Cavity area and bags
Insulation calculator
Studs and joists are not insulated, so the area you buy for is smaller than the wall you measured. Enter the area and your framing spacing to get the real cavity area, then a bag count from the coverage on your wrapper.
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You are not insulating the whole wall
A stud is 1.5 inches of wood every 16 inches, which is 9.375 percent of the wall. At 24 inches on center it is 6.25 percent. That is arithmetic rather than an estimate, and it is why buying insulation for the gross area leaves you with bags to return.
The deduction here counts the regular framing only. Top and bottom plates, headers over openings, blocking and corner studs add more, so a real wall runs a few points higher — treat the cavity figure as the minimum you need, which is the safe direction to be wrong in.
Framing deduction by spacing
| Spacing | Member width | Framing | Cavity left |
|---|---|---|---|
| 16 in o.c. | 1.5 in | 9.4% | 90.6% |
| 24 in o.c. | 1.5 in | 6.3% | 93.8% |
| 16 in o.c. | 3.5 in | 21.9% | 78.1% |
| Open attic floor | — | 0% | 100% |
Coverage is on the wrapper, and it is conditional
Batts state the square feet in a bag. Loose fill states square feet at a given depth, and that qualifier is the whole thing: the same bag covers roughly twice the area at half the depth, and at half the depth you get roughly half the R-value. Blowing an attic thin to make the bags go further is the most common way an insulation job quietly fails.
This is why coverage is a field here rather than a number we picked. It varies by manufacturer, by material and by the R-value you are buying, and any figure printed on this page would be wrong for most of the people reading it.
What R-value you need is a code question
Required R-values depend on your climate zone and on which part of the building you are insulating, and they are set by the energy code your jurisdiction has adopted — which is not always the current one. We do not publish a table of them here, because a stale table would be worse than no table: someone would insulate an attic to a number that was withdrawn two code cycles ago.
Get the target from your local building department or your energy code, then buy the product that reaches it and enter its coverage above.
Compression matters too. A batt rated for a 5.5 inch cavity squeezed into 3.5 inches does not deliver its printed R-value, and the loss is permanent. Match the batt to the cavity rather than forcing a thicker one in.
Common questions
How do I calculate how much insulation I need?
Measure the gross wall or ceiling area, subtract the framing, then divide by the coverage printed on the bag. This calculator does the framing deduction from your spacing and gives you the bag count.
How much of a wall is framing?
About 9.4 percent for 1.5 inch studs at 16 inches on center, and 6.3 percent at 24 inches. Plates, headers and blocking add a few points more on a real wall.
What R-value do I need?
It depends on your climate zone and which assembly you are insulating, and it is set by the energy code your jurisdiction has adopted. Ask your building department rather than trusting a table online, including ours — this is why we do not publish one.
Can I compress a thicker batt into a thinner cavity?
You can fit it, but you lose R-value permanently and you will not get the number on the label. Buy the batt made for the cavity depth you have.
Does loose fill coverage depend on depth?
Yes, and that is the important part. Bag coverage is stated at a specific settled depth. Spreading the same bags thinner covers more area at a lower R-value.
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