Blown-In Insulation Calculator

Calculate how many bags of blown-in cellulose or fiberglass you need to bring an attic to R-38, R-49 or R-60, plus the depth to blow and the coverage per bag.

By CalcBuilt Editorial TeamUpdated September 9, 2026Formulas checked against manufacturer specs, see how we calculate.

Quick answer: To find how many bags of blown-in insulation you need, divide the R-value you are adding by the R per inch to get the depth, multiply by the attic area for cubic feet, then multiply by the settled density and divide by the bag weight. A 40 x 30 ft attic going from bare joists to R-38 with cellulose needs 10.9 in of insulation, which is 73 bags of 25 lb with 5% waste.

Your measurements

Cross-section: volume is the area covered times the depth of the layer.

Attic lengthftAttic widthftTarget R-valueAsk the building department which code year applies. Most existing attics are topped up to R-49 or R-60; the extra bags are the cheapest R you will ever buy.Existing R-value0 for bare joists. Figure about R-3 per inch of old fiberglass or cellulose that is still fluffy; measure at 3 or 4 spots away from the hatch. Matted, wet or rodent-fouled insulation counts for nothing and comes out.MaterialWaste allowance%5% covers dust lost through the blower and uneven depth around the eaves.

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Bags to buy73 bagsAll results

Results

Bags to buy: 73 bags; 25 lb bags of cellulose with 5% waste, for 1,200 sq ft
Depth to add: 10.9 in; Adds R-38 at R-3.5 per inch, settled; blow cellulose to about 13 in installed, it settles to this. Note: 2021 IECC calls for R-49 in zones 2 to 3 and R-60 in zones 4 to 8; R-38 meets 2015 and older codes
Insulation weight: 1,737 lb; 1,086 cu ft at 1.6 lb per cubic foot settled
Coverage per bag: 17.3 sq ft; At this depth, before waste

Blown-In Insulation formula

R-value to add = Target R-value - Existing R-value
Depth (in) = R-value to add / R per inch  (cellulose 3.5, fiberglass 2.5)
Weight (lb) = Area (sq ft) x Depth (in) / 12 x settled density (lb per cu ft)
Bags = Weight / bag weight x (1 + waste %)

Worked example

A 25 x 32 ft attic with about R-11 of old fiberglass, brought up to R-49 with blown fiberglass: 25 x 32 = 800 sq ft. 49 - 11 = R-38 to add. 38 / 2.5 = 15.2 in of new material. 800 x 15.2 / 12 = 1,013.3 cubic feet. At 0.7 lb per cubic foot that is 709.3 lb, and 709.3 / 28 = 25.3 bags. With 5% waste, 26.6, so buy 27 bags.

The existing R-11 is about 4.4 in at R-2.5 per inch, so the finished depth is about 19.6 in. Mark that height on the rafters or on attic rulers before you start blowing.

Quick reference

Depth and bags per 1,000 sq ft of attic from bare joists (5% waste)
Target R-value	Cellulose depth	Cellulose bags (25 lb)	Fiberglass depth	Fiberglass bags (28 lb)
R-30	8.6 in	48	12 in	27
R-38	10.9 in	61	15.2 in	34
R-49	14 in	79	19.6 in	43
R-60	17.1 in	96	24 in	53

How to calculate blown-in insulation

Measure the attic floor at the outside walls and multiply for square feet; do not subtract anything but a large stairwell. Decide the target R-value for your climate zone, subtract what is already there, and divide by the R per inch of the product to get the settled depth to add. Every bag lists a coverage chart with the number of bags per 1,000 sq ft at each R-value; the calculator reproduces that chart from the settled density and bag weight, so compare the two and use the larger number.

Blown-in works over existing batts or old loose fill as long as the old material is dry. Kraft facing can stay because it is already on the warm side, but never lay faced batts or poly on top. Wet, moldy or rodent-fouled insulation comes out first, typically $1 to $2 per sq ft with a vacuum truck. Vermiculite (gray-brown granules that look like mica) may contain asbestos and must not be disturbed without testing. Cellulose is the cheaper R and the heavier load: a 1,200 sq ft attic to R-49 is about 95 bags, 2,400 lb, two pickup loads or a store delivery.

Before you blow

Air seal the attic floor first: canned foam at top plates, wiring and plumbing holes and the bath fan housing, sheet metal and fire caulk at the chimney chase. Weatherstrip the hatch lid. Any bath fan dumping into the attic gets ducted through the roof (IRC M1501.1) before it gets buried. Frame a dam around the hatch out of 2x12s or foam board taller than the finished depth, and the same around a furnace platform. Staple a baffle in every rafter bay that has a soffit vent; R806.3 wants 1 in of air space to the sheathing. Non-IC recessed cans get a fire-rated cover or a box with 3 in clearance. Keep 2 in from a masonry chimney (R1003.18) and 1 in from a B-vent flue, or whatever the vent listing says.

Staple cardboard rulers to the trusses, one per 300 sq ft (IRC N1101.10.1.1), marked at the installed depth from the bag chart. For cellulose that is about 20 percent above the settled depth on this page. Pick up the blower when you buy the bags: it is free for the day with 10 or more at the big box stores and typically $100 to $150 otherwise. The 100 ft hose is the reach limit, so plan the route from the driveway. Two people: one feeds the hopper and breaks up clumps, one runs the hose, talking by phone. Set the gate about half open for cellulose so it fluffs instead of clumping. Work from the far corners back to the hatch and lob the material 6 to 8 ft rather than pointing the hose down. Wear a respirator, eye protection and long sleeves, and work on cool mornings in summer.

What this calculator leaves out

Bags only. Not included: baffles (one per rafter bay, typically $2 to $3 each), foam and caulk for air sealing, and the hatch dam and an insulated hatch cover. Nor fire-rated covers for non-IC cans, attic rulers, the R-value card the inspector wants at the hatch, the blower if you are under 10 bags, or a respirator. Also removal of wet or rodent-fouled insulation, typically $1 to $2 per sq ft, and knee walls, which take batts or netted dense-pack, not loose fill.

Mistakes that cost money

Blowing cellulose to the settled depth: it settles about 20%, so R-38 is blown at about 13 in, not 10.9; read the installed column.
No baffles before blowing: loose fill slides into the eaves, chokes the soffit vents and the sheathing frosts; staple a baffle in every rafter bay.
Burying a non-IC recessed can: the thermal cutout cycles the light or the housing overheats; box it with 3 in clearance or swap to IC-rated.
No dam at the hatch: every trip rakes insulation into the opening; box it with 2x12s or foam board taller than the finished depth.
Buying 9 bags: the blower is free with 10 or more and typically $100 to $150 a day otherwise; round up and return unopened bags.
Running the blower alone: the hopper starves or bridges and the hose clogs; one feeds, one sprays, and you talk by phone or radio.

Key facts

The 2021 IECC (Table R402.1.3) requires R-49 attics in climate zones 2 and 3 and R-60 in zones 4 through 8; states on 2015 or older codes use R-38 and R-49.
Blown cellulose is about R-3.2 to R-3.8 per inch and settles to roughly 1.5 to 1.6 lb per cubic foot; blown fiberglass is about R-2.2 to R-2.7 per inch at 0.5 to 0.8 lb per cubic foot.
IRC N1101.10.1.1 requires a depth marker for every 300 sq ft showing the installed depth, and N1101.14 a certificate near the attic access stating the installed R-value.
Most big box stores lend the blowing machine free for the day when you buy 10 or more bags, which is why bag counts are rounded up to whole bags.
Cellulose can settle up to 20 percent from its blown depth, so the bag chart lists a thicker installed depth than the settled depth that delivers the rated R-value.

Frequently asked questions

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

From bare joists to R-38, about 61 bags of 25 lb cellulose or 34 bags of 28 lb blown fiberglass with 5% waste. Adding R-19 over existing R-19 takes about half that.

How deep should blown-in attic insulation be?

About 10.9 in of cellulose or 15.2 in of fiberglass for R-38, and 14 in of cellulose or 19.6 in of fiberglass for R-49. The bag chart lists a slightly deeper installed depth to allow for settling.

Is cellulose or fiberglass better for blown-in?

Cellulose gives more R-value per inch and fills gaps better, but it is dustier and heavier, about 1.6 lb per cubic foot settled. Blown fiberglass is lighter, does not settle much, and takes fewer bags at the same R-value because the bags are less dense.

Do I need baffles before blowing in attic insulation?

Yes, in every rafter bay that has a soffit vent below it. Loose fill slides into the eaves and chokes the vents, and IRC R806.3 requires a 1 in air space between the insulation and the sheathing there. Foam or cardboard baffles staple to the rafters and cost typically $2 to $3 each; a 40 x 30 ft attic with trusses 24 in on center takes about 40. Build the hatch dam at the same time.

Sources and references

Next steps for this project

The usual order for a insulation & hvac project. See all insulation & hvac calculators.

Results are estimates based on standard formulas and typical product specifications. Confirm quantities with your supplier and local code before ordering.