2.1 Climate Zones, Moisture Regimes & County Lookup (R301)

2.1 Climate Zones, Moisture Regimes & County Lookup (R301)

Quick Answer: Open Figure R301.1 or Table R301.1, find the state and county, and record the climate-zone number plus the moisture letter (A moist, B dry, C marine). If the county is missing, R301.3 assigns the zone from HDD and CDD. Treat Marine 4 (4C) as the 5 and Marine 4 envelope row, not 4 except Marine. An asterisk means Warm Humid. Independent OpenExamPrep material covering the 2021 IECC residential provisions — this is not an ICC publication.

Exam 79 is a national open-book inspector/plans-examiner exam. ICC administration rules and the before-exam materials make the same point the codebook does: you are tested on all U.S. climate and geographic conditions, not only the county where you inspect during the week. A New Jersey 4A job, a King County, Washington 4C job, a Summit County, Colorado Climate Zone 7 job, a Maricopa County, Arizona 2B job, and a Kauai, Hawaii 1A* job can all appear on the same 50-question form. Do not import a home-state amendment, a local energy stretch code, or a New Jersey UCC overlay as if it were the national 2021 IECC residential text.

R301.1 — Figure, table, then Chapter 4

R301.1 is short and it is the gate. Climate zones from Figure R301.1 or Table R301.1 shall be used for determining the applicable requirements from Chapter 4. Locations not indicated in Table R301.1 shall be assigned a climate zone in accordance with Section R301.3. That sentence does three jobs for a plans examiner:

1. The envelope, fenestration, air-leakage, and additional-efficiency numbers in Chapter 4 are meaningless until the climate zone is correct.
2. The legal lookup is the figure or the county table, not a weather-station nickname, a utility territory, or the climate zone printed on a window sticker from another state.
3. If the site is truly not in the table (a new county name, an unlisted territory, or a location outside the tabulated states), you do not borrow the nearest listed county. You run R301.3.

Table R301.1 is organized by U.S. state, then county, and it also lists territories. Some entire states are one line (Connecticut 5A (all), Delaware 4A (all), Hawaii 1A (all)*). Other states mix zones inside the same political boundary. New Jersey in the 2021 table is not all 4A: Atlantic, Burlington, and Ocean are 4A, while Bergen, Hunterdon, Morris, Passaic, Somerset, Sussex, and Warren are 5A. Using "New Jersey is Climate Zone 4" on a Morris County dwelling is a failed lookup even though the project is still in New Jersey. The national table — not a state energy subcode amendment — is what Exam 79 expects you to read.

Worked county lookups (2021 Table R301.1)

The Puerto Rico exception is the table teaching in miniature: read the line, do not assume a whole territory is one climate. The Alaska and Arizona lines teach the same national-exam rule: dry desert, marine coast, moist interior, and arctic Interior Alaska are all in scope.

Moisture regimes — A, B, C, and the missing letter

Table R301.1 footnote (key) is the decoder ring:

• A — Moist
• B — Dry
• C — Marine
• Absence of a moisture designation indicates the moisture regime is irrelevant for that listing (typical of Climate Zones 7 and 8).
• An asterisk (*) indicates a Warm Humid location.

R301.2 repeats the asterisk rule in one sentence: in Table R301.1, warm humid counties are identified by an asterisk. That mark is not decoration. Table R402.1.3 footnote f (the Warm Humid basement-wall note) states that basement wall insulation is not required in Warm Humid locations as defined by Figure R301.1 and Table R301.1. The matching U-factor table note sends Warm Humid basement walls to a U-factor not exceeding 0.360. If you strip the asterisk and apply a Climate Zone 3 basement R-value to a 3A* Gulf county, you will require insulation the R-value table does not require in that location.

Moisture letters also change how you talk about Climate Zone 4. 4A (Delaware, much of New Jersey's coastal and southern counties, the District of Columbia) is 4 except Marine. 4B (many Arizona and New Mexico upland counties, for example Yavapai County, Arizona) is still Climate Zone 4, Dry, and is still not Marine. 4C is Marine 4. Only 4C belongs with the Marine-4 envelope grouping.

The Exam 79 trap: Marine 4 versus 4 except Marine

Chapter 4 envelope tables do not give Climate Zone 4 one row. They split it:

• 4 except Marine — used for 4A and 4B locations.
• 5 and Marine 4 — used for all of Climate Zone 5 (5A, 5B, and 5C) and for Marine 4 (4C).

That grouping is the classic miss. A dwelling in King County, Washington (4C) is geographically "Climate Zone 4," but the R-value and U-factor tables that apply to most envelope components are the 5 and Marine 4 row. A dwelling in Sussex County, Delaware (4A) is also Climate Zone 4, and it does use 4 except Marine. Floor R-value is one of the numbers that moves: the 2021 R-value table calls for R-19 floors in 4 except Marine and R-30 floors in 5 and Marine 4. Mass-wall U-factors move as well (0.087 in 4 except Marine versus 0.065 in 5 and Marine 4 when more than half the insulation is interior). Ceiling R-60 appears on both of those rows in 2021; do not let a matching ceiling number talk you into treating the rows as identical.

A second Marine-specific note sits in the fenestration SHGC footnotes: there are no SHGC requirements in the Marine Zone. Table R402.1.2 even prints NR in the SHGC column for the 5 and Marine 4 row. When the project is truly Marine (the C letter), do not invent a 0.40 SHGC cap from the neighboring 4 except Marine row. When the project is Climate Zone 5 moist or dry (5A or 5B), follow the table row and footnotes as printed — Chapter 4 will teach the SHGC numbers in detail. This chapter's job is to stop you from parking a 4C building on the wrong row.

Field scenario — Summit County versus Marine 4. A contractor who just closed a house in Summit County, Colorado (Climate Zone 7) takes a weekend inspection in Clatsop County, Oregon. Clatsop is 4C. If that inspector thinks "Zone 4 means 4 except Marine" because the number 4 is on the page, the floor insulation, basement-wall options, crawl-space options, and Marine SHGC note will all be wrong. Summit County, Colorado is 7. Clatsop County is Marine 4, grouped with Climate Zone 5. Summit County, Ohio is a third place with the same county name and a 5A listing. State plus county is the entire method.

Unlisted locations — R301.3 HDD and CDD

R301.3 is the algorithm for locations not listed in the county table. You do not average two neighboring counties. You:

1. Determine the thermal climate zone, 0 through 8, from Table R301.3 using HDD and CDD for the location.
2. Determine the moisture zone (Marine, Dry, or Humid) from the R301.3 definitions, using monthly data if you have it, or the CDD shortcuts for Marine 3C/4C/5C when only annual or degree-day data exist.
3. Apply the Marine (C) definition (coldest-month mean between 27°F and 65°F, warmest-month mean less than 72°F, at least four months with mean temperatures over 50°F, and a dry season in summer as defined), the Dry (B) precipitation equations, or Humid (A) as whatever is left after Marine and Dry are ruled out.

Table R301.3 thermal criteria (IP units)

Marine shortcuts in R301.3 when you are assigning 3C, 4C, or 5C from degree-days include CDD50°F ≤ 4,500 for Marine 3C, CDD50°F ≤ 2,700 for Marine 4C, and CDD50°F ≤ 1,800 for Marine 5C, each paired with the matching thermal zone. If only degree-day information is available and the site is not Marine, R301.3 states it is not possible to assign Humid versus Dry. That limitation matters on a plans comment: you cannot invent a B or an A from HDD/CDD alone.

Tropical climate region — R301.4 and the R407 door

R301.4 defines the tropical region as:

1. Hawaii, Puerto Rico, Guam, American Samoa, U.S. Virgin Islands, Commonwealth of Northern Mariana Islands; and
2. Islands in the area between the Tropic of Cancer and the Tropic of Capricorn.

That definition is geography, not a compliance path. R401.2.4 is the path that points to R407. R407.1 then limits the tropical option to residential buildings in the tropical region at elevations less than 2,400 feet (731.5 m) above sea level. A house on Hawaii island at 1,800 feet is in the R301.4 tropical region and below the R407 elevation cap, so R407 is a path to evaluate. A house at 2,800 feet is still in the tropical region for R301.4 mapping, but it is not eligible for the R407 option. Hawaii's Table R301.1 listing remains 1A (all)* for Chapter 4 envelope criteria when the project is not using R407. Do not tell the applicant that "Hawaii is exempt from climate zones."

Inspector and plans-examiner workflow

On construction documents, the first energy mark a plans examiner should be able to defend is the climate zone string (for example 4C, 5A, 2B, 7, 1A*). Check that the string matches Table R301.1 for the permit address county, not the builder's office county. Check whether an asterisk is present if basement walls are shown as uninsulated. Check whether a 4C project is using 5 and Marine 4 tables. Check whether an R407 request is both inside R301.4 geography and below 2,400 feet. In the field, the same string still governs insulation thickness, fenestration labels, and which leakage limits later chapters will apply. Wrong zone at permit is the error that quietly fails every later inspection.

Keep Chapter 3 of the IECC residential provisions in the national first-printing frame: R301 through R303 set location, design temperatures, and product identification. They do not replace Chapter 4. They tell you which Chapter 4 row you are allowed to open.