Climate Zone Calculator
What climate zone am I in? This free climate zone calculator answers it instantly — type your ZIP code or choose your state and county and it returns your IECC climate zone, from Zone 1 (very hot) to Zone 8 (subarctic), plus the moisture regime suffix (A moist, B dry, C marine).
Your climate zone is the master input for every energy-code decision in a remodel: how much attic, wall, and floor insulation you need, what window U-factor and SHGC to spec, and how to size HVAC. The calculator pairs your zone with the exact 2021 IECC minimum R-values and maximum window U-factors so you can move straight from "what zone am I in" to "what do I actually buy."
Zones come from the U.S. Department of Energy / PNNL county dataset, which mirrors IECC Table R301.1 and ASHRAE Standard 169. Because zones are assigned by county, ZIP lookups resolve to the county first. Always confirm the IECC edition your local building department has adopted — that is what sets the final numbers.
View material estimation guides →
The zone map orients you before you look up your own zone: the country splits into eight zones from warm to cold, and the colder the zone, the more insulation the energy code requires. That single relationship — zone drives R-value — is why this calculator’s whole job is pinning down which of the eight you are in.
Climate Zone Calculator
Find your IECC climate zone by ZIP code or county, then see the code-required insulation R-values and window U-factors for your zone. Free, no signup.
Find your IECC climate zone
IECC climate zones are assigned at the county level (per ASHRAE Standard 169), so your county determines the zone.
Zone-by-zone insulation requirements
Once you know your zone, open its guide for the full 2021 IECC R-value requirements, wall assemblies, and code citations.
How climate zones work
Two things trip people up: zones are assigned by county (not city), and a colder zone means the energy code demands more insulation. Here's both, at a glance.
The county-assignment diagram is why the calculator resolves your ZIP to a county rather than reading the map by state. Zones are assigned county-by-county, so one state can span two or three zones and a neighboring town can fall in a colder one. That ZIP-to-county-to-zone lookup is what keeps the answer from being a rough regional guess.
The R-value ladder is why pinning down your zone matters: it is the number the energy code enforces. Attic and wall minimums climb with the zone and then plateau in the coldest ones, so the ladder is where your zone becomes the concrete R-values you insulate to. Read your zone across and it gives the targets every insulation estimate should hit.
Calculation Formulas
The IECC does not compute a zone from a street address — it assigns one zone to each U.S. county (and county-equivalent). This calculator matches your ZIP to its county, then returns that county's published zone. ZIP boundaries can cross county lines, so the county is the authoritative key.
Example:
ZIP 98101 → King County, WA → Zone 4C (Mixed–Marine).
ASHRAE 169 sets the underlying number from long-term weather data. Cooling-dominated zones are bracketed by cooling degree-days (base 10°C); heating-dominated zones by heating degree-days (base 18°C). The IECC then adopts the result county-by-county.
Example:
Zone 1: 5000 < CDD10°C ≤ 6000. Zone 2: 3500 < CDD10°C ≤ 5000. Zone 5: 3000 < HDD18°C ≤ 4000. Zone 7: 5000 < HDD18°C ≤ 7000.
The letter suffix is the moisture regime. 'B' (Dry) is assigned when annual precipitation falls below a temperature-dependent threshold. 'C' (Marine) is assigned to coastal climates meeting ASHRAE's seasonal temperature and summer-dryness tests. Everything else is 'A' (Moist). Zones 7 and 8 carry no suffix.
Example:
Denver, CO → Zone 5B (Cool–Dry). Atlanta, GA → Zone 3A (Warm–Moist). Seattle, WA → Zone 4C (Mixed–Marine).
Marine climates get their own envelope rules because mild, wet winters change the heating load and vapor-drive direction. In the IECC, Marine Zone 4 (4C) follows the Zone 5 insulation bracket rather than the Zone 4 bracket.
Example:
The Pacific Northwest coast (Seattle, Portland) and Northern California coast fall in 3C/4C.
Once the zone is known, the 2021 IECC prescribes minimum R-values for ceiling, wood-frame wall, floor, basement wall, crawlspace wall, and slab edge. R-values are minimums. Marine 4 uses the Zone 5 row.
Example:
Zone 5 → ceiling R60, wood-frame wall R30 (or R20+5ci or R13+10ci), floor R30, basement wall R15/19.
Wall rows list interchangeable assemblies. 'ci' = continuous insulation (unbroken by framing, e.g. exterior foam). 'R13+5ci' means R-13 cavity plus R-5 continuous. Continuous insulation also reduces thermal bridging through studs.
Example:
A Zone 4 wall can be R30 cavity, or R20 cavity + R5 ci, or R13 cavity + R10 ci.
Windows and skylights are limited by maximum U-factor (heat flow) and, in cooling-dominated zones, maximum SHGC (solar heat gain). Northern zones drop the SHGC limit because winter solar gain is beneficial.
Example:
Zone 5: window U-factor ≤ 0.30, skylight U ≤ 0.55, SHGC ≤ 0.40. Zones 6–8: SHGC not required.
The IECC relaxes the window U-factor slightly where impact-rated or high-altitude glazing is required, recognizing the limited product availability.
Example:
A Zone 6 home above 4,000 ft elevation may use U-0.32 windows instead of U-0.30.
Standard Constants
| Constant | Value | Description |
|---|---|---|
| Number of IECC climate zones | 8 (1–8) | Zone 1 (very hot) through Zone 8 (subarctic/arctic). ASHRAE 169 also defines Zone 0, which does not occur in the U.S. |
| Moisture regimes | A / B / C | A = Moist, B = Dry, C = Marine. Appended to zones 1–6; zones 7 and 8 carry no suffix. |
| U.S. counties mapped | ~3,235 | Every county and county-equivalent (including territories) is assigned exactly one IECC zone. |
| Zone 1 threshold | 5000 < CDD10°C ≤ 6000 | Very hot. Southern Florida, Hawaii, Guam, Puerto Rico, the U.S. Virgin Islands. |
| Zone 4 threshold | CDD10°C ≤ 2500 and HDD18°C ≤ 3000 | Mixed. The transitional band across the mid-Atlantic, mid-South, and lower Midwest. |
| Zone 8 threshold | 7000 < HDD18°C | Subarctic/arctic — interior and northern Alaska only. |
| Marine 4 (4C) insulation rule | Uses Zone 5 bracket | Marine Zone 4 follows the Zone 5 insulation requirements in IECC Table R402.1.3, not the Zone 4 row. |
| Ceiling R-value range | R30 (Z1) → R60 (Z4–8) | Attic/ceiling minimums climb with heating load. 2021 IECC raised Zone 4–8 ceilings to R60. |
| Window U-factor range | 0.50 (Z1) → 0.30 (Z3–8) | Maximum whole-window U-factor by zone, 2021 IECC. |
| SHGC limit | 0.25 (Z1–3), 0.40 (Z4–5), NR (Z6–8) | Maximum solar heat gain coefficient; not required in the coldest zones. |
Note: All calculations include appropriate waste factors based on project complexity and material type. Results are estimates and should be verified by professionals before purchasing materials.
International Energy Conservation Code — Climate Zones(IECC Table R301.1 / Figure R301.1)
View StandardAssigns each U.S. county to one of eight climate zones plus a moisture regime, and is the basis for all residential energy-code requirements.
Key Requirements:
- •Zones numbered 1 (very hot) through 8 (subarctic)
- •Moisture regimes A (moist), B (dry), C (marine)
- •Zone assigned by county, not ZIP or address
- •Marine designation overrides for coastal climates
ASHRAE Standard 169 — Climatic Data for Building Design(ANSI/ASHRAE 169)
View StandardDefines the heating/cooling degree-day thresholds and moisture criteria that the IECC adopts to set each zone.
Key Requirements:
- •Cooling degree-days (base 10°C) bracket the hot zones
- •Heating degree-days (base 18°C) bracket the cold zones
- •Precipitation/temperature test sets the Dry (B) regime
- •Seasonal temperature test sets the Marine (C) regime
2021 IECC — Insulation & Fenestration Requirements(IECC Table R402.1.3)
View StandardPrescribes the minimum insulation R-values and maximum window U-factor/SHGC for each climate zone — the numbers this calculator returns alongside the zone.
Key Requirements:
- •Ceiling, wall, floor, basement, crawlspace, and slab R-value minimums
- •Continuous-insulation (ci) alternatives for walls
- •Maximum window and skylight U-factors
- •Maximum SHGC in cooling-dominated zones
International Residential Code — Energy Efficiency(IRC Chapter 11 [RE] (N1101–N1104))
View StandardDuplicates the IECC residential energy provisions inside the IRC, so one-and-two-family dwellings can comply via either code path.
Key Requirements:
- •Same climate-zone map as the IECC
- •Same R-value and fenestration tables
- •Adopted by most states for residential work
DOE / PNNL — Guide to Determining Climate Zone by County(DOE Building America Program)
View StandardThe U.S. Department of Energy's authoritative county-to-zone dataset, maintained by Pacific Northwest National Laboratory, including the 2021 updates.
Key Requirements:
- •County-level zone and moisture-regime assignments
- •Covers all 50 states, DC, and territories
- •Documents the counties reassigned in the 2021 edition
- •Aligns Building America zones with IECC zones
ENERGY STAR & DOE Zero Energy Ready Home(ENERGY STAR v3.2 / DOE ZERH)
View StandardAbove-code programs that layer stricter, climate-zone-specific insulation and window targets on top of the IECC minimums.
Key Requirements:
- •Climate-zone-indexed insulation and fenestration targets
- •Often one zone 'colder' than the IECC minimum
- •Whole-house air-sealing and HVAC criteria
Standards Disclaimer: Standards and codes are subject to periodic updates. Always verify current requirements with local building authorities and professional engineers before beginning construction. Links provided are for reference only.
Code edition adopted varies by state
2009 IECC through 2021 IECC are all in force somewhere
A county's zone barely changes between editions, but the R-values and U-factors attached to that zone tightened over time. The minimums shown here are 2021 IECC; a state still on the 2015 or 2018 IECC will have slightly lower targets.
Regional Examples:
2021 edition reassigned some counties
A handful moved one zone warmer or colder
The 2021 IECC adopted updated ASHRAE 169 data, shifting a small number of counties between zones. The DOE/PNNL guide lists the changes. For most addresses the zone is identical across editions.
Regional Examples:
Marine zones follow different rules
The 'C' suffix changes the insulation bracket
Marine climates (3C, 4C) are split out because mild, wet winters change heat-flow and moisture-drive. Notably, Marine Zone 4 (4C) uses the Zone 5 insulation requirements, so a Seattle wall is insulated like a Chicago wall even though the heating load is milder.
Regional Examples:
Dry (B) vs Moist (A) affects assemblies, not just R-value
Vapor-control strategy flips by moisture regime
Two counties can share a thermal number but differ by moisture regime, which drives where (or whether) you place a vapor retarder and how walls dry. The R-value targets are usually the same; the wall design is not.
Regional Examples:
High-altitude and coastal window exceptions
Zones 5–8 above 4,000 ft or in wind-borne debris regions
The IECC permits a slightly higher window U-factor (0.32 vs 0.30) where high-altitude or impact-rated glazing limits product choice. Coastal high-wind zones add separate impact and pressure requirements from the IRC/IBC.
Regional Examples:
Local amendments can override the model code
Cities and counties add their own stretch requirements
Many jurisdictions amend the IECC or layer a 'stretch' or 'reach' energy code on top, pushing insulation, air-sealing, and renewable requirements beyond the base values shown.
Regional Examples:
Before You Build
- •Contact your local building department for specific requirements
- •Verify frost line depths, wind zones, and seismic requirements for your area
- •Check if permits are required and schedule required inspections
- •Consult with a local contractor familiar with local codes
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How to Use This Calculator
- Enter your 5-digit ZIP code and click "Find my zone" for an instant match.
- No ZIP, or a ZIP that crosses county lines? Pick your state and then your county from the dropdowns instead.
- Read your IECC climate zone — the number (1–8) plus the moisture regime letter (A moist, B dry, C marine).
- Review the 2021 IECC code minimums shown for your zone: insulation R-values for ceiling, walls, floor, basement, crawlspace, and slab, plus maximum window U-factor and SHGC.
- Jump to the linked insulation, spray-foam, window, and HVAC calculators — each one uses the R-values and U-factors for your zone.
- Confirm the adopted code edition and any local amendments with your building department before ordering materials.
Why Climate Zone Is Assigned by County, Not ZIP
The IECC and ASHRAE Standard 169 assign exactly one climate zone to each U.S. county based on long-term heating and cooling degree-day data, not to individual ZIP codes or addresses. A single ZIP can straddle two counties, so this tool resolves your ZIP to its primary county and returns that county's published zone. If your ZIP sits on a county line — or near a zone boundary — use the county dropdown to confirm, and check the edition your jurisdiction enforces. Zones change very rarely between code editions, but the 2021 IECC reassigned a small number of counties and tightened several R-value and U-factor targets (notably raising Zone 4–8 ceilings from R49 to R60).
Frequently Asked Questions
What climate zone am I in?
Enter your ZIP code (or pick your county) above and the calculator returns your IECC climate zone instantly. Zones run from 1 (very hot — south Florida, Hawaii, Puerto Rico) to 8 (subarctic — interior Alaska), each with a moisture suffix. For reference: Miami is Zone 1A, Houston 2A, Atlanta 3A, Philadelphia 4A, Seattle 4C, Denver 5B, Chicago 5A, Minneapolis 6A, and Fairbanks Zone 8. The zone map shows the eight zones.
Is climate zone based on ZIP code or county?
The IECC assigns one climate zone to each U.S. county, not to ZIP codes or addresses. ASHRAE Standard 169 sets the zone from each county's long-term heating and cooling degree-day data. Because a single ZIP can cross county lines, this tool resolves your ZIP to its primary county first, then returns that county's published zone. If your ZIP sits on a county line, use the county dropdown to confirm. The county assignment diagram shows why zones are assigned by county.
What do the letters A, B, and C mean after my zone number?
The letter is the moisture regime. A = Moist (the default east of about the 100th meridian), B = Dry (the arid West and Southwest), and C = Marine (mild, wet coastal climates like the Pacific Northwest). So 5B is 'Cool–Dry' (Denver) and 5A is 'Cool–Moist' (Chicago). Zones 7 and 8 are cold enough that no moisture suffix is used. The suffix mainly drives your wall's vapor-control strategy, not the R-value target.
What insulation R-value do I need for my climate zone?
Under the 2021 IECC (Table R402.1.3), ceiling minimums run R30 in Zone 1 up to R60 in Zones 4–8. Wood-frame walls range from R13 (Zones 1–2) to R30 or R20+5ci or R13+10ci (Zones 4–8), where 'ci' means continuous insulation. Floors run R13 to R38. The calculator prints the full set — ceiling, wall, floor, basement, crawlspace, and slab — for your specific zone, then links to the insulation and spray-foam calculators so you can convert R-values into batts or board feet. The R-value ladder diagram shows the target by zone.
What window U-factor and SHGC does my zone require?
Maximum window U-factor under the 2021 IECC is 0.50 in Zone 1, 0.40 in Zone 2, and 0.30 in Zones 3–8 (skylights are higher). The SHGC (solar heat gain) cap is 0.25 in Zones 1–3 and 0.40 in Zones 4–5, and is not required in Zones 6–8 because winter solar gain is beneficial there. Zones 5–8 above 4,000 ft elevation or in wind-borne-debris regions may use U-0.32. The calculator shows your zone's exact maximums next to a link to the window calculator.
Did climate zones change in the 2021 IECC?
The map is very stable — most counties have carried the same zone since the 2009 IECC. The 2021 edition adopted updated ASHRAE 169 data and reassigned a small number of counties (the DOE/PNNL guide lists them), and it tightened several requirements, most notably raising Zone 4–8 ceiling insulation from R49 to R60. The bigger variable is which edition your state has adopted: the 2009, 2012, 2015, 2018, and 2021 IECC are all still in force somewhere, so confirm with your building department.
Are IECC climate zones the same as ASHRAE climate zones?
Effectively yes for buildings. The IECC adopts its climate zones directly from ASHRAE Standard 169, so the zone number and moisture letter match. ASHRAE 90.1 (commercial) and the IECC (residential) therefore use the same 1–8 / A-B-C system. ASHRAE 169 also defines a Zone 0 (extremely hot), but it does not occur anywhere in the United States, so U.S. lookups only ever return 1 through 8.
Does this calculator include pricing or contractor costs?
No. It is a free reference tool — it returns your IECC climate zone and the code-required R-values and window specs for that zone, with no pricing, labor, or material costs. The numbers shown are model-code minimums; your jurisdiction may enforce a different IECC edition, local amendments, or an above-code 'stretch' program that requires more. Always verify with your local building department before ordering materials.