Climate zones by state

Washington Climate Zones by County

Washington's 39 counties span climate zones 4C to 6B under the 2021 International Energy Conservation Code (IECC). Zone 5B is the most common, with 18 of the 39. For reference, Seattle is in zone 4C and Spokane and Yakima are in 5B.

Insulation minimums for each zone in Washington

ZoneCountiesAtticWood-frame wallsFloors over unheated spaceBasement walls
4C14R-60R-30, or 20+5ci, or 13+10ci, or R-20ciR-30R-15ci, or R-19, or 13+5ci
5B18R-60R-30, or 20+5ci, or 13+10ci, or R-20ciR-30R-15ci, or R-19, or 13+5ci
5C4R-60R-30, or 20+5ci, or 13+10ci, or R-20ciR-30R-15ci, or R-19, or 13+5ci
6B3R-60R-30, or 20+5ci, or 13+10ci, or R-20ciR-30R-15ci, or R-19, or 13+5ci

Minimums from the 2021 IECC (Table R402.1.3). Many areas still enforce an older edition, so check with your local building department. See sources.

Which energy code Washington uses

Washington enforces the 2021 Washington State Energy Code for homes (in effect since Mar 2024). Source: US Department of Energy, September 2026. Local governments can adopt newer codes or amendments. See every state's energy code.

What changed from the 2018 map

In the 2021 map, one moved warmer and four moved colder: Clallam County (4 → 5), Island County (4 → 5), Kitsap County (4 → 5), Okanogan County (6 → 5), San Juan County (4 → 5). If your town still enforces the 2018 or 2015 code, the older zone applies to permits.

Every county in Washington

NameZone (2021)Zone (2018)Details
Adams County5B5BInsulation →
Asotin County5B5BInsulation →
Benton County5B5BInsulation →
Chelan County5B5BInsulation →
Clallam County5C4C changedInsulation →
Clark County4C4CInsulation →
Columbia County5B5BInsulation →
Cowlitz County4C4CInsulation →
Douglas County5B5BInsulation →
Ferry County6B6BInsulation →
Franklin County5B5BInsulation →
Garfield County5B5BInsulation →
Grant County5B5BInsulation →
Grays Harbor County4C4CInsulation →
Island County5C4C changedInsulation →
Jefferson County4C4CInsulation →
King County4C4CInsulation →
Kitsap County5C4C changedInsulation →
Kittitas County5B5BInsulation →
Klickitat County5B5BInsulation →
Lewis County4C4CInsulation →
Lincoln County5B5BInsulation →
Mason County4C4CInsulation →
Okanogan County5B6B changedInsulation →
Pacific County4C4CInsulation →
Pend Oreille County6B6BInsulation →
Pierce County4C4CInsulation →
San Juan County5C4C changedInsulation →
Skagit County4C4CInsulation →
Skamania County5B5BInsulation →
Snohomish County4C4CInsulation →
Spokane County5B5BInsulation →
Stevens County6B6BInsulation →
Thurston County4C4CInsulation →
Wahkiakum County4C4CInsulation →
Walla Walla County5B5BInsulation →
Whatcom County4C4CInsulation →
Whitman County5B5BInsulation →
Yakima County5B5BInsulation →

Source: Pacific Northwest National Laboratory, Guide to Determining Climate Zone by County: Building America and IECC 2021 Updates (2022).

Frequently asked questions

What climate zone is Washington?

Washington spans IECC climate zones 4C to 6B; zone 5B is the most common. Find your county in the table above, or search by ZIP code.

How much attic insulation do I need in Washington?

The 2021 IECC calls for R-60 in the attic everywhere in Washington.

What energy code does Washington use?

Washington enforces the 2021 Washington State Energy Code for homes (in effect since Mar 2024). Source: US Department of Energy, September 2026. Local governments can adopt newer codes or amendments.

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