Work out how many bags of blown-in insulation your attic needs. Enter the area, existing and target R-values, and material to get the required depth and bag count to buy.
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Insulation is rated by R-value, and the amount you need is set by the gap between what you have and what your climate zone calls for. Enter your attic or floor area, measure the existing insulation's R-value (depth in inches times its R-per-inch, or zero for a bare attic), and pick a target R-value. The calculator subtracts to find the R-value to add, divides by the material's R-per-inch to get the required depth in inches, and divides the area by your bag's coverage at the target R-value to give the number of bags to buy, with a waste allowance for settling and awkward edges.
Three numbers drive the whole estimate. First, the R-value to add: target R minus existing R. Topping up an attic that already has R-19 to reach R-49 means adding R-30, roughly half the bags of a bare attic. Second, the required depth: R to add divided by the material's R-per-inch. Blown cellulose delivers about 3.5 R per inch, blown fiberglass about 2.5, and fiberglass batts about 3.2. So R-49 over a bare attic needs 49 / 2.5 = 19.6 inches of blown fiberglass, but only 49 / 3.5 = 14 inches of cellulose. Third, the bag count: area divided by the bag's coverage at your target R-value, times one plus the waste allowance, rounded up. A 1,000 sq ft attic at R-49 with blown fiberglass covering 55 square feet per bag needs ceil(1,000 / 55 × 1.05) = 20 bags.
Start by entering the area to insulate in square feet, usually the attic floor: length times width of the ceiling below it. Next, estimate the existing R-value: measure the current depth in inches in several spots, multiply by the R-per-inch of what is up there (about 2.5 for loose fiberglass, 3.5 for cellulose), and use zero for a bare attic. Pick your target R-value from the climate zone guidance below. Choose the insulation material you plan to buy. Then read the coverage per bag at your target R-value from the bag's printed chart, which the manufacturer is required to show, and enter it. Set a small waste allowance for settling and awkward edges. The calculator returns the R-value to add, the required depth, and the bags to buy.
Recommended attic R-values rise with colder climates. In hot Zone 1, attics target about R-30. Zones 2 and 3 target R-49. Zones 4 through 8, covering most of the cold and northern states, target R-60 for uninsulated attics. If the attic already has 3 to 4 inches of old insulation, the targets step down: add R-25 in Zone 1, R-38 in Zones 2 to 3, and R-49 in Zones 4 to 8. Exterior walls typically need R-13 to R-23, floors over unheated space need R-19 to R-30, and crawl space or basement walls need R-11 to R-25. These are cost-effectiveness targets for retrofitting existing homes, not code minimums; confirm requirements with your local building department before starting.
Blown cellulose has the highest R-value per inch at about 3.5, fills around obstacles, and is usually the most cost-effective choice for attics at $0.25 to $0.50 per square foot in DIY materials. Blown fiberglass carries about 2.5 R per inch, so it needs roughly 40 percent more depth for the same R-value, but it is lighter, settles less, and suits humid attics at $0.30 to $0.55 per square foot. Fiberglass batts at about 3.2 R per inch are pre-cut rolls that are cheaper and DIY-friendly for standard joist spacing with no blower needed, but they fit poorly around obstructions. For a retrofit attic, blown-in insulation is usually the practical choice; for small, simple attics with open joist bays, batts are simplest. Note that bag counts differ a lot: reaching R-49 over 1,000 square feet takes about 26 bags of blown fiberglass but 84 bags of cellulose, because the bags are different sizes.
Loose-fill insulation goes in slightly fluffed and settles over the first few weeks, so manufacturers rate coverage at the settled depth, which is lower than the installed depth. Cellulose charts list both an initial installed thickness and a lower settled thickness; the rated R-value applies at the settled figure. Install to the fluffier initial height so the layer relaxes onto your target rather than below it. Mark the target depth on the joists or on rulers stapled across the attic so you can sweep for thin spots after blowing. Going by depth alone is the classic mistake: fluffing loose fill thicker with fewer bags gives a lower R-value, because the label requires both the bag count and the thickness. Blown-in insulation also settles over years, so topping up a few years after install is normal.
Blown-in attic insulation is one of the easiest DIY projects in home improvement. Rent a blower machine from a home improvement store; the rental is often free with a purchase of 10 to 20 bags. A two-person crew can blow a 1,000 sq ft attic in 2 to 4 hours: one person feeds the machine, one directs the hose in the attic. Keep insulation 3 inches away from recessed lights not rated for insulation contact, use baffles at the eaves so soffit vents stay open, and wear a dust mask and protective clothing. As a benchmark, DIY blown-in materials run roughly $0.50 to $1.50 per square foot at R-49, while a professional install runs about $1 to $2 per square foot. Under-insulated attics can waste 10 to 30 percent of heating and cooling energy, so the payback is often just a few years.
Most homes need R-38 to R-49 in the attic: about R-30 in hot Zone 1, R-49 in Zones 2 to 3, and R-60 in the coldest Zones 4 to 8. Exterior walls run R-13 to R-23 and floors over unheated space R-19 to R-30. If your attic already has some insulation, you only need to add the shortfall, not the full target.
It depends on the material and target R-value. A 1,000 sq ft attic at R-49 needs about 20 bags of blown fiberglass (55 square feet per bag at that depth) or about 84 bags of cellulose, since cellulose bags are smaller. At R-38, the same attic needs about 14 bags of blown fiberglass. Enter your numbers in the calculator for an exact count.
Depth follows from the R-value and material. For R-49, blown fiberglass needs about 19.6 inches of depth, while cellulose needs about 14 inches. For R-38, the figures drop to about 15.2 inches of fiberglass and 10.9 inches of cellulose. Measure depth in several spots and use the average, since old insulation compresses unevenly.
Batts are pre-cut rolls that are cheaper and DIY-friendly for standard joist spacing with no blower required. Blown-in loose fill covers odd spaces, attics, and areas around obstacles much faster, but it needs a rented machine. For a retrofit attic, blown-in is usually the practical choice; for small straightforward attics, batts are simplest.
Yes, R-values add. Measure the existing depth and estimate its R-value, then the calculator fills only the shortfall to your target. For example, topping up 3.5 inches of old fiberglass batts (about R-11) to R-49 needs only R-38 of new material, roughly halving the bags. If the old insulation is wet or compressed, remove it first.
Yes. Blown-in insulation is a beginner-friendly DIY project: rent the blower (often free with 10 to 20 bags), work with a partner, and a 1,000 sq ft attic takes 2 to 4 hours. Keep insulation away from non-rated recessed lights, keep soffit vents open with baffles, and seal air leaks first for the best result.
DIY blown-in materials typically cost $0.50 to $1.50 per square foot at R-49, so a 1,000 sq ft attic runs about $500 to $1,500 in materials. Professional installation runs roughly $1 to $2 per square foot installed. Adding insulation to an under-insulated home can cut heating and cooling costs by 10 to 30 percent, so payback is often just a few years.
Divide your area by the bag's coverage at your target R-value. Coverage shrinks as R-value rises: blown fiberglass covers about 150 square feet per bag at R-13, 88 at R-30, 71 at R-38, and 55 at R-49. Always read the coverage chart printed on your specific bag, since it varies by brand and bag weight.