How much insulation do I need for a wall?

The cavity area rather than the wall area. Take the gross area, subtract the windows and doors, then subtract the framing: 1.5 inches of stud every 16 is 9.4% geometrically, and plates, headers and corners add more. Divide what is left by the square feet per package, which the label must state under 16 CFR 460.12(b)(1).

Will R-19 fit in a 2x6 wall?

Not without compressing. A nominal 2x6 dresses to 5 1/2 inches and an R-19 batt is thicker than that. Two separate rules exist because of it: 16 CFR 460.13(f) requires the fact sheet for an R-19 batt to disclose its R-value in a shallower cavity, and the energy code requires an R-19 batt compressed into a nominal 2x6 to be marked with the compressed value where the loss is R-1 or more.

What R-value do I get if I compress a batt?

Less than the label, and the figure is published rather than estimated. For R-19 batts the fact sheet has to carry it, so read it there. For any other R-value there is no such required disclosure, which is a good reason to buy a batt that fits the cavity instead.

Can I put two batts in one stud bay?

You can, and the second one is largely wasted, because the cavity is a fixed depth and both batts end up compressed. The way to more R-value in the same wall is continuous insulation over the framing, which is what the newer energy code wall requirements are asking for.

Because it fits. R-21 batts are made at 5 1/2 inches to suit a nominal 2x6 cavity, which is the problem R-19 has in the same wall. If you are insulating 2x6 framing, comparing R-19 and R-21 on price alone ignores that one of them will be compressed and the other will not.

Do I subtract the studs when buying batts?

Yes. Batts go between the framing, so you are buying cavity area. The geometric figure is 9.4% at 16 inch spacing and 6.3% at 24, and the real figure is larger once plates, headers, corners and window framing are counted. This page starts from the geometry and lets you add the rest.

What is on an insulation label by law?

For batts and blankets of any type: the R-value, length, width, thickness, and square feet of insulation in the package, under 16 CFR 460.12(b)(1). Loose fill has a different and longer list. Retailers selling to do-it-yourself customers must also have fact sheets available.

Is fiberglass or rock wool cheaper?

The published comparison on this page puts fiberglass batts at $0.80 to $2.60 per square foot installed and rock wool at $1.10 to $3.10. Read that as a comparison between materials rather than a rate: the source states no R-value or thickness for any of its rows, and every one of those products costs more at a higher R-value.

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Insulation Calculator

Two federal rules name R-19 batts, and nothing else

Insulation labelling is written in general terms, and then twice it is not. The Federal Trade Commission requires an extra disclosure for R-19 batts specifically, and the energy code separately requires an R-19 batt compressed into a nominal 2x6 cavity to be marked with its compressed value. Two instruments, neither citing the other, both reaching for the same product in the same cavity. The reason is that a nominal 2x6 gives you five and a half inches and an R-19 batt is thicker than that.

Cavity area, packages, and whether the batt fits

Wall length

Wall height

Or enter the area directly (0 to use the dimensions)

sq ft

Windows and doors

sq ft

Framing

Framing spacing

Framing added for plates and corners (ours)

Batt R-value (from the label)

Batt thickness (from the label, 0 to skip the fit check)

Square feet per package (from the label)

sq ft

Cutting allowance (ours)

Priced as (the source's own row labels)

Every figure this page asks for is on the package. 16 CFR 460.12(b)(1) requires a batt label to carry the R-value, length, width, thickness and square feet in the package, and a retailer selling to do-it-yourself customers has to have the fact sheets as well.

Specification summary

Generated from on . Reopen that address to reproduce these figures exactly.

Entered

Result

The batt against the cavity it has to go into

Worked example

A 40 ft by 9 ft wall with 60 sq ft of openings, nominal 2x6 at 16 in on centre, R-19 batts 6 1/4 in thick, 49 sq ft per package.

Gross. 40 × 9 = 360 sq ft, less 60 of openings = 300 sq ft.
Framing. 9.4% geometric plus 15% for plates and corners = 24.4%.
Cavity. 300 × 75.6% = 227 sq ft, which is what you buy.
With cutting. 250 sq ft, so 6 packages.
The fit. 6.25 in of batt into a 5.5 in cavity: it compresses by 0.75 in.
The R-value. Not R-19. Compressed, and lower.

That last line is why both rules exist. The wall is the commonest exterior wall in America, the batt is the commonest wall batt, and the two do not fit each other. 16 CFR 460.13(f) makes the manufacturer publish the compressed value for exactly this case, so it is a number you can go and read rather than one anybody should estimate. Change the batt to a 5 1/2 in R-21 and the warning disappears, which is the whole reason R-21 exists.

The formula

You buy cavity area, not wall area, and then you check whether the batt fits the cavity:

cavity = (A − openings) × (1 − f) packages = ⌈ cavity(1 + k) / p ⌉

A: gross wall area
f: framing fraction. 1.5 / spacing is exact; the rest is yours
k: cutting allowance. Ours, an input
p: square feet per package, which the label must state
t > d: batt thicker than cavity, which is where the two rules bite

The framing fraction cuts both ways and people usually apply it once. It reduces the batt you buy, which this page does. It also reduces the wall's thermal performance, which is a different calculation on a different page, and the same percentage does both jobs.

Why one R-value gets named twice

Regulations do not usually name a product. 16 CFR 460.12(b)(1) covers batts and blankets of any type in one sentence, and every other disclosure in the rule is written the same way.

Then 460.13(f) singles out R-19 batts and requires the fact sheet to disclose the R-value when the batt is thicker than the cavity. Not R-13, not R-21, not R-30. One product.

And the energy code, independently and in a footnote to a completely different document, requires an R-19 batt compressed into a nominal 2x6 cavity to be marked with the compressed value where the loss reaches R-1.

The reason both landed on the same product is dimensional. A nominal 2x6 dresses to 5 1/2 inches and an R-19 batt is thicker than that, so the most common exterior wall in the country squashes the most common wall batt as a matter of routine. Two sets of rule-writers looked at that and decided it had to be said out loud.

Nominal framing against the cavity it gives you
Nominal framing	Cavity depth	What fits without compressing
2x4	3.5 in	3.5 in of batt fits
2x6	5.5 in	5.5 in of batt fits	the one both rules name
2x8	7.25 in	7.25 in of batt fits
2x10	9.25 in	9.25 in of batt fits
2x12	11.25 in	11.25 in of batt fits

Read 2026-09-10. Cavity depths are dressed lumber dimensions, the same ones the framing pages here use. The last column marks the cavity both rules name.

Download this table as CSV. Same cells as printed above, nothing added.

Two rules, written by different people, both name R-19

Insulation labelling is written in general terms. Every batt has to disclose the same five things:

For batts and blankets of any type: the R-value, length, width, thickness, and square feet of insulation in the package.

Then, two paragraphs later, the Federal Trade Commission stops being general:

For R-19 insulation batts, the fact sheet must also disclose the insulation's R-value when installed in wall cavities where the insulation's thickness exceeds the depth of the cavity.

One R-value, by name. Not R-13, not R-21, not R-30. And the energy code does the same thing independently, in a footnote to its own insulation table:

R-19 batts compressed into a nominal 2x6 framing cavity such that the R-value is reduced by R-1 or more shall be marked with the compressed batt R-value in addition to the full thickness R-value.

A federal trade regulation and a model energy code, neither citing the other, both reaching for R-19 batts in a nominal 2x6 cavity.

The reason is dimensional, and it is in the framing rather than in the insulation:

Cavity depth by nominal framing
Nominal framing	Cavity depth	What fits without compressing
2x4	3.5 in	3.5 in of batt fits
2x6	5.5 in	5.5 in of batt fits	the one both rules name
2x8	7.25 in	7.25 in of batt fits
2x10	9.25 in	9.25 in of batt fits
2x12	11.25 in	11.25 in of batt fits

A nominal 2x6 gives you 5.5 inches, and an R-19 batt is thicker than that. So the commonest exterior wall in America squashes the commonest wall batt, at scale, every day, and both rule-writers decided that had to be said out loud rather than left to the installer.

This page will not tell you how thick your batt is. The rule requires the package to, which makes it another figure guaranteed to exist and guaranteed to be in your hand. Enter it above and the calculator will tell you whether it fits.

The gap is worse than the compression

Compression loses some R-value and the rules make somebody publish how much. A gap loses far more and nobody publishes anything, because it is not a property of the product.

A batt cut short at the top plate leaves a void where air can move, and moving air carries heat by a mechanism the R-value does not describe at all. The same is true of a batt that has been pushed behind a wire rather than split around it, or one that bridges over an electrical box instead of being cut to fit behind it.

Friction fit means friction fit. A batt should touch all six surfaces of the bay: both studs, the sheathing, the drywall, the top plate and the bottom plate. Where it does not, the cavity is doing less than the label says by an amount nobody can quantify from a page.

Which is the practical hierarchy: a batt that fits the bay and touches everything beats a higher-numbered batt stuffed in badly, every time, and it is why the industry grades installation quality separately from R-value.

Reading a batt label, and the three numbers that matter

The label is regulated, so it is worth knowing what has to be on it and which parts you actually use.

Square feet in the package is the number this calculator divides by, and it already accounts for the batt width. A package sized for 16 inch framing does not go as far on a 24 inch wall, so check the width matches your framing before comparing two packages on price.

Thickness is the one people never look at, and it is the one that decides whether the R-value on the front is the R-value you get. Compare it with the cavity depth before you buy, not after.

R-value is the headline and it is a laboratory figure for the material lying undisturbed at its full thickness. Everything above is about the distance between that and your wall.

The rule also requires the length, the width, and for retailers selling to do-it-yourself customers, a fact sheet available on request. That fact sheet is where the compressed figure lives for an R-19 batt, and asking for it is a reasonable thing to do.

What compression actually does, and what it does not

Squashing a batt does not throw its R-value away in proportion. It loses some, and it loses less than the thickness ratio suggests, because the material gets denser as it gets thinner and density is not useless.

But it does lose some, and the loss is the reason both rules exist. The energy code sets the disclosure trigger at a reduction of R-1 or more, which is a small enough loss to be worth naming.

The figure is on the fact sheet, not on this page. 460.13(f) makes the manufacturer publish the compressed value for exactly this case, so it is a number you can go and read rather than one anybody should be estimating. Ask the retailer for the fact sheet; 460.14 requires them to have it.

Two practical consequences.

You cannot make up the difference with more batt. A cavity is a fixed depth. Stuffing two batts into one bay compresses both and the second one is largely wasted; the way to more R in the same wall is continuous insulation over the framing, which is what the energy code's newer wall cells are asking for.

And compression is not the same as a gap. A batt cut short, or pushed around a wire, leaves a void, and a void is far worse than compression because air moves in it. The code's own line on this is blunt:

R-values are minimums. Where insulation is installed in a cavity that is less than the label or design thickness of the insulation, the installed R-value of the insulation shall not be less than the R-value specified in the table.

You buy less batt than the wall has square feet

Batts go between the framing, so the square footage you buy is the cavity area rather than the wall area, and the two differ by whatever the framing occupies.

The geometric part is exact: a 1.5 inch stud every 16 inches is 9.4% of the wall, and every 24 inches is 6.3%. The rest of the framing, plates, headers, corners, window and door framing, is real and is not something a general figure covers, so the calculator starts from the geometry and lets you add to it.

Then subtract the openings. A window is not a cavity, and on a wall with a lot of glass that correction is larger than the framing one.

Then add back for cutting. Batts come in fixed widths for 16 and 24 inch framing and in fixed lengths, and every non-standard bay, every rim joist, every stud pack at a corner is cut from a full piece. What is left over is usually the wrong shape for anywhere else.

Packages are counted from the square feet the package states, which 460.12(b)(1) requires it to. That figure already accounts for the batt width, so a package sized for 16 inch framing does not go as far on a 24 inch wall.

The sources, and what the label guarantees you

Read 2026-09-10.

16 CFR Part 460, Labeling and Advertising of Home Insulation, Federal Trade Commission, sections 460.12(b)(1) and 460.13(f). Requires every batt package to disclose R-value, length, width, thickness and square feet in the package, and then singles out R-19 batts alone for an additional disclosure of the R-value when the batt is thicker than the cavity it goes in.

2009-2021 IECC and IRC Minimum Insulation Requirements for New Homes, Building America Solution Center, US Department of Energy, Pacific Northwest National Laboratory, footnotes to the insulation table. Carries the energy code's own footnote on the same product: R-19 batts compressed into a nominal 2x6 cavity such that the R-value is reduced by R-1 or more must be marked with the compressed value as well as the full thickness one.

The useful thing about insulation is that the label is regulated. Unlike aggregate density or a supplier's price, the figures this calculator needs are required by federal rule to be printed on the package you are holding, and a retailer selling to do-it-yourself customers is required to have the fact sheets available as well.

Which is why this page asks for them instead of publishing an average. The one line worth reading on any of those labels is the one the rule makes every fact sheet carry:

To get the marked R-value, it is essential that this insulation be installed properly.

What R-value you need is a different question, answered on the R-value calculator, and it depends on your climate zone and on which edition of the energy code your jurisdiction enforces.

Frequently asked questions

How much insulation do I need for a wall?: The cavity area rather than the wall area. Take the gross area, subtract the windows and doors, then subtract the framing: 1.5 inches of stud every 16 is 9.4% geometrically, and plates, headers and corners add more. Divide what is left by the square feet per package, which the label must state under 16 CFR 460.12(b)(1).
Will R-19 fit in a 2x6 wall?: Not without compressing. A nominal 2x6 dresses to 5 1/2 inches and an R-19 batt is thicker than that. Two separate rules exist because of it: 16 CFR 460.13(f) requires the fact sheet for an R-19 batt to disclose its R-value in a shallower cavity, and the energy code requires an R-19 batt compressed into a nominal 2x6 to be marked with the compressed value where the loss is R-1 or more.
What R-value do I get if I compress a batt?: Less than the label, and the figure is published rather than estimated. For R-19 batts the fact sheet has to carry it, so read it there. For any other R-value there is no such required disclosure, which is a good reason to buy a batt that fits the cavity instead.
Can I put two batts in one stud bay?: You can, and the second one is largely wasted, because the cavity is a fixed depth and both batts end up compressed. The way to more R-value in the same wall is continuous insulation over the framing, which is what the newer energy code wall requirements are asking for.
Why is R-21 a thing?: Because it fits. R-21 batts are made at 5 1/2 inches to suit a nominal 2x6 cavity, which is the problem R-19 has in the same wall. If you are insulating 2x6 framing, comparing R-19 and R-21 on price alone ignores that one of them will be compressed and the other will not.
Do I subtract the studs when buying batts?: Yes. Batts go between the framing, so you are buying cavity area. The geometric figure is 9.4% at 16 inch spacing and 6.3% at 24, and the real figure is larger once plates, headers, corners and window framing are counted. This page starts from the geometry and lets you add the rest.
What is on an insulation label by law?: For batts and blankets of any type: the R-value, length, width, thickness, and square feet of insulation in the package, under 16 CFR 460.12(b)(1). Loose fill has a different and longer list. Retailers selling to do-it-yourself customers must also have fact sheets available.
Is fiberglass or rock wool cheaper?: The published comparison on this page puts fiberglass batts at $0.80 to $2.60 per square foot installed and rock wool at $1.10 to $3.10. Read that as a comparison between materials rather than a rate: the source states no R-value or thickness for any of its rows, and every one of those products costs more at a higher R-value.

Check these numbers yourself

16 CFR Part 460, Labeling and Advertising of Home InsulationFederal Trade Commission, sections 460.12(b)(1), 460.13(f) and 460.14, read 2026-09-10. Requires every batt package to disclose R-value, length, width, thickness and square feet in the package, and then singles out R-19 batts alone for an additional disclosure of the R-value when the batt is thicker than the cavity it goes in. Section 460.14 requires retailers selling to do-it-yourself customers to have the fact sheets available.
2009-2021 IECC and IRC Minimum Insulation Requirements for New HomesBuilding America Solution Center, US Department of Energy, Pacific Northwest National Laboratory, read 2026-09-10. Carries the energy code's own footnote on the same product: R-19 batts compressed into a nominal 2x6 framing cavity such that the R-value is reduced by R-1 or more shall be marked with the compressed batt R-value in addition to the full thickness R-value.
This Old House, how much does spray foam insulation costWritten by Stephanie Minasian-Koncewicz, updated 13 April 2026, read 2026-09-10. Its comparison table gives installed cost per square foot for fiberglass batts, blown-in, rock wool and spray foam. It states no R-value or thickness for any of the four rows, so it is quoted here as a comparison between materials rather than as a rate that can be held to.

How every figure here is verified: Sources & Method. Who builds this: About. Found something wrong? Tell us and it gets fixed or removed.

Figures on this page last checked against the source documents on 2026-09-10. Codes are amended locally; confirm against the edition your jurisdiction enforces.

Related calculators

Every figure this calculator asks for is on the package by federal rule, and this page prints none of them as an average. Batt thickness, R-value and square feet per package are label disclosures under 16 CFR 460.12(b)(1); the compressed R-value for an R-19 batt is a fact sheet disclosure under 460.13(f). The framing addition and the cutting allowance are this page's own figures and are inputs. The quoted prices state no R-value or thickness and are a comparison rather than a rate. What R-value you need is a separate question answered on the R-value calculator.