Insulation Calculator: Blown-In Attic Bags and Wall Batts

How to use the insulation calculator

Blown-in attic mode

1. Pick your climate zone. The hint under the menu shows the ENERGY STAR attic target for that zone: one figure for an empty attic, and one for the amount to add when 3–4 in is already there.
2. Pick a material and enter the attic floor area. Use the footprint of the heated ceiling below.
3. Set the target R-value. This is the total you want once the job is done, not just the amount you are adding.
4. Measure the existing depth. Push a ruler to the drywall in a few spots and average them. The calculator multiplies by 3 to estimate the R-value already there, which is the rule of thumb ENERGY STAR gives.

The result is the number of bags to buy, the settled depth to blow in on top, and the weight of material.

Batts for walls mode. Pick a batt size, then enter the total length of the walls, their height, and the area of doors and windows. The calculator subtracts the openings and the studs, adds your waste allowance, and counts both pieces and bags.

How the numbers are worked out

Blown insulation is sold by coverage, not volume. Every bag has a chart showing how many bags cover 1,000 sq ft at each R-value, and how deep the layer has to be once it settles. For blown fiberglass, we use the Owens Corning AttiCat chart, which uses 27.5 lb bags. Reaching R-49 takes 25 bags per 1,000 sq ft and a settled depth of 17 in. For cellulose, we use National Fiber’s loose-fill attic chart, which uses 25 lb bags. There, R-49 covers 16 sq ft per bag at 13.1 in settled, or about 62.5 bags per 1,000 sq ft.

Cellulose is denser, so it takes more bags but a thinner layer. Between chart rows, the calculator interpolates in a straight line.

When you are topping up an existing layer, the calculator works out the R-value you still need and prices only that amount. That is an approximation, since the charts assume a bare ceiling. Old insulation that is matted, wet or patchy is worth less than depth × 3 suggests. If yours looks like that, enter a smaller depth.

For walls, a framed wall is not all cavity. A 1½ in stud every 16 in takes about 9% of the wall, and at 24 in spacing it takes about 6%. The calculator removes that before counting batts. The sq ft on a batt bag is the area of the batts themselves. In the Johns Manville table, 11 pieces of 15 × 93 in R-13 come to 106.56 sq ft.

Worked examples

Empty attic, 1,000 sq ft, R-60 with blown fiberglass

The chart gives 31.5 bags per 1,000 sq ft at R-60, so you need 32 bags with no extra and 34 with 5%. The settled depth is 20½ in, which is taller than many attic hatches and baffles. Install rafter vents and a hatch dam before you start blowing.

Topping up 4 in to R-49, 1,200 sq ft

4 in × 3 ≈ R-12 already there, so you need R-37 more. With fiberglass, that is 18.45 bags per 1,000 sq ft, or 22.1 bags for 1,200 sq ft. Round up to 23 bags, adding about 13 in on top. With cellulose, the same job takes 55 bags but adds only about 10 in.

800 sq ft attic, empty, to R-38 with cellulose

At 21.4 sq ft per bag, 800 ÷ 21.4 = 37.4, so you need 38 bags, about 950 lb of material, settled at 10.2 in.

Garage wall, 40 ft × 8 ft, R-13 batts

320 sq ft minus 30 sq ft of openings leaves 290 sq ft. Taking out studs at 16 in and adding 5% gives 276 sq ft of cavity. That is 29 pieces of 15 × 93 in, or 3 bags of 11.

How much insulation do I need? R-values by climate zone

ENERGY STAR’s retrofit table for wood-framed homes gives these targets. “Add on top” applies when 3–4 in of insulation is already there.

For walls, ENERGY STAR recommends that when siding comes off an uninsulated wood-frame wall, you blow insulation into the empty cavities. It also recommends adding insulating sheathing under the new siding: R-5 in zone 3, and R-5 to R-10 in zones 4 to 8. The Department of Energy’s homeowner guide gives a similar table, listing ranges instead of single figures. Its range for an empty attic in zones 2 and 3, for example, is R-38 to R-60.

As a rough guide, the common loose materials run about R-3 to R-3.5 per inch. Fiberglass batts are rated by thickness and density, so a 3½ in batt can be R-11, R-13 or R-15.

Before you blow insulation into an attic

ENERGY STAR’s attic guide lists the steps a good insulation job includes. They are worth following whether you hire the work out or do it yourself.

• Seal air leaks first. Seal the chase around the plumbing vent pipe. Seal the gap between a flue and the chimney chase with metal flashing and high-temperature caulk. Weatherstrip and insulate the attic hatch. Sealing is much easier before the insulation goes in.
• Cover old can lights. Older recessed fixtures that stick up into the attic should be covered and sealed with covers made for them before insulation goes over them.
• Install baffles. Fit rafter vents at the eaves so the new, deeper layer does not block the soffit vents.
• Test fuel-burning appliances. If a gas water heater or furnace is in the attic, have an HVAC contractor do combustion safety testing after air sealing.
• Leave old insulation in place. You can usually add new insulation on top unless the old insulation is wet or contaminated.

Planning a ceiling or wall repair at the same time? The drywall calculator counts sheets, screws and mud, and the paint calculator covers the finish coats.

Sources & assumptions

How this calculator gets its numbers. Full method: our methodology.

• Blown fiberglass bags per 1,000 sq ft and settled depths follow the Owens Corning AttiCat data sheet (27.5 lb bags). Blown cellulose follows the National Fiber loose-fill attic chart (25 lb bags). R-values between chart rows are interpolated in a straight line, and the chart tops out at R-60.
• Existing insulation is estimated at R-3 per inch, the ENERGY STAR rule of thumb. Only the remaining R-value is priced, which approximates a top-up with full-depth charts.
• Wall batts: openings are subtracted, then 1½ in studs (1.5 ÷ stud spacing) are removed from the wall area. Pieces and bags use the Johns Manville kraft-faced batt sizes and sq ft per bag. Extra studs and headers are covered by the waste allowance (default 5%).
• Climate-zone targets are the ENERGY STAR retrofit recommendations for wood-framed homes. Your local energy code may differ.

Sources

1. ENERGY STAR (U.S. EPA), Recommended Home Insulation R-Values. Attic, floor and wall-sheathing targets by climate zone.
2. ENERGY STAR (U.S. EPA), Attic Insulation (ENERGY STAR Home Upgrade). ≈ R-3 to 3.5 per inch (depth × 3); air sealing, baffles, can-light covers, combustion testing.
3. U.S. Department of Energy, Insulation consumer guide (ENERGY STAR / DOE). R-value ranges by climate zone.
4. Owens Corning, AttiCat PINK Blown-In Insulation product data sheet. 27.5 lb bag; bags per 1,000 sq ft and settled thickness, R-13 to R-60.
5. National Fiber, Expanded Bag Coverage Chart for National Fiber’s Cellulose Insulation. Loose-fill attic, 25 lb bag: sq ft per bag and settled depth by R-value.
6. Johns Manville, Fiberglass Manufacturers Fact Sheet. Kraft-faced batt sizes, pieces and sq ft per bag.

Last reviewed October 25, 2026. Product figures vary by brand; the label on what you buy wins.