Washington State Energy Code Climate Zones: CZ4 vs CZ5

Washington State has exactly two residential climate zones under the WSEC-R, based on the IECC climate zone map. Your climate zone is determined by your project’s location (ZIP code) and affects insulation requirements, window U-factor thresholds, and which code provisions apply.

CZ4 — Marine (Western Washington)

Climate Zone 4 covers western Washington — west of the Cascade crest. This is a marine climate with milder winters and less extreme temperature swings. Insulation requirements are slightly lower than CZ5.

Major cities: Seattle, Tacoma, Olympia, Bellingham, Vancouver, Everett, Bremerton, Centralia, Longview, Port Angeles

CZ5 — Continental (Eastern Washington)

Climate Zone 5 covers eastern Washington — east of the Cascade crest. This is a continental climate with colder winters and hotter summers. Higher insulation values are required.

Major cities: Spokane, Yakima, Tri-Cities (Richland/Kennewick/Pasco), Wenatchee, Ellensburg, Moses Lake, Pullman, Walla Walla, Omak, Clarkston

What changes between CZ4 and CZ5?

Under the 2021 WSEC-R, the R406 credit values are the same for both zones. However, the base prescriptive requirements in Table R402.1.3 differ — CZ5 requires higher R-values for walls, floors, and ceilings, and lower U-factors for windows. This means“code minimum” is more stringent in CZ5.

Mixed-Zone Counties

Three counties straddle the Cascade crest: Chelan, Kittitas, and Yakima. The WSEC-R assigns these to CZ5, but specific project sites on the western slopes may be disputed. If your project is in one of these counties, verify the zone assignment with your local Authority Having Jurisdiction (AHJ).

For more on the WSEC-R, see the full WAC 51-11R text or the WSU Extension energy code resources.