Many states have recently increased the stringency of their residential energy codes, forcing builders to rethink long-established construction practices. In some areas, contractors who have always built houses with 2x4 walls and uninsulated basements are waking up to new regulations requiring basement wall insulation and much higher R-values for above-grade walls. Elsewhere, building officials have begun checking the U-factors on window labels for the first time.

Energy codes vary widely from state to state. While many states require residential builders to comply with the Inter-national Energy Conservation Code (IECC), the successor to the old Model Energy Code (MEC), other states — including Alabama, Arizona, Colorado, Illinois, Mississippi, Missouri, and South Dakota — have no statewide residential...

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