How much insulation does my attic need?

Start with the ceiling R-value in the code edition your jurisdiction enforces, then convert it to inches using the chart on your insulation bag. In Climate Zone 5 that is R-38 under the 2009 IECC, R-49 under the 2012 through 2018 editions and R-60 under the 2021, which at the 3.7 R per inch of the chart used on this site is 10.3, 13.2 and 16.2 inches settled, the last two by this site's extrapolation. Multiply by the heated ceiling area only, with the hatch taken out.

How many inches of blown insulation is R-60?

It depends on the product, and the figure is on the bag rather than on any web page. The coverage chart used on this site prints no R-60 row, but its rows run at 3.7 R per inch with installed thickness 11.1 percent above settled, and extending those constants, which is this site's arithmetic, gives 16.2 inches settled and 18.0 inches as installed. Federal labeling rules only require a chart to print rows up to R-49, so R-60 may not appear on yours.

What R-value does code require for an attic?

That depends on your climate zone and, more importantly, on which code edition your jurisdiction enforces. The same Zone 5 ceiling cell reads R-38, R-49 and R-60 across the 2009, the 2012 to 2018 and the 2021 IECC. A jurisdiction is bound by the edition it has adopted, so ask your building department which edition they are on before buying anything.

Why is the insulation thinner near the eaves?

Near the wall the roof sits too low above the ceiling to fit the full depth plus the 1 inch airspace IRC R806.3 keeps open at the vent. Add that inch to your settled depth and divide by the pitch as a ratio, which is 4 divided by 12 on a 4:12 roof, to get how far in the shortfall reaches. At R-60 on that roof, by this site's arithmetic at 3.7 R per inch, it is 4.3 feet in from each wall under an eave.

How many bags of insulation do I need for a 1,000 square foot attic?

On the chart used for the worked figures here, 56 bags at R-38 from its printed row. It prints no R-49 or R-60 row; this site's extrapolation from its other rows gives 72 at R-49 and 88 at R-60. Your own bag prints the number for its product directly, as bags per 1,000 square feet at each R-value, and that figure is the one to divide your area by.

Do I have to leave a gap at the eaves?

Where eave or cornice vents are installed, yes. IRC R806.3 requires not less than a 1 inch space between the insulation and the roof sheathing and at the vent, and blocking, bridging and insulation must not block the free flow of air. Baffles hold that gap open and have to be fitted before the insulation goes in, because nobody reaches the eaves afterwards.

How big does an attic access opening have to be?

Not less than 22 inches by 30 inches under IRC R807.1, required wherever the attic has 30 inches or more of vertical height over an area of not less than 30 square feet, and located somewhere with ready access. Treat the panel as part of the ceiling: board insulation on its back and a gasket on its frame.

Can an attic have too much insulation?

The sources used here set no upper limit on R-value. What they do set is the one inch of clearance R806.3 keeps open above the insulation at a vented eave, and the weight involved: R-60 across 1,000 square feet is a little over a ton of loose fill, on this site's extrapolation from one cellulose coverage chart. Past that, the limits are the space under the roof and what the ceiling can carry.

How Much Attic Insulation Do I Need?

Two numbers answer this and only one of them is in the code. The code sets an R-value for the ceiling, your product sets how many inches of it that is, and the roof decides how much of the ceiling can hold those inches at all. The third number is the one nobody prints, and on a shallow roof it is the largest of the three.

By Lucas Pradella, builder and editor. Checked 2026-09-16.

The question splits into three, and three different documents answer it

How much insulation an attic needs is really three questions. How deep, which comes from a table in the energy code and a chart on the bag. Over what area, which is the ceiling plane below the attic and not the footprint of the house. And how much of that area can hold the depth at all, which is decided by the roof and is the one nobody prints.

Each has its own source. The prescriptive ceiling R-value is IECC Table R402.1.3 in the 2021 edition, Table R402.1.2 in the 2015 and 2018, and Table R402.1.1 in the 2009 and 2012. The inches per R come from the coverage chart 16 CFR 460.12(b)(2) requires on every loose-fill package sold in the United States. The clearance that caps the depth at the eave is IRC R806.3. Getting one of the three right and the other two wrong is how an attic ends up correctly specified and badly insulated.

The required R-value changed twice in twelve years

A ceiling in Climate Zone 5 is R-38 under the 2009 IECC, R-49 under the 2012, 2015 and 2018 editions, and R-60 under the 2021. Zone 4 outside Marine moves on the same three steps. Zones 6, 7 and 8 held at R-49 through 2018 and went to R-60 in 2021. Zone 1 has not moved in twelve years.

A code edition applies only where a jurisdiction has adopted it, and a jurisdiction can amend what it adopts. So the useful question is not what R-value an attic needs. It is which edition your building department enforces, and that is one phone call before anything is ordered.

The table number moves with the edition too, which is why a citation with no edition on it is worth nothing. The IRC prints the same requirements under its own numbering, Table N1102.1 in 2009 through Table N1102.1.3 in 2021, and the DOE source flags three 2009 cells where the IRC and the IECC give different numbers for the same component in the same year: basement walls in Zones 6, 7 and 8, and floors in Zones 7 and 8.

The cross-edition table, all seven zone rows and all seven components, sits on the R-value calculator here rather than being repeated on this page.

R-value is not inches, and the conversion is printed on the bag

Insulation is specified in R and sold in bags, and federal trade regulation puts the bridge between them in your hand. 16 CFR 460.12(b)(2) requires every loose-fill package to carry 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, at R-13, 19, 22, 30, 38 and 49.

Notice where that list stops. The six R-values the rule names end at 49, and the 2021 IECC asks for R-60 across Zones 4 through 8. For a new attic in most of the country, the row you actually need is not one the label is required to print, so you will be reading it off the rows that are there.

That is safer than it sounds, because a coverage chart is generated from two constants. On the US GreenFiber Cocoon INS550LD cellulose chart carried on this site, a page dated 2005 and used here for its arithmetic rather than as current product data, all twelve rows, R-11 to R-50, divide out at 3.7 R per inch of settled thickness to within rounding, and every installed thickness is 1.111 times its settled one. Twelve rows, two numbers, everything else arithmetic. That chart prints R-50 as its top row and has no R-49 or R-60 row. Other products run on other constants, so the figures to use are the ones on your own package.

At 3.7 per inch, this site's arithmetic puts R-38 at 10.3 inches settled, R-49 is 13.2 and R-60 is 16.2. The installed thicknesses, which is what you blow to on the day, are 11.4, 14.7 and 18.0 inches. The chart prints the R-38 pair, 10.3 and 11.4, directly; the R-49 and R-60 pairs are extrapolated at the same two constants.

Every time the code went up, more of the ceiling stopped being able to meet it

Here is the part no attic insulation page found from here carries. Toward each eave the underside of the roof drops closer to the ceiling joists, so a band runs around the whole perimeter where the target depth does not physically fit. That band gets wider every time the required depth goes up. The code has moved twice. The geometry has not moved at all.

The width is a division: the depth you want, plus the one inch R806.3 keeps open above it, divided by the tangent of the roof pitch. Run it for all three ceiling requirements on an ordinary truss.

Our arithmetic: the DOE cross-edition ceiling requirement, converted at the Cocoon chart's 3.7 R per inch, plus the 1 inch R806.3 clearance, divided by the pitch. Share and average are for a 40 by 25 ft ceiling at 4:12 with eaves on all four walls, as on a hip roof, and no raised heel, with depth tapering straight to nothing at the clearance line. The average is area-weighted R, an average of depth rather than of heat flow
Zone 5 ceiling requirement	Settled depth	Band at 4:12	Band at 8:12	Share of the ceiling in the band, 4:12	Area-weighted average, 4:12
R-38, 2009 IECC	10.3 in	2.8 ft	1.4 ft	33%	R-30.9
R-49, 2012 to 2018 IECC	13.2 in	3.6 ft	1.8 ft	41%	R-37.8
R-60, 2021 IECC	16.2 in	4.3 ft	2.2 ft	49%	R-43.8

Read the last two columns together. The requirement rose 58 percent between 2009 and 2021 and the area-weighted average rose 42 percent, because the extra inches can only be laid where there is room for them. Under the 2009 code a third of a 40 by 25 ft hip-roofed ceiling at 4:12 was in the tapered band. Under the 2021 code it is nearly half.

No code publishes those widths. They are trigonometry on sourced inputs, and anyone can confirm them at their own eave with a rule held vertical against a rafter.

A raised heel truss is the one design change that makes the band disappear, and it has to be specified before the trusses are built. A heel of 17.2 inches, the 16.2 inch settled depth plus the 1 inch clearance, erases the band at R-60, and it does so at any pitch, because the heel and the rise the insulation needs are the same quantity measured from the same place.

Once the roof is framed the options shrink to two. Seal the perimeter thoroughly, since the shallow band lies directly over the top plates of the exterior walls. Or change material at the edge alone, to something that packs more R into fewer inches, and keep the cheaper fill for the open field.

The inch above the insulation is not yours to fill

IRC R806.3 requires not less than a 1 inch space between the insulation and the roof sheathing, and at the location of the vent, wherever eave or cornice vents are installed. The section is quoted in full on the roof ventilation calculator here, which owns it.

That inch is why the band above is computed at depth plus one rather than depth. It is also one way a vent system that was sized correctly on paper stops working: fill gets pushed or blown right to the wall plate, it slumps into the soffit opening, and the problem stays invisible from the hatch.

A baffle stapled to the underside of the deck at each rafter bay keeps that inch clear, and it has to be fitted while the bay is still empty.

How much venting has to be there is a different figure from a different section. R806.2 sets the minimum net free ventilating area at 1/150 of the vented space, with 1/300 available as an exception only where both of two conditions are met: a Class I or II vapor retarder on the warm-in-winter side of the ceiling in Climate Zones 6, 7 and 8, and not less than 40 percent and not more than 50 percent of the required area provided by ventilators in the upper portion, not more than 3 feet below the ridge, with the balance in the bottom one third. The upper share is a band with a ceiling as well as a floor, so too much upper venting fails the exception just as surely as too little.

What that depth weighs, and how many bags it is

Inches settle and bags do not, so the order is best written as a bag count. The same arithmetic gives the load going onto the drywall.

R-38 and R-50 rows as printed on the Cocoon INS550LD chart, with bags for a 1,000 sq ft attic rounded up. R-49 and R-60 are this site's extrapolation, not the manufacturer's figures: that chart prints no row for either. Across its twelve printed rows, weight per square foot is proportional to R to within rounding, so we take 0.03378 lb per square foot per R from the R-50 row (1.689 divided by 50), multiply by the target R, divide the 23.25 lb bag by that weight for coverage per bag, and divide 1,000 by coverage for bags per 1,000 sq ft
Target	Bags per 1,000 sq ft	Coverage per bag	Bags for a 1,000 sq ft attic	Weight on the ceiling
R-38, printed	55.2	18.1 sq ft	56	1,284 lb
R-49, our extrapolation	71.2	14.0 sq ft	72	1,655 lb
R-50, printed	72.7	13.8 sq ft	73	1,689 lb
R-60, our extrapolation	87.2	11.5 sq ft	88	2,027 lb

On that extrapolation, R-60 on a 1,000 square foot ceiling is a little over a ton of material, 88 bags of a 23.25 lb product. That is a figure to have in hand before anyone assumes an old ceiling will carry it.

Two multiplications prove any coverage chart is internally right before you order from it. Maximum coverage per bag times bags per 1,000 square feet has to come to 1,000. Maximum coverage per bag times minimum weight per square foot has to come to the bag's net weight. If either product misses by more than rounding, one of the printed figures is wrong, and you want to find that at the store. Every printed row on the Cocoon chart passes both to within rounding.

The same PDF also prints a chart for a different product, Cocoon INS510LD in a 30 lb bag, which does reach R-60, at 18.0 inches installed and 16.1 settled. Its weights and bag counts belong to that bag and are not used in the table above.

One more figure from the same rule is worth carrying. 16 CFR 460.8 allows no individual specimen to be more than 10 percent below the R-value shown on the label, and sets no matching limit above.

HomeAdvisor's attic insulation guide puts blown-in at $1 to $4 per square foot, batt at $2 to $4, loose-fill at $2 to $5 and spray foam at $2 to $5, with removal of old insulation at $1 to $1.50 per square foot and air sealing at $250 to $750 for the job. Its table is headed average cost per square foot and states no R-value or depth on any row, so a square foot there is a square foot of something unspecified. Last price check: 2026-09-13.

The area is the ceiling below the attic, not the plan of the house

Reaching for the house's square footage can overstate the job, and it does so in three specific places.

Second floors over first floors. A two story house has attic only above its upper level, and that level is often a good deal smaller than the one below it.

Sloped ceilings. A vaulted room has its insulation in the roof assembly itself, with no open attic above it to blow into, so it drops out of this count.

Unheated spaces under a shared roof. An attached garage or a covered porch normally lies outside the conditioned boundary, even when one continuous attic spans it. Draw the line where the heating stops.

A tape in the attic itself gives a better number than a plan, and the same trip gets you the roof pitch and the heel height that the band table above needs.

The hatch is a hole the code requires you to leave

IRC R807.1 requires an attic access opening of not less than 22 inches by 30 inches wherever the attic has a vertical height of 30 inches or greater over an area of not less than 30 square feet, located in a hallway or other location with ready access. That is 4.6 square feet of opening in the ceiling plane, and it is not insulated area, so it comes off the total.

By area that is under half a percent of a 1,000 square foot ceiling. By performance it can be the weakest point in the whole plane, since a loose panel lets warm indoor air straight up into a space built to stay cold.

Glue foam board to the attic side of the panel, run a gasket around the frame, and add a catch that holds the panel down on that gasket. Folding ladder units take the same treatment and need more of it.

Installing it: the sequence is set by what cannot be undone afterwards

Establish the edition first. On a 1,000 square foot ceiling the difference between R-38 and R-60 is 32 bags on the chart used here, by this site's extrapolation, and it decides the whole order.
Air seal the ceiling plane. Top plates, wire holes, plumbing penetrations, recessed fittings and the hatch. Loose fill blown over an unsealed ceiling buries every one of them where nobody will find them again.
Baffle the eaves. R806.3 requires the inch to stay open at the vent, and a baffle fitted after the insulation is a job that does not happen.
Mark the depth on the framing, at the installed thickness rather than the settled one, before any material moves.
Blow to the installed figure on your chart. On the chart carried here that is 11.1 percent deeper than the settled target, because the chart's figures are based on settled thickness, and skipping it is how a finished attic reads short once it settles.
Insulate and weatherstrip the hatch last, from the warm side, and check that it still closes on its seal.

None of the documents quoted here set that order, and this page will not pretend otherwise. What sets it is that several of those steps are far harder, or no longer possible, once the insulation is down.

What this guide does not decide

Your climate zone. This page does not print the zone map. The permit desk will give it to you along with the edition.

Which edition binds you. Same call, and it is the input you cannot get anywhere else.

Whether to insulate the ceiling or the roofline. A roofline assembly brings the attic inside the thermal envelope and stops being a vented attic, which puts it under different sections entirely. IRC R806.5 covers unvented assemblies, and nothing on this page applies to one.

Your product's R per inch. The chart behind the arithmetic here belongs to one manufacturer, is dated 2005, and is on this site to show what a coverage chart is made of. Yours is on the bag you are about to buy.

Run the numbers

Frequently asked questions

How much insulation does my attic need?: Start with the ceiling R-value in the code edition your jurisdiction enforces, then convert it to inches using the chart on your insulation bag. In Climate Zone 5 that is R-38 under the 2009 IECC, R-49 under the 2012 through 2018 editions and R-60 under the 2021, which at the 3.7 R per inch of the chart used on this site is 10.3, 13.2 and 16.2 inches settled, the last two by this site's extrapolation. Multiply by the heated ceiling area only, with the hatch taken out.
How many inches of blown insulation is R-60?: It depends on the product, and the figure is on the bag rather than on any web page. The coverage chart used on this site prints no R-60 row, but its rows run at 3.7 R per inch with installed thickness 11.1 percent above settled, and extending those constants, which is this site's arithmetic, gives 16.2 inches settled and 18.0 inches as installed. Federal labeling rules only require a chart to print rows up to R-49, so R-60 may not appear on yours.
What R-value does code require for an attic?: That depends on your climate zone and, more importantly, on which code edition your jurisdiction enforces. The same Zone 5 ceiling cell reads R-38, R-49 and R-60 across the 2009, the 2012 to 2018 and the 2021 IECC. A jurisdiction is bound by the edition it has adopted, so ask your building department which edition they are on before buying anything.
Why is the insulation thinner near the eaves?: Near the wall the roof sits too low above the ceiling to fit the full depth plus the 1 inch airspace IRC R806.3 keeps open at the vent. Add that inch to your settled depth and divide by the pitch as a ratio, which is 4 divided by 12 on a 4:12 roof, to get how far in the shortfall reaches. At R-60 on that roof, by this site's arithmetic at 3.7 R per inch, it is 4.3 feet in from each wall under an eave.
How many bags of insulation do I need for a 1,000 square foot attic?: On the chart used for the worked figures here, 56 bags at R-38 from its printed row. It prints no R-49 or R-60 row; this site's extrapolation from its other rows gives 72 at R-49 and 88 at R-60. Your own bag prints the number for its product directly, as bags per 1,000 square feet at each R-value, and that figure is the one to divide your area by.
Do I have to leave a gap at the eaves?: Where eave or cornice vents are installed, yes. IRC R806.3 requires not less than a 1 inch space between the insulation and the roof sheathing and at the vent, and blocking, bridging and insulation must not block the free flow of air. Baffles hold that gap open and have to be fitted before the insulation goes in, because nobody reaches the eaves afterwards.
How big does an attic access opening have to be?: Not less than 22 inches by 30 inches under IRC R807.1, required wherever the attic has 30 inches or more of vertical height over an area of not less than 30 square feet, and located somewhere with ready access. Treat the panel as part of the ceiling: board insulation on its back and a gasket on its frame.
Can an attic have too much insulation?: The sources used here set no upper limit on R-value. What they do set is the one inch of clearance R806.3 keeps open above the insulation at a vented eave, and the weight involved: R-60 across 1,000 square feet is a little over a ton of loose fill, on this site's extrapolation from one cellulose coverage chart. Past that, the limits are the space under the roof and what the ceiling can carry.

Check these numbers yourself

2009-2021 IECC and IRC Minimum Insulation Requirements for New HomesBuilding America Solution Center, US Department of Energy, Pacific Northwest National Laboratory. Tabulates the prescriptive insulation minimums from the 2009 through 2021 editions of the IECC and the IRC side by side, names the table number each edition uses, and marks the cells where the two codes disagree. Used because codes.iccsafe.org returns HTTP 403. Read 2026-09-09.
16 CFR Part 460, Labeling and Advertising of Home Insulation (the R-value Rule)Federal Trade Commission, sections 460.8 and 460.12. Requires every loose-fill package to carry a coverage chart giving minimum settled thickness, initial installed thickness, maximum net coverage, bags per 1,000 square feet and minimum weight per square foot at R-13, 19, 22, 30, 38 and 49, and sets the 10 percent tolerance below the labeled R-value. Read 2026-09-10.
Loose Fill Insulation Coverage Chart, Cocoon INS550LDUS GreenFiber, a printout of the Loose Fill Insulation Coverage Chart page from cocooninsulation.com for Cocoon loose-fill cellulose INS550LD, 23.25 lb bag, with each page footer dated 1/27/2005, hosted by the National Association for State Community Services Programs. It prints twelve rows from R-11 to R-50 and no R-49 or R-60 row. Source of the 3.7 R per inch, the 1.111 installed to settled ratio, the 23.25 lb bag and the R-38 and R-50 rows; the R-49 and R-60 figures on this page are this site's extrapolation from it. It is twenty-one years old and is not presented as current product data. Read 2026-09-10.
2018 Seattle Residential Code, Chapter 8 Roof-Ceiling ConstructionSeattle Department of Construction and Inspections, adopting the 2018 International Residential Code. Prints IRC Chapter 8 in full: Section R806.2 with both conditions of the 1/300 exception and the 40 to 50 percent band, Section R806.3 with the 1 inch clearance, and Section R807.1 with the 22 by 30 inch attic access opening. Used because codes.iccsafe.org returns HTTP 403. Read 2026-09-09.
HomeAdvisor, how much does attic insulation costWritten by HomeAdvisor staff writer, published 20 June 2026. Source of every dollar figure on this page. Its table is headed average cost per square foot and states no R-value or depth on any row, and it lists blown-in and loose-fill as separate lines although blown-in insulation is loose fill blown by machine. Read 2026-09-13.

Figures here last checked against the source documents on 2026-09-16. Codes are amended locally; confirm against the edition your jurisdiction enforces.

About the author

Lucas Pradella builds and maintains Nails & Numbers. He is not a tradesperson: every figure on the site is traced to a named, dated document, and the page links it so you can check it yourself. More about how these pages are made.

Every guide on this site: Guides.