Attic Insulation Calculator

Reads the coverage chart the FTC requires on the bag instead of a rule of thumb, because bags are not proportional to R-value. Subtracts what is already up there, and reports the load it puts on the ceiling.

The attic

The space

This is the footprint of the heated space underneath, not the area of the roof above it, which is larger because it slopes. If you are only doing part of the attic, enter that part. Coverage charts are written per 1,000 square feet for the same reason.
Measure at several places away from the hatch and take the shallowest, because old insulation settles unevenly and the thin spot is what the ceiling loses heat through. Measure from the top of the drywall, not from the top of the joist.
A rule of thumb is unavoidable here, because nobody knows what was blown into their attic in 1978. The Department of Energy puts loose fill fiberglass at roughly 2.2 to 2.7 per inch and loose fill cellulose at roughly 2.9 to 3.4, and calls both approximate. Old settled material is at the low end of its range, so 2.5 is a deliberately cautious default.

What you are adding

The two behave differently and not only in price. Cellulose reaches a given R-value in less depth, which helps in a shallow attic, and weighs roughly two and a half times as much doing it. Switch between them with the weight line showing and the trade is visible in one number.
The 2021 IECC asks for R-30 in zones 0 and 1, R-49 in zones 2 and 3, and R-60 from zone 4 up. R-38 is on the list because Section R402.2.1 allows one step down where the full height of uncompressed insulation carries over the wall top plate at the eaves.

How it is bought

Bags are not a standard size, so this is the one field worth correcting from the product in front of you. The reference charts behind this answer use a 33.5 lb fiberglass bag and a 30 lb cellulose bag. Weight per square foot does not change with bag size, so entering yours simply recounts the bags.
Lower than most trades because there are no offcuts. What it covers is the machine holding material at the end, uneven depth across a long attic, and the fact that coverage on a label is a maximum rather than an average.

Prices

Many suppliers waive the blower rental above a bag count, which can make the larger order cheaper in total than the smaller one. Ask before comparing two quotes on bag price alone.
The order
28 bags
Depth to add 16.5 in
Finished depth 16.5 in
Total added weight 866 lb
Bags per 1,000 sq ft
21.6
Against the label
Check bags per 1,000 sq ft against the chart on your bag

The detail

Existing R-value 0
R-value to add 49
Coverage per bag 46.4 sq ft
Added weight 0.72 lb per sq ft
Material cost $0

Every figure here is a planning estimate, not a professional takeoff. Yields, waste factors and code requirements vary by product, supplier and jurisdiction, so confirm quantities against the manufacturer’s data and your local building department before you order.

Generated by Mathterial

Blown insulation is specified in R-value and sold in bags, and the thing that connects the two is a chart the government requires on the package. This calculator reads that chart rather than multiplying by a rule of thumb, because the relationship between R-value and bags is not the straight line almost everybody assumes it is.

The chart on the bag is the specification

Under the FTC’s R-value Rule at 16 CFR 460.12, a bag of loose fill has to carry a chart. For R-13, 19, 22, 30, 38 and 49 it must print the minimum settled thickness, the initial installed thickness, the maximum net coverage area, the number of bags per 1,000 square feet, and the minimum weight per square foot.

That is five columns, and the one nobody reads is the last. Depth can be faked by spreading too few bags too thin, and the result looks right from the hatch and performs nothing like the label. Weight per square foot is the column that cannot be faked, which is why this calculator works from it.

Why bags are not proportional to R-value

Take a real fiberglass chart. A bag covers about 198 square feet at R-13. R-60 is 4.6 times the R-value, so if coverage were proportional the same bag would do about 42.9 square feet at R-60. The chart says 36.9.

Cellulose is worse. A bag covering about 78.9 square feet at R-13 should manage 17.1 at R-60 on the same reasoning, and the chart says 13.0.

The reason is that loose fill settles under its own weight, so a deeper layer needs more material per inch than a shallow one. Scale from one row to another and you order 14 percent too few bags of fiberglass, or 24 percent too few of cellulose. That is the difference between finishing the attic and a second trip to the store.

So the answer here interpolates between chart rows instead. Enter a target the chart lists and it uses that row exactly; enter one between rows, which is what happens the moment you subtract existing insulation, and it works out the point in between.

Fiberglass or cellulose is a question about your ceiling

The usual framing is price and green credentials. The figure that actually decides it is on the chart, and it is the weight.

At R-49 over 1,200 sq ftLoose fill fiberglassCellulose
Settled depth16.5 in13.3 in
Weight per sq ft0.72 lb1.81 lb
Total on the ceiling866 lb2,172 lb

Cellulose gets there in about three inches less depth, which matters in a shallow attic or where you are trying to clear the top plate without blocking the vent gap. It does it at roughly two and a half times the weight, and that weight lands on the drywall and the ceiling joists below.

Neither number is a reason to rule a material out. They are the reason to ask the question before the truck arrives rather than after, and on an older house with a ceiling of unknown construction, the heavier option is the one worth checking with somebody who can look at the framing.

Topping up is the normal case

Almost nobody insulates a bare ceiling. There is usually something up there already, and the job is to work out what it is worth and add the difference, so the existing depth field is on by default.

Measure in several places away from the hatch and take the shallowest reading, not the average. Old loose fill settles unevenly and drifts, and the thin spot is where the ceiling actually loses its heat. Measure from the top of the drywall rather than the top of the joist, since the joist is not the floor of the insulation.

Then the R per inch of whatever is already there, which is the one genuinely uncertain input on the page. Nobody knows what was blown into their attic in 1978. The Department of Energy puts loose fill fiberglass at roughly 2.2 to 2.7 per inch and loose fill cellulose at roughly 2.9 to 3.4, and calls both figures approximate. Settled material sits at the low end, so the default here is 2.5 and it is deliberately cautious. Guessing low means buying a few bags too many, which is the cheaper mistake.

Which R-value your zone asks for

Attic Insulation R-Value by Climate Zone · the table, and the eave detail it depends on

Read the guide

R-60 may not be on your bag

Here is a gap worth knowing about before you are standing in the aisle. The 2021 IECC asks for R-60 in climate zones 4 through 8, which is most of the country by land area. The FTC’s mandated chart rows stop at R-49.

Manufacturers are free to print more, and many do carry an R-60 row. But they are not required to, so the row you need may simply not be there, and the answer on this page says so rather than quietly extrapolating past the end of the chart. If your bag stops at R-49, ask the supplier for the full coverage chart for that product. It exists even when the package is abbreviated.

Bags are not a standard size

The reference charts behind this answer use a 33.5 pound fiberglass bag and a 30 pound cellulose bag. Yours may well differ, and bag weight is the one field worth correcting from the product in front of you.

Correcting it is safe, because weight per square foot does not change with bag size. It is a property of the material at that R-value. Enter your bag weight and the coverage per bag and the bag count are recalculated from it, and the depth stays exactly where it was.

The answer prints bags per 1,000 square feet for the same reason. That is the figure the FTC requires on the package, in those words, so it is the one line of this calculator you can hold against the bag in your hands and check.

What it does not count

  • Baffles, soffit dams and the vent gap at the eaves. Those decide whether the insulation can run out over the top plate at all, and the R-value guide covers the detail.
  • Air sealing, which should happen before anything is blown and is the work that makes the rest of it pay. Insulation slows heat; it does not stop air.
  • The volume lost to joists. Coverage charts are published both ways and this uses the unadjusted figure, which is the conservative one. Adjusted for framing, a bag covers slightly more floor.
  • Batts. This is a loose fill tool, and rolls are counted by area and length rather than by weight per square foot.
  • Rim joists, which the rim joist insulation calculator already handles in bays, foam board and spray kits.

One last thing worth doing before the first bag goes in. Staple a ruler upright to a joist near the middle of the attic, where you can see it from the hatch. It costs nothing, it settles every future argument about how deep the insulation actually is, and it is the only way to check the depth after the machine has gone back to the store.