How Much Blown-In Insulation Do I Need? Bags, Depth & Coverage

The answer depends on three things: the size of your attic floor, how much insulation is already there, and which material you choose. A 1,500 square foot attic with nothing in it needs about 40 bags of blown fiberglass to reach R-49, or about 50 bags of cellulose, or about 46 bags of mineral wool. The same attic with 6 inches of old fiberglass already in place needs far fewer bags, because the old layer is already doing part of the job. This guide walks through the measurement, the math, and the bag counts for the common attic sizes in Waukegan and Lake County, so you can check any quote before you sign it.

Step 1: Measure the Attic Floor Area

The attic floor area is the length times the width, measured inside the attic at floor level. A 50 by 30 foot attic is 1,500 square feet. Most attics in Waukegan are between 1,000 and 2,000 square feet, and the floor is usually a rectangle, which makes the math easy. If the attic is L-shaped or has a wing, split it into rectangles, work out each area, and add them together. Measure to the inside of the walls, not the outside of the house, because the insulation only covers the floor inside. If you have the house plans, the attic floor area is usually close to the footprint of the top floor, but measure it anyway, because dormers and knee walls change the number.

Do not guess the area from the square footage of the house: a two-story home with 2,400 square feet of living space usually has a 1,200 square foot attic, because the attic sits over one floor, not both. The blown-in insulation calculator takes either the length and width or a direct area, so use whichever measurement you trust more.

Step 2: Measure the Existing Depth

Go into the attic with a tape measure and a flashlight, and measure the insulation depth at five or six spots: the middle, each corner, and near the hatch. Use the shallowest reading, not the average, because insulation settles unevenly and the thin spots are where the heat escapes. If the old layer is a mix of batts and blown-in material, measure the blown-in depth and note the batts separately. A ruler pushed down to the ceiling drywall gives the true depth, but do not step on the drywall itself: walk only on the joists or on boards laid across them.

Once you have the depth, convert it to an R-value. Multiply the inches by the R-value per inch of the old material: 2.5 for fiberglass, 3.5 for cellulose, 3.0 for mineral wool. Six inches of old fiberglass is R-15. Ten inches of old cellulose is R-35. If you do not know what the old material is, look at it: fiberglass is pink, yellow, or white and fluffy; cellulose is grey and looks like shredded paper; mineral wool is brown or grey and feels dense and springy. If you truly cannot tell, use 2.5 per inch, which is the most conservative figure, and the calculator will ask for a little more material than you strictly need.

Step 3: Work Out the Bags

The bag count comes from the coverage charts. At R-49, a bag of blown fiberglass covers 37.5 square feet, a bag of cellulose covers 30 square feet, and a bag of mineral wool covers 33 square feet. But you only need to cover the R-value that is missing, so the formula is the attic area times the needed R-value, divided by the square-foot R-value each bag delivers. For fiberglass that divisor is 1,837.5, for cellulose it is 1,470, and for mineral wool it is 1,617. The result is always rounded up, because you cannot buy part of a bag and a short attic is worse than a spare bag.

Attic size	Fiberglass bags (R-49)	Cellulose bags (R-49)	Mineral wool bags (R-49)
1,000 sq ft	27	34	31
1,200 sq ft	32	40	37
1,500 sq ft	40	50	46
2,000 sq ft	54	67	61

That table is for an empty attic. If you already have insulation, the numbers fall fast. A 1,200 square foot attic with 6 inches of old fiberglass (R-15) needs 34 more R-value, which takes 28 bags of cellulose, not 40. A 2,400 square foot attic with 10 inches of old cellulose (R-35) needs only 14 more R-value, which takes 21 bags of mineral wool, not 61. The calculator on the pillar page does this subtraction for you, but the table above is the quick check to use when a quote arrives: if a contractor quotes 60 bags for a 1,500 square foot attic that already has 6 inches of fiberglass, the count is far too high, and you should ask why. Run your own numbers through the blown-in insulation calculator before the quote arrives.

How Deep Should It Be?

Depth and bags are two sides of the same number. At R-49, blown fiberglass needs 19.6 inches, cellulose needs 14.0 inches, and mineral wool needs 16.3 inches, when you start from nothing. Those depths are the final settled depths, and they are measured from the ceiling drywall to the top of the insulation. If you already have 6 inches of fiberglass, you add 9.7 inches of cellulose on top, bringing the total to 15.7 inches, and the R-value to 49. The crew should measure the finished depth in several places before they leave, and you should be there to watch them do it. A depth check takes two minutes and it is the only proof the job was done right.

Cellulose settles a little in the first year, so the crew should blow it slightly deep and check the settled depth later. Fiberglass and mineral wool do not settle, so the depth on the day of the install is the depth you keep. In a low attic, depth matters twice: the insulation must fit under the roof deck without blocking the soffit vents, and the baffles at the eaves hold it back. If the full R-49 depth will not fit at the eaves, the crew can use R-38 in the tight bays and R-49 in the open middle, or switch to cellulose, which reaches R-49 in 14.0 inches instead of 19.6.

Related reading: blown-in insulation calculator, plus our guides on blown-in insulation R-value and blown-in insulation cost, and the roofing calculators hub.

Frequently Asked Questions