Attic R-value by climate zone

Zones run from 1, the hottest, to 8, the coldest. They are set by county, not by state. Several states cover three of them, so a state map is not a lookup. Put your five-digit ZIP into the climate zone by ZIP tool and it returns the county-resolved answer.

Two soft edges in that lookup, which you should know about before you trust a result. About 1,044 of the 33,635 ZIP areas in the data sit across a zone line, and take the zone of the county holding most of their land. And roughly 7,000 USPS ZIPs are PO boxes or single addresses with no mapped area at all. Those fall back to their neighbors in the same three-digit prefix. The tool flags both cases when it hits one. Where a boundary ZIP leaves you between two zones, the table below usually settles it, because most neighboring zones carry the same target.

The R-value chart is the short version of all this. This page is the detail under it. Each zone gets its own section: what it recommends, what the model code sets, how far apart those two sit, and how many inches of each material the target takes.

The first two columns are ENERGY STAR's published recommendation for an existing home. The third is the model 2021 IECC ceiling minimum for new and permitted work. Most pages print one number and call it code. These are three different things, and for a homeowner topping up an attic only the first two apply.

Recommended levels from ENERGY STAR, "Recommended Home Insulation R-Values". ENERGY STAR says that guidance is based on 2021 IECC Table R402.1.3. The model code column is that same table, as the International Code Council publishes it. Treat the last column as orientation, not a lookup. Zones follow county lines.

Read the first two columns against each other and the shape of the guidance shows up. The bare-attic column matches the model code number in every zone. The top-up column sits one step below it: 5 points lower in zone 1, and 11 points lower in every other zone. ENERGY STAR is not saying a topped-up attic is worth less. It is saying the last step buys the least, so it stops asking for it once the attic is already working.

The steps between zones are not even either. Going from zone 1 to zone 2 raises the bare target by 19 points, which is the biggest jump in the table. Zone 3 to zone 4 raises it by 11. After zone 4 the recommendation stops moving at all: zones 4, 5, 6, 7 and 8 carry identical numbers, so a house in Ohio and a house in Fairbanks aim at the same R-value.

One number in that table is easy to misread. In zones 4 through 8 the top-up target is R-49, and the model code separately allows R-49 to satisfy R-60. Same number, two unrelated reasons. Where the 2021 IECC asks for R-60 in the ceiling, R-49 is enough on one condition. The insulation has to run all the way out over the top of the outside wall at its full depth, with nothing squashing it. That normally takes a raised-heel truss, which lifts the roof to make room. The allowance is about how the insulation lands at the eave. The retrofit target is about what you already have.

Then there is what your state enforces. That is a third number again. Only 12 of 51 jurisdictions are on the 2021 IECC or newer, and 8 have no mandatory statewide code for homes at all. What states have adopted runs from a voluntary 1989 basis in Wyoming to the 2024 IECC in Rhode Island. Source: ACEEE State Policy Database, reviewed November 2024. It is second-hand and it goes stale between checks. Energy codes cover new building and work done under a permit. They do not usually cover adding insulation to an attic you already have. Which edition applies where you live depends on your state, and often on your city. Your local building department is the only authority on it. The state energy code tracker lists all 51 rows.

Measure the depth in three or four spots away from the hatch, and use the typical reading rather than the deepest one. Then find your row. The last column is where this page parts company with most of the market.

The verdict, plainly. If your attic is bare or nearly bare, insulating it is one of the better buys in the house, and the zone number in the second column is what to aim at. If it holds 3 to 4 inches, top up to the third column and stop there. If you are already within about R-10 of your target, more depth is the wrong purchase, and air sealing is the right one.

That R-10 threshold is our own estimate. It is not measured, and it is not anybody's published rule. The ENERGY STAR table steps in jumps of about 11 points. One step is the finest grain the source itself draws, so that is where we set the line.

Two things outrank the zone number whatever your reading says. Wet, moldy, rodent fouled or possibly vermiculite material is a removal question before it is a depth question, and the remove-or-add-over guide runs that test. And if you are about to redo the roof, the wiring or the vents, wait. Work that disturbs insulation spends the money twice. For the full sequence from measuring to buying, see how much insulation do I need.

Zone 1 is the only zone where the target stops at R-30. The top-up target sits 5 points below the model code figure, the narrowest gap in the country. Heat here moves into the house for most of the year, so the attic's job is keeping it out.

Radiant barrier is sold hard in this zone, and it is the one product that cannot be added to the number above. A radiant barrier reflects radiant heat rather than resisting conduction, so it has no useful R-value per inch and cannot be counted toward an R-value target. It is measured differently from bulk insulation and the two do not add together. It can still help under a hot roof, but it is a separate decision from depth. See radiant barrier. If your ducts run through the attic, sealing them is usually worth more here than the last few inches on the floor.

Crossing from zone 1 into zone 2 raises the bare-attic target by 19 points, the largest single step in the whole table. So a house on the wrong side of that line by a county is aiming much higher than its neighbor. The top-up target is 11 points under the model code figure here.

Humidity is the other half of the job in most of zone 2. An attic here runs hot and damp, and air leaking up from the house carries moisture with it. Seal the ceiling before you bury it. Attic ventilation matters more in this zone than the last inch of depth does.

Zone 3 carries the same targets as zone 2, so the line between them changes nothing for your attic. What changes is the weather on both sides of the year. Summer heat and a real heating season both show up here, which means the insulation earns its keep twice.

Zone 3 also holds the 3C strip along the coast of California, where the letter marks a marine climate. The letter does not move your attic number. It does change how a house handles moisture, and it changes which materials suit the rest of the shell. The types guide covers that choice.

Zone 4 is where the bare-attic target reaches R-60 and then stops rising. It is also where the eave starts to decide the job. R-60 in the middle of the attic and four squashed inches over the outside wall is not R-60 at the edge.

This is the first zone where the model code's raised-heel allowance is worth knowing, because contractors quote it both ways. Where the 2021 IECC asks for R-60 in the ceiling, R-49 is enough on one condition. The insulation has to run all the way out over the top of the outside wall at its full depth, with nothing squashing it. That normally takes a raised-heel truss, which lifts the roof to make room. On an existing house you rarely get to change the truss, so the practical move is baffles at the eaves and depth carried as far out as the roof line allows. See baffles.

Zone 5 carries the same numbers as zone 4. What is different is how the failure shows up. Winter is long enough here that heat leaking through the ceiling melts snow on the roof, and the melt refreezes at the cold eave. That is an ice dam, and it is an insulation and air sealing problem wearing a roofing costume.

The Department of Energy puts the order plainly for this: seal the ceiling plane, then insulate, then ventilate. Depth alone does not fix a dam, because the heat that makes one often rides up through gaps rather than through the material. The ice dam guide has the full sequence.

The attic numbers do not move in zone 6. One other rule starts here. The model code lets an attic use a smaller vent area, the 1 to 300 ratio, but only when two conditions both hold. The first one names this zone. Here it is, in the code's own words. In climate zones 6, 7 and 8, a Class I or II vapor retarder is installed on the warm-in-winter side of the ceiling.

Source: Model International Residential Code, Section R806 (roof ventilation), 2021 edition. These come from the model International Residential Code. Your state, and often your city, decides which edition applies where you live and whether it was changed. Your local building department is the only authority on that. And these rules cover new building and permitted work, not a top-up you choose to do on an attic you already have.

What that means for a top-up. Cold zones punish a warm, damp ceiling: moisture that rides into the attic on leaking air can condense on the cold roof deck and wet the insulation from above. Sealing the ceiling is the fix, and it comes before depth. If someone proposes adding a plastic sheet under existing insulation, get advice specific to your house first.

In zone 7 every target in the table is deeper than the joists. A 2x10 joist is 9.25 inches deep, and the shallow end of the blown fiberglass column below is past that. So the material has to cover the wood, not just fill between it.

Burying the joists matters for a reason the R-value cannot show. Wood carries heat better than the insulation beside it, so bare joist tops are a shortcut through the ceiling. Deep loose fill over the whole floor beats batts of the same rated R fitted between the framing. That is one reason blown-in wins most cold-zone attics.

Zone 8 is the coldest zone in the country. It recommends exactly what zone 4 does. Alaska holds nearly all of it. Alaska is also one of the 8 states with no mandatory statewide energy code for homes: No mandatory statewide code; BEES (2018 IECC basis) for financed projects. So here the model number is guidance for most work, not a rule. The recommendation itself does not change.

One data note for zone 8 readers. The county file we use publishes no zone for the Kusilvak Census Area, so our lookup assigns it zone 8, matching the tables for its former name and every neighboring borough. It is the only value in the file that is not straight from the source.

We do not sell insulation and we do not install it, which is why this section can exist. Your zone target describes the whole ceiling, not the deepest patch of it. And the value inside that target is not spread evenly. The first inches over a bare deck do most of the work, and the last ones do the least.

Heat moving through the insulation falls as one over R, so each step buys less than the one before it. These are the steps, computed rather than asserted.

Arithmetic on the definition of R, not measured savings. It describes conduction through the insulated plane only. Air leakage, heat taking the shortcut through the joists, duct losses, climate and fuel price all move a real bill. This site holds no energy price or degree day data, so it publishes no dollar figure.

Read the two ends of that table together. The first 20 points over a nearly bare ceiling cut the heat flow by about 87 percent. The last 10, from R-50 to R-60, cut it by about 17 percent. Even the whole top-up step, R-49 to R-60, is worth about 18 percent. So chasing the final points of your zone number is the least valuable thing you can buy in that attic.

Three ways that changes what you should do. If you measure within about R-10 of your target, air seal instead of adding depth. If your depth is uneven, even it out before you raise the middle: a thin strip at the eaves leaks more than a deep center saves. And if the attic is spray foamed at the roof and heated like the rest of the house, none of these floor targets apply to you at all.

A target is a thing you can buy against. It turns a vague quote into a checkable one: the contractor should write down the material, the settled depth in inches, and the R-value it lands at. If two bids disagree by 20 points of R, they are not the same job. Our attic insulation calculator turns your zone and square footage into a quantity, and the cost guide shows how a price is built from wage and coverage data.

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Tell us the job and your ZIP. We pass it to a licensed contractor who works in your area. Bring your zone target and your measured depth to the call.

A marketing service, not a licensed contractor. Insulation work is performed by independent licensed local contractors.

What does it cost?

A 1,200 square foot attic blown to R-49 runs about $2,600 to $3,200 installed, from this site's own cost model dated August 2026. Two of its parts are estimates, so read it as a range and not a quote. See the full working and the price by attic size.

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• The R-value chart: every zone and material on one screen
• How much insulation do I need? The full sequence
• R-value per inch by material, and how a label R is tested
• Climate zone by ZIP
• How much insulation do I have?
• What your state's energy code actually says
• What insulation does to an energy bill