% — so it's rated at the settled thickness and you blow deeper. Either way, settling does NOT change the bag count: the FTC coverage chart's bags-per-1,000-sq-ft already accounts for settling. Settling only changes the depth you blow to, not how many bags you buy.
What R-value should my attic be?
The ENERGY STAR / 2021 IECC recommendation for a bare attic 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). If you already have some insulation, you top up the difference. Note that many local codes still enforce the older 2015/2018 IECC, which required only R-38/R-49 — so your legal minimum may be lower than the recommendation. This calculator sets the target from your climate zone (or a value you enter) and subtracts existing insulation.
Fiberglass or cellulose blown-in insulation — which is better?
It's a depth-vs-weight trade-off. Cellulose reaches a target R-value in less depth (denser, higher R per inch) and packs tightly around obstructions, but it weighs about three times more per square foot and must be kept dry. Fiberglass is lighter, non-combustible, and moisture-tolerant, but needs more depth. Cellulose wins where eave clearance is tight; fiberglass wins on a light ½" ceiling at 24" o.c., in humid attics, and for the lowest ceiling load. For a deep R-60 fill over thin drywall, fiberglass is the safer structural choice.
Can blown-in insulation be too heavy for my ceiling?
Yes — and it's an overlooked risk with 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; 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.
Can I add blown-in insulation over existing insulation?
Yes — blown-in loose-fill goes right over old batts or existing loose-fill, as long as the existing insulation is dry and not compressed or moldy. 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. Before blowing, install baffles at the eaves so you don't block soffit vents, and keep insulation 3" clear of any non-IC-rated recessed lights.