How Much Blown-In Insulation Do I Need?

Wondering how much blown in insulation do I need? Find your bag count in two numbers — your attic square footage and your R-value target — then read the count straight off the cross-brand table below. Every figure is computed from the Owens Corning AttiCat and Greenfiber SANCTUARY coverage charts, the same constants behind our Blown-In R-Value Calculator.

Blown-In Insulation Bags by Attic Size and R-Value

This is the fast answer for two blown-in products side by side: Owens Corning AttiCat blown fiberglass (27.5 lb bag) and Greenfiber SANCTUARY blown cellulose (25 lb bag). Counts are the base bag count for a bare attic floor — bags = ceil(area ÷ sqft-per-bag) — computed from each manufacturer's published coverage chart. Add about 10% for HVAC platforms, joists, and knee walls. If your bag's chart lists different sqft/bag values, treat the bag label as authoritative.

Per 1,000 sq ft, that works out to a coverage rate of 20 AttiCat or 53 SANCTUARY bags at R-38; 26 or 71 bags at R-49; and 32 or 87 bags at R-60. These are base counts — buy about 10% more. This is a fiberglass-vs-cellulose coverage comparison, not a brand ranking: both products carry the same DOE-recognized R-values at their labeled depths.

How to Calculate Blown-In Insulation Bags

If you already have insulation and are topping up, you do not need full-R bags — you need the delta. Measure your existing depth, estimate its R-value, subtract from the target, and buy only enough to close the gap. AttiCat blown fiberglass adds about 2.79 R per inch; SANCTUARY cellulose about 3.7 R per inch. Delta math is easy to get wrong by hand, so run your existing depth and target through the Blown-In R-Value Calculator, or read the dedicated topping-up attic insulation guide (a 6-inch existing layer can cut your bag count by a third or more).

Why 1,000 Sq Ft Takes 20 AttiCat Bags but 53 SANCTUARY Bags

The 20-versus-53 gap at R-38 surprises most first-time buyers, and it is the single most useful thing to understand before you shop. It is not a quality difference — both products deliver a labeled R-38 at their rated depth. It is a coverage-per-bag difference that falls straight out of the two manufacturer charts.

One AttiCat bag spreads across 52.6 sq ft at R-38; one SANCTUARY bag spreads across 19.0 sq ft at R-38, per the Greenfiber SANCTUARY coverage chart. That is roughly 2.8 times fewer square feet per bag, which is why you buy roughly 2.6 times as many cellulose bags. The difference is a physics-of-density story: AttiCat is an expanding blown fiberglass that fluffs to its R-38 depth (about 13.5 inches) at low installed density, while cellulose reaches R-38 at a shallower depth (about 10.3 inches) but at a much higher installed mass per square foot. Each bag weighs about the same (27.5 lb AttiCat vs 25 lb SANCTUARY), so the denser cellulose covers far less floor per bag.

The practical takeaways: cellulose gets you to target in less depth (useful in shallow joist bays and low-headroom attics), while fiberglass gets you there in fewer bags and less lifting. Neither is the correct answer for every attic — availability, blower compatibility, and whether moisture is a concern usually decide it. Both charts report their numbers on the settled-depth basis required by the FTC R-Value Rule (16 CFR Part 460), so the bag counts above are directly comparable.

What About Blown-In Insulation for Walls?

The bags-per-sq-ft table above is for attic floors, where loose-fill sits horizontally and is sized by area and depth. Walls are a different calculation. A vertical wall cavity has to be dense-packed — blown to roughly 3.5 lb per cubic foot — so the fill grips the cavity and resists settling behind the drywall. That means wall bag count is driven by cavity volume (stud depth × wall area), not floor area, and the coverage rates in the attic table do not apply.

A 2x4 wall cavity dense-packed with cellulose lands around R-13, and a 2x6 cavity around R-20 — but the bag math and the drill-and-fill retrofit method are their own topic. For the dense-pack density figures, open-wall net-and-blow versus drill-and-fill, and cavity-R-by-stud-depth math, see the dedicated blown-in wall insulation guide (2x4 ≈ R-13, 2x6 ≈ R-20) rather than reusing this attic table.

Which R-Value Should You Target?

Pick your R-value before you read the bag count. The U.S. Department of Energy publishes attic targets by IECC climate zone, aligned with the 2021 IECC edition: roughly R-30 to R-49 in the warm South (zones 1 to 3), and R-60 from mixed zone 4 (including the 4A, 4B, and 4C sub-zones) through the cold zones 5 to 8 for an uninsulated attic. Look up your zone by ZIP at energystar.gov or on the DOE map at energy.gov/energysaver/insulation. These are cost-effectiveness targets, not code minimums — confirm the adopted IECC edition and any R-value minimum with your local building department before a permitted project. For the full 8-zone table with existing-insulation scenarios and energy-savings estimates, see how much insulation do I need in my attic (DOE zones 4 to 8 land at R-60).

What Does Blown-In Insulation Cost?

Material cost follows bag count, so cellulose and fiberglass can land close on total dollars even though the bag counts look very different. As of 2026, expect roughly $65 to $70 per bag for AttiCat fiberglass and roughly $10 to $16 per bag for SANCTUARY cellulose — verify pricing at your local Home Depot or Lowe's, since retail prices, bulk breaks, and promotions drift by region and season. The table below runs a 1,000 sq ft attic at R-38 both ways.

Per-bag prices below are approximate as of 2026 and vary by region and retailer — confirm pricing at your store before purchasing.

Cellulose usually wins on material cost at the same R-value; fiberglass wins on bags to lift and haul. Blower-rental terms — free with a bag-count minimum at Home Depot or Lowe's, and which blower each product needs — can swing the total either way. For those rental thresholds and machine logistics, see the blown-in insulation by retailer guide and the insulation blower rental guide (20 AttiCat bags typically clears the free-blower minimum).

Before You Buy: A 6-Step Bag-Count Checklist

Follow these in order so your bag count is right the first time and you are not making a second trip mid-blow.

1. Measure the attic floor, do not eyeball it — Walk the space and multiply length by width for each rectangle; subtract the attic hatch and any floored storage you are keeping. Guessing area is the number-one cause of a short order.
2. Pick your R-value target — Use the DOE zone target for your ZIP from energy.gov; most attics land at R-38, R-49, or R-60. Higher zones need higher R.
3. Account for any usable existing insulation — Measure current depth at several spots. Existing R subtracts from your target, so you buy delta bags, not full-R bags. Run it through the Blown-In R-Value Calculator.
4. Read the coverage rate off the bag you will actually buy — Find the R-value row on that bag's coverage chart; that sqft/bag figure is authoritative for your purchase. Do not swap AttiCat's rate onto a SANCTUARY bag or vice versa.
5. Add roughly 10% buffer — HVAC platforms, joists, knee walls, and settling-around-objects eat into coverage. A heavily obstructed attic may need 15% or more.
6. Install rafter baffles before blowing near the eaves — Baffles keep loose-fill out of the soffit airway so the attic can still breathe (attic ventilation per IRC §R806 (verify locally-adopted edition with your local building department) ). Set depth gauge sticks across the deck so you hit the target depth evenly.

Common Blown-In Bag-Count Mistakes

These four errors are why people either run short mid-blow or finish under their R-value target.

• Blowing cellulose to the settled depth instead of the initial depth — The most common way to finish under target. SANCTUARY cellulose settles about 7%, so if you stop at the settled number your attic drops below the R you paid for. Blow to the initial depth the coverage chart lists; the bag count already assumes settled coverage.
• Eyeballing the square footage — A 10% area error is a 10% bag error. At 1,000 sq ft R-38 that is 2 AttiCat or 5 SANCTUARY bags off. Measure each rectangle and add them; do not estimate from the house footprint, which includes non-attic area.
• Ignoring existing insulation R-value — Buying full-R bags when you already have 6 inches of old fill wastes money and overfills the attic. R-values add, so subtract what is there from your target and buy only the delta.
• Skipping the bag's own coverage chart — Private-label and older bags can list different sqft/bag values than the numbers here. Product lines also get renamed. Read the coverage chart on the actual bag you are buying and let it override any table, including this one.

Frequently Asked Questions

How many bags of blown-in insulation do I need for 1,000 sq ft?

At R-38 over a bare 1,000 sq ft attic: 20 bags of AttiCat fiberglass (52.6 sqft/bag) or 53 bags of SANCTUARY cellulose (19.0 sqft/bag) per the manufacturer coverage charts. At R-49 it is 26 AttiCat or 71 SANCTUARY bags. Add about 10% for obstructions, and confirm against the chart on your bag.

How much blown-in insulation do I need for walls?

Walls are dense-packed to roughly 3.5 lb per cubic foot, so bag count is driven by cavity volume, not floor area — the attic bags-per-sq-ft table does not apply. A 2x4 cavity lands around R-13 and a 2x6 around R-20. See the blown-in wall insulation guide for the dense-pack math.

How much blown-in insulation do I need for a garage?

Use the same attic math on the garage ceiling area. A 400 sq ft garage attic at R-38 takes about 8 AttiCat or 22 SANCTUARY bags. Check that the ceiling is built to carry loose-fill and that recessed fixtures are IC-rated before blowing.

How do I calculate blown-in insulation bags?

Divide attic area by the coverage per bag at your R-value and round up: bags = ceil(area / sqft-per-bag). Read the coverage rate off the bag's chart — for example 52.6 sqft/bag for AttiCat at R-38, or 19.0 sqft/bag for SANCTUARY at R-38 — then add about 10%.

How much cellulose vs fiberglass do I need for the same R-value?

More cellulose bags than fiberglass, because each 25 lb SANCTUARY bag covers less area at the labeled R than a 27.5 lb AttiCat bag. At R-38 over 1,000 sq ft that is 53 SANCTUARY vs 20 AttiCat bags — a coverage-per-bag difference from the two charts, not a claim that either product performs better.

Do I blow to the settled depth or deeper?

Blow cellulose to the initial depth its chart lists (it settles about 7%); blow fiberglass to the marked depth (it does not settle appreciably). Both coverage charts already report bags on the settled-depth basis required by the FTC R-Value Rule.

Estimates only — verify quantities against your bag's coverage chart and confirm requirements with your local building authority before starting. See our full disclaimer.