IECC and IRC minimums, all four editions, from US DOE
A ceiling in Climate Zone 5 is R-38 under the 2009 energy code, R-49 under the 2012, 2015 and 2018 editions, and R-60 under the 2021. All three are enforced somewhere in the United States right now, so a contractor quoting R-38 is not wrong, they are quoting the 2009 code. The second half of this page is the other gap: a wall with R-21 batts is not an R-21 wall, because the studs are not insulation.
What the code requires, and what your assembly delivers
R
R
in
%
R/in
The edition is an input because it has to be. Adoption lags publication by years and varies by state, by county and sometimes by city, and this page will not guess which one your building department enforces.
Specification summary
Generated from on .
Reopen that address to reproduce these figures exactly.
Entered
Result
The two heat paths, and what each one is worth
Worked example
A vented attic in Climate Zone 5 under the 2021 code: R-30 between 9 1/4 in joists at 24 in on centre, with R-30 blown over the top of them.
Required. Zone 5 ceiling, 2021 IECC: R-60.
Other editions. The same cell is R-38 in 2009 and R-49 in 2012 through 2018.
Stud fraction. 1.5 in every 24 is 6.3%, plus 6% for plates and ends.
Cavity path. 30 + 30 = R-60.
Framing path. 9.25 × 1.25 + 30 = R-41.6.
Whole assembly. Parallel path gives R-56.9, not R-60.
Three points fall out of that. The framing costs 5.2% here, which is small because the R-30 blown over the joists covers them. Take that layer away and the same attic drops to R-25.1, so the second R-30 is worth R-31.8 to the assembly rather than R-30. And the finished assembly is R-56.9 against a requirement of R-60, which is not a compliance failure, because the code cell is a component R-value for the insulation rather than a whole-assembly figure. It is the gap between what the bag says and what the ceiling does.
The formula
Two paths in parallel, and you cannot average the R-values because resistance does not add that way:
U = f / Rframing + (1 − f) / Rcavity Rassembly = 1 / U
f
framing fraction: the share of the area that is wood, not insulation
Rcavity
cavity insulation plus any continuous layer
Rframing
depth × R per inch of wood, plus the same continuous layer
1.5 / oc
the geometric stud fraction, and it is not the whole story
U
conductance. Averaging U is correct; averaging R is not
The reason you have to average U rather than R is that heat takes the easy path. Two paths of R-20 and R-5 in equal area do not give R-12.5; they give R-8, because the R-5 path passes four times as much heat and dominates the total. That is the entire mathematics of a thermal bridge, and it is why continuous insulation is specified separately.
Why the edition is an input and not a footnote
Almost every page that answers "what R-value do I need" gives one number per climate zone. That is a defensible thing to do only if the number has not changed, and it has changed twice in twelve years.
Ceilings went R-38, then R-49, then R-60 in Zones 4 and 5. Walls went from R-13 to R-20 to a 2021 cell offering four different assemblies. Basement and crawl space walls moved in 2012 and again in 2021. Only mass walls and floors held still.
None of that would matter if everybody enforced the newest edition, and nobody does. Adoption is by state, sometimes by county, occasionally by city, it lags publication by years, and several states amend individual cells or write their own energy code outright.
So the honest form of this page is a cross-edition table with the edition as an input, and the honest instruction is to ring the building department. The US Department of Energy publishes exactly that table, which is why it is the source here rather than the code itself.
Ceiling R-value, every edition still enforced somewhere
Climate zone
2009 IECC
2012, 2015 and 2018 IECC
2021 IECC
2021 against 2009
1
R-30
R-30
R-30
same
2
R-30
R-38
R-49
63% higher
3
R-30
R-38
R-49
63% higher
4 except Marine
R-38
R-49
R-60
58% higher
5 and Marine 4
R-38
R-49
R-60
58% higher
6
R-49
R-49
R-60
22% higher
7 and 8
R-49
R-49
R-60
22% higher
Read 2026-09-09. Values from the DOE Building America cross-edition table. The last column is this page dividing one column by another.
The number changed twice, and your code did not change with it
Ask what R-value an attic needs and you will get an answer with no edition attached to it. Here is the same cell of the same table across the editions that are still enforced somewhere in the United States:
Prescriptive minimum ceiling R-value by climate zone and code edition
Climate zone
2009 IECC
2012, 2015 and 2018 IECC
2021 IECC
2021 against 2009
1
R-30
R-30
R-30
same
2
R-30
R-38
R-49
63% higher
3
R-30
R-38
R-49
63% higher
4 except Marine
R-38
R-49
R-60
58% higher
5 and Marine 4
R-38
R-49
R-60
58% higher
6
R-49
R-49
R-60
22% higher
7 and 8
R-49
R-49
R-60
22% higher
Climate Zone 5 went from R-38 to R-49 to R-60. A contractor quoting R-38 is not wrong; they are quoting the 2009 code. A blogger quoting R-49 is quoting 2012 through 2018. Both are current somewhere.
Which is why this page makes you pick an edition, and why it will not pick one for you. The only reliable way to find out is to ring the building department and ask which edition they enforce, and adoption lags publication by years and varies by state, by county and sometimes by city.
Note the shape of it too. Zone 1 has not moved at all in twelve years, and Zones 2 and 3 jumped hardest in 2021. The change is not a uniform tightening; it is concentrated where the modelling said the savings were.
One table, seven section numbers
The second reason a citation without an edition is worthless is that the table itself keeps moving. The DOE source spells it out:
This table is adapted from Table R402.1.1 in the 2009 and 2012 International Energy Conservation Code (IECC), which is Table R402.1.2 in the 2015 and 2018 IECC, and Table R402.1.3 in the 2021 IECC, as well as Table N1102.1 in the 2009 International Residential Code (IRC), Table 1102.1.1 in the 2012 IRC, Table N1102.1.2 in the 2015 and 2018 IRC, and Table N1102.1.3 in the 2021 IRC.
Where the same table lives in each edition
Edition
Code
Table
2009
IECC
Table R402.1.1
2012
IECC
Table R402.1.1
2015
IECC
Table R402.1.2
2018
IECC
Table R402.1.2
2021
IECC
Table R402.1.3
2009
IRC
Table N1102.1
2012
IRC
Table 1102.1.1
2015
IRC
Table N1102.1.2
2018
IRC
Table N1102.1.2
2021
IRC
Table N1102.1.3
So "IECC Table R402.1.2" is a 2015 or 2018 citation and nothing else, and a page that cites it while printing 2021 numbers has copied from two places without noticing.
And the two codes do not always agree. The source flags it plainly:
The IRC code requirement differs from the IECC code requirement, as noted.
In the 2009 edition, a basement wall in Zones 6, 7 and 8 is 15/19 in the IECC and 10/13 in the IRC, and a floor in Zones 7 and 8 is R-38 in the IECC and R-30 in the IRC. Same publisher, same year, same component, different number, and which one binds depends on which document your jurisdiction adopted.
Where the climate zone comes from, and why it is not on this page
Climate zones are assigned by county, from a map and a table in the code itself, and there are eight of them plus moist, dry and marine designations.
This page does not reproduce that list. It is roughly three thousand counties, it is revised between editions, and a county list that goes stale is worse than no list at all: somebody would use it, and it would be wrong for exactly the jurisdictions that had changed.
Your building department knows your zone in one phone call. So does the permit desk, and so does any local insulation contractor, and all three of them also know which code edition you are on, which is the input you cannot get anywhere else.
The Marine designation matters more than people expect. Marine 4 is grouped with Zone 5 in this table, not with Zone 4, which means a coastal Pacific Northwest house has meaningfully higher requirements than an inland house at the same latitude.
Batts, blown and board, and why the calculator asks for depth
The cavity R-value and the cavity depth are separate inputs because the relationship between them is a property of the material, and getting it wrong is the most common way this arithmetic goes astray.
The calculator prints the R per inch your two inputs imply, and warns when it exceeds anything in common residential use. R-38 in a 3 1/2 inch stud bay is about eleven per inch, which no ordinary insulation reaches, and it is what you get by typing an attic requirement into a wall assembly.
A deep loose-fill attic is not a cavity assembly at all. Once the insulation is deeper than the joists, the part above them is continuous by definition, and that is how it should be entered: the depth level with the framing as cavity, everything above it as continuous. That is also why a blown attic has such a small framing penalty and a batted wall has a large one.
And compressed insulation loses R-value. The source is explicit that where insulation is installed in a cavity thinner than its label thickness, the installed R-value is what counts. An R-19 batt stuffed into a 2x4 bay is not R-19 and the label on the bag does not change that.
13+5 is not R-18, and 0+20ci is not R-20 of anything you can stuff
Half the cells in this table are not numbers. They are assemblies, and the notation is where most insulation pages quietly go wrong.
The first value is cavity insulation; the second value is continuous insulation. Therefore, as an example, “13+5” means R-13 cavity insulation plus R-5 continuous insulation.
Cavity insulation goes between the studs. Continuous insulation goes over them. They are not interchangeable and they do not simply add, because the studs themselves are a thermal bridge: a wall with R-13 batts is not an R-13 wall, it is R-13 between the studs and about R-4 at every stud. Continuous insulation is the only part of the assembly that covers the framing, which is why the code specifies it separately rather than asking for a bigger number.
The 2021 edition writes the alternatives out in full:
“15ci or 19 or 13+5ci” means R-15 continuous insulation (ci) on the interior or exterior surface of the wall or R-19 cavity insulation on the interior side of the wall; or R-13 cavity insulation on the interior of the wall in addition to R-5 continuous insulation on the interior or exterior of the wall.
Three ways to satisfy one requirement, and they are not equivalent in cost, in thickness or in how the wall dries. This page prints the cell as the code prints it and does not reduce it to a single figure, because reducing it is the error.
One more piece of notation worth knowing: on basement and crawl space walls, 10/13 means R-10 continuous OR R-13 in the cavity, which is a choice rather than a sum, and reads exactly like the cavity-plus-continuous notation used elsewhere in the same table.
What the table is, and four things it is not
It is the prescriptive path. The energy codes also allow a performance path and a total UA alternative, where a house that misses a prescriptive number can comply by doing better somewhere else. A house that fails this table is not necessarily non-compliant; it is not compliant by this route.
It is a minimum, not a target. The source says so:
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.
Note the second sentence. Compressing an R-19 batt into a 2x6 cavity does not give you R-19, and the installed value is what counts.
It is not the whole envelope. Fenestration U-factor and SHGC, air leakage, duct sealing and duct insulation all live in the same chapter and none of them are here. An attic at R-60 above a leaky ceiling plane is money spent on the wrong layer.
And several cells carry conditions. Basement wall insulation is not required in warm-humid locations, defined by a separate figure and table. A heated slab needs R-5 under the full slab area in the 2018 and 2021 editions, on top of the edge value shown. Read the footnotes on the source before relying on a zero.
The source, and why it is not the code itself
Read 2026-09-09. 2009-2021 IECC and IRC Minimum Insulation Requirements for New Homes, Building America Solution Center, US Department of Energy, Pacific Northwest National Laboratory. Tabulates the prescriptive insulation minimums from four editions of the IECC and the IRC side by side, names the table number each edition uses, and marks the cells where the IRC and the IECC disagree. Used because codes.iccsafe.org returns HTTP 403, and because the cross-edition comparison is the whole point.
codes.iccsafe.org returns HTTP 403 and cannot be fetched from here, which is a standing problem on this site and the reason city and government publications are used throughout. In this case the substitute is better than the original would have been: the code itself only ever shows you one edition, and one edition is exactly what hides the finding.
Your jurisdiction may amend any of this. States adopt editions years apart, many amend individual cells, and a few write their own energy code outright. Washington and California both do. The building department is the only authority on which numbers apply to your permit.
Climate zone assignment is by county, from a map and table in the code itself, and is not reproduced here because a county list is not something this page can keep current.
Frequently asked questions
What R-value do I need for my attic?
It depends on your climate zone and on which edition of the energy code your jurisdiction enforces, and the second half of that is the part most pages leave out. A Climate Zone 5 ceiling is R-38 under the 2009 IECC, R-49 under the 2012, 2015 and 2018 editions, and R-60 under the 2021. All three are current somewhere. Ring the building department and ask which edition they are on.
What does 13+5 mean on an insulation table?
R-13 cavity insulation between the studs plus R-5 continuous insulation over them. It is an assembly, not R-18. The two are specified separately because continuous insulation is the only layer that covers the framing, and the framing is where a large share of the heat leaves.
What is the difference between 13+5 and 10/13?
They look almost identical and mean opposite things. "13+5" is cavity plus continuous, both required. "10/13", used on basement and crawl space walls, is R-10 continuous OR R-13 in the cavity, a choice between two ways of complying. Reading a slash as a plus is a real and expensive mistake.
Is a wall with R-21 batts an R-21 wall?
No. The studs are about R-1.25 per inch, so a 2x6 stud is roughly R-6.9 against R-21 of insulation beside it, and heat takes the easy path. With a realistic framing fraction that wall comes out near R-16 before you count sheathing, cladding and air films. Adding continuous insulation over the studs is worth more than its own R-value, because it is the only layer that closes that path.
What is the framing factor of a wall?
The share of the wall area that is wood rather than insulation. The geometric figure is small: 1.5 inches of stud every 16 is 9.4%. The real figure is much larger, because plates, headers, corners, partition intersections and window framing are all wood too. This page starts from the geometry, which is exact, and lets you add the rest, which is not something any code publishes.
Which code edition applies to my house?
Whichever one your jurisdiction has adopted, which is a question only the building department can answer. Adoption lags publication by years, it varies by state and often by county or city, and several states amend individual cells or write their own energy code. There is no national answer and this page will not invent one.
Do the IRC and the IECC give the same R-values?
Almost always, and not quite. The DOE source flags two cells where they diverge in the 2009 edition: a basement wall in Zones 6 through 8 is 15/19 in the IECC and 10/13 in the IRC, and a floor in Zones 7 and 8 is R-38 in the IECC and R-30 in the IRC. Which one binds depends on which document your jurisdiction adopted.
Does my house have to meet these numbers?
Not necessarily by this route. These are the prescriptive path. The energy codes also allow a performance path and a total UA alternative, where a house that misses one prescriptive number complies by doing better elsewhere. A house that fails this table is not automatically non-compliant, it is not compliant by this table.
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, read 2026-09-09. Tabulates the prescriptive insulation minimums from four editions of the IECC and the IRC side by side, names the table number each edition uses, and marks the two cells where the IRC and the IECC disagree. Used because codes.iccsafe.org returns HTTP 403, and because the cross-edition comparison is the entire point: the code itself only ever shows you one edition, and one edition is what hides the finding.
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 2026-09-09. Codes are amended locally; confirm against the edition your jurisdiction enforces.
These are prescriptive minimums and they depend on which edition your jurisdiction enforces, which this page cannot know and will not guess. Values are quoted from the US Department of Energy's cross-edition table as read on 9 September 2026. Several states amend individual cells or write their own energy code. The energy codes also allow a performance path and a total UA alternative, so failing a number here is not automatically non-compliance. The framing addition and the R per inch of wood are this page's own figures and are inputs; the geometric stud fraction is arithmetic. Climate zone assignment is by county, from a table in the code, and is not reproduced here.