How Much Insulation Do I Need? Batts and Blown-In Bags

Measure the area, subtract the R you already have from your target, then divide by what the product covers. Batts: area ÷ package sq ft, per layer. Blown-in: area ÷ 1,000 × bags per 1,000 from the bag chart. Round up.

A 30 × 40 ft attic is 1,200 sq ft. To reach R60 from bare joists with the AttiCat chart, 1,200 ÷ 1,000 × 31.5 = 37.8, so 38 bags. With two layers of R-30 batts at 53.33 sq ft per package, it is 23 packages per layer, or 46.

How do you work out how much insulation you need?

1. Measure the area. For an attic or floor, multiply length by width. Split odd shapes into rectangles (see how to calculate square feet). Take out a hatch or chimney chase only if you will not insulate it.
2. Pick the target R-value. The attic R-value guide has the ENERGY STAR and DOE tables by climate zone. As an example, ENERGY STAR suggests R60 for an uninsulated attic in zone 4A. New construction follows the energy code adopted where you build.
3. Subtract what you already have. R-values of layers add up, so the R to add is the target minus the existing R.
4. Read the product label or chart. Batts list the square feet in a package. Blown-in bags come with a chart of bags per 1,000 sq ft for each R-value.
5. Divide and round up. You buy whole packages and whole bags.

What is the formula?

R to add          = max(0, target R − existing R)
-- batts and rolls (one layer at a time)
packages per layer = ceil( area ÷ package sq ft )
layers             = R to add ÷ R per layer
-- blown-in
bags               = ceil( area ÷ 1,000 × bags per 1,000 sq ft at the R you are adding )
minimum thickness  = from the same row of the chart

Package coverage, R per layer, bags per 1,000 sq ft and thickness all come from the product you buy, so none of them is a general fact. If you want a cushion, add an extra percentage to the area before rounding. No source in our research gives a standard allowance for batts or bags. The manufacturers say to divide by the coverage and round up, so the examples below use no extra.

How many batts or rolls do you need?

Divide the area by the square feet in one package, then round up. A second layer goes across the first. The DOE says to fit the first layer between the joists and lay the second perpendicular to it, which covers the tops of the joists and reduces heat loss through the framing. Each layer needs its own count. Packaging varies a lot, as this example product sheet shows.

These are example rows from one manufacturer’s fact sheet (May 2026). Other brands, and other sheets, show different counts, so read the label on the package. Two R-19 products in the table cover 77.5 and 48.96 sq ft per package. The label also decides the thickness, and a batt does not give its full R-value if you squeeze it into a thinner space.

How many bags of blown-in insulation do you need?

Blown-in (loose-fill) bags come with a coverage chart. Find the row for the R-value you are adding, then:

bags = ceil( area ÷ 1,000 × bags per 1,000 sq ft )
or, with the "maximum net coverage" column:
bags = ceil( area ÷ sq ft per bag )

Here is a chart for one product, the Owens Corning AttiCat (27.5 lb nominal bag), as an example.

Owens Corning’s rule is that you reach the stated R-value by installing at least the required number of bags per 1,000 net sq ft at a thickness not less than the minimum on the label. If the installer meets only one of the two, the R-value comes out lower. So the bag count is only half the check: measure the depth as you go.

The numbers change by product. This chart is for the AttiCat machine. Greenfiber’s Sanctuary chart for the same R-30 shows 38.6 bags per 1,000 sq ft, against 14.6 here, because the bag weight and density differ. Always use the chart for the exact product and machine you will use.

How many bags by attic size?

These counts use the example AttiCat rows above, rounded up. They show the method. They are not a general rule for blown-in insulation.

The R-value here is the R you are adding. If the attic already has R-19 and you want R-49, you add R-30, so use the R-30 column. For the targets themselves, see the R-value guide.

Worked examples: a 1,200 sq ft attic

The attic is 30 × 40 ft, so the area is 30 × 40 = 1,200 sq ft.

• Two layers of R-30 batts to reach R60 from bare joists. Package of 10 batts at 16 × 48 in covers 53.33 sq ft. 1,200 ÷ 53.33 = 22.50, so 23 packages per layer. Two layers: 46 packages, and 30 + 30 = R60. With a 5% extra the count per layer would be 23.6, so 24 (48 in total).
• Blown-in AttiCat to R-60 from bare joists. 1,200 ÷ 1,000 × 31.5 = 37.8, so 38 bags. Cross-check: 1,200 ÷ 31.8 sq ft per bag = 37.7. Minimum thickness: 20.5 in.
• Blown-in AttiCat to R-30. 1,200 ÷ 1,000 × 14.6 = 17.52, so 18 bags, with a minimum thickness of 10.75 in.
• Existing R-19 (about 6 in of fiberglass), target R-38, Greenfiber Sanctuary. R to add = 38 − 19 = 19. The R-19 row shows 44.2 sq ft per bag. 1,200 ÷ 44.2 = 27.15, so 28 bags, with a minimum installed thickness of 5.7 in to add. Greenfiber’s own example prints 27 for the same division because it rounds down. Rounding up is the safer count: 27 bags at 44.2 sq ft cover 1,193.4 sq ft, which leaves about 7 sq ft bare.

Greenfiber’s chart also has a gross-coverage column for 2 × 6 framing at 16 in on center. At 48.8 sq ft per bag, the same job is 1,200 ÷ 48.8 = 24.59, or 25 bags. The net figure is the cautious default, since it gives more bags.

How much insulation do walls need?

Use the gross wall area minus openings. A batt package already counts the stud space, so you do not add a framing factor. Example: a 20 ft × 8 ft wall is 160 sq ft. Two 3 × 4 ft windows are 24 sq ft and a 3 × 6.67 ft door is about 20 sq ft, so the openings are 44 sq ft and the net area is 160 − 44 = 116 sq ft. With R-13 batts, 16 × 96 in, 11 pieces per package (117.34 sq ft, from the example sheet), 116 ÷ 117.34 = 0.99, so 1 package. With 5% extra it becomes 1.04, which rounds up to 2. For the wall area itself, see the drywall guide, which uses the same gross-minus-openings method.

Assumptions, common errors and limits

• Coverage, R per layer, bags per 1,000 sq ft and thickness are product-specific. The tables here are examples from named charts, so check your product’s chart.
• Using the full target instead of the R to add. Subtract the existing R first.
• Meeting only the bag count. Blown-in also has to reach the minimum thickness.
• Forgetting to round up. You buy whole packages and bags.
• Compressing batts. Squeezing a batt into a thinner cavity lowers its R-value.
• Skipping the prep. Air-seal first, keep attic vents clear with baffles, keep insulation at least 3 in away from recessed lights unless they are IC-rated, and have knob-and-tube wiring checked before covering it. Follow local code for vapor retarders, and use no vapor barrier between layers.
• The R-value targets come from ENERGY STAR and DOE guidance. They are not code. This is a quantity estimate, not an energy audit, and spray foam and rigid board are not covered.

Common questions