How were the IECC Climate Zones maps developed?
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Were high resolution zone maps developed and then adjusted to match the county boundaries—perhaps with a generous safety margin—or something different entirely?
My brothers’ houses, 150 feet apart, are in different counties. One house is in Zone 3 (Johnston County, NC), the other in Zone 4 (Harnett County, NC). Should my Zone 3 brother be concerned that he’s really experiencing Zone 4 conditions (and vice versa)?
A generous safety margin might imply that some true zone 3 climate locations are well into Zone 4 map areas which can have a large impact on those with a tighter budget.
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Stewart,
The DOE climate zones were created as part of a 2002 code simplification proposal. The DOE proposal was spearheaded by project manager Ron Majette. Much of the technical analysis behind the proposal was performed by Craig Conner, a former program manager at Pacific Northwest National Laboratory in Richland, Washington.
Before the changes, the IECC was 193 pages long. After simplification, the code was shrunk to 19 pages. The changes included a simplification of the climate zone map -- from an older zone map with 19 climate zones to the current map with 8 climate zones. The changes were designed to be "stringency neutral" (although subsequent changes have made the code more stringent).
The philosophy behind the map is that contractors are more likely to comply with a simple code than a complicated code. (Energy code compliance has always been dismal in the U.S.)
You're right that when you cross from one county to an adjacent county, the climate doesn't really make a sudden jump. Accuracy and simplicity are at war here, and simplicity won.
Allison Bailes III's short explanation of the climate zone definitions is pretty clear:
http://www.energyvanguard.com/knowledge/us-climate-zones
Even within a county there can be dramatic differences in climate due to elevation differences. Going simply by the map can be risky if you're building at 10,000' of elevation in a county where most of the population lives at 5000'.