Attic Insulation Calculator

How to estimate attic insulation

Blown-in insulation is bought by the bag, and every bag on the shelf carries a coverage chart that says how many square feet one bag covers at each R-value. Federal rules require it. The whole estimate is reading the right row of that chart and dividing.

R-value to add = target R-value − (existing depth × R per inch)
bags = attic floor area ÷ square feet per bag at that R-value, rounded upThe FTC's R-value rule, 16 CFR 460.12(b)(2), requires every loose-fill label to state "the minimum settled thickness, initial installed thickness, maximum net coverage area, number of bags per 1,000 square feet" at each R-value.

Measuring the attic floor

Measure the floor you will cover, which on a single-story house is close to the house's outside footprint. On a two-story house it is the ceiling of the top floor only. Leave out a garage unless it is heated and you mean to insulate over it. Odd shapes go in as rectangles added together.

Choosing the target

ENERGY STAR's current table, based on the 2021 International Energy Conservation Code, recommends R-30 for an uninsulated attic in climate zone 1, R-49 in zones 2 and 3, and R-60 in zones 4 through 8. The zone map is on the same ENERGY STAR page.

Counting what is already there

You only buy for the difference. ENERGY STAR's method for the existing layer is to measure its depth and multiply by a figure for the material: 2.5 for loose fiberglass, 3.2 for fiberglass batts, 2.8 for rock wool and 3.7 for cellulose. Six inches of old batts works out to about R-19, so reaching R-49 means adding about R-30.

That multiplication is an estimate, and it is the only part of this page that uses one. Old insulation compresses and gets disturbed, so measure in several places and use the average. New insulation is not estimated that way. Its R-value comes from the chart.

Reading the chart

Owens Corning's AttiCat chart, the fiberglass product sold for homeowner rental machines, gives 39.9 square feet per bag at R-49, blown 17 inches deep. Greenfiber's SANCTUARY cellulose chart gives 14.1 square feet per bag at R-49, installed at 14.3 inches and settling to 13.3. Fiberglass charts list installed and settled depth as the same number because, in Owens Corning's words, "settling is negligible". Cellulose settles about 7 percent, so its chart shows both.

The chart is a minimum in two ways at once. Owens Corning states that failing to provide both the required number of bags and at least the minimum thickness will result in a lower R-value. Hitting the depth with fewer bags means the layer is fluffier than tested. Using all the bags at a thinner depth means it is packed too tight. The depth rulers in the buy list are how you check the second half.

The rounding trap

Charts print two columns that look like the same thing: bags per 1,000 square feet, and square feet per bag. On AttiCat at R-49 the first column says 25. Dividing 1,000 by 39.9 says 25.06, which means buying 26. The coverage column is the more exact one, so this page divides by it and rounds up.

Between the rows

A target of R-49 over R-19 already in place means adding R-30, which is a chart row. Other combinations land between rows. Here the page interpolates in a straight line between the two nearest rows, for both bags and depth. That step is ours; the manufacturer does not publish it. The error is small because the charts change smoothly from one row to the next.

Why other calculators disagree

For R-49 over 1,000 square feet, the coverage charts of four fiberglass makers say 22 to 26 bags. We tested calculators that instead convert R-value to a depth with a flat R-per-inch figure, then convert depth to volume, then guess a bag yield. They said 48 and 67 bags. Three approximations stacked on each other drift a long way. The FTC rule tells professional installers the same thing when they write up a job: use the data the manufacturer gives you, and "do not multiply the R-value for one inch by the number of inches you installed."

Fiberglass or cellulose

The bag counts are not comparable. Cellulose bags hold denser material, so the same attic at R-49 takes 26 bags of AttiCat and 71 bags of SANCTUARY. Compare them by price per square foot covered. The cost field above multiplies whatever price you type by the count for the product you picked.

A quick cost check

A 1,000 square foot attic with nothing in it, going to R-49 with AttiCat, is 26 bags. At $40 a bag that is $1,040. With 6 inches of old batts already there, about R-19, the amount to add drops to about R-30, which the chart covers at roughly 69 square feet per bag: 15 bags, or $600.

Common estimating mistakes

• Buying for the full target over existing insulation. Subtract what is there first.
• Using bags per 1,000 instead of square feet per bag. The rounded column can leave you a bag short.
• Blowing to depth with too few bags. The chart requires both numbers.
• Mixing up two products' charts. Each product has its own. The count on this page belongs to the product named next to it.
• Converting through R-per-inch. Use the chart for new material and keep R-per-inch for estimating the old layer.

When to call a pro

Several attic conditions are not estimation problems. The EPA says to assume vermiculite insulation contains asbestos and not to disturb it, which rules out blowing anything over it. ENERGY STAR lists knob-and-tube wiring as a fire hazard in contact with insulation, along with wet insulation, mold, rot, and bathroom or dryer vents that exhaust into the attic, as reasons to consult a professional before starting. Keep insulation at least 3 inches from recessed lights unless they are rated for contact, never cover soffit vents, and keep the clearances around flues and chimneys that your local code requires. If any of those apply, a home energy auditor or insulation contractor should look before you buy.