Utah’s heating-dominated climate creates vapor drive from warm interior toward the cold exterior in winter. IECC requires a Class II vapor retarder (kraft-faced batt or equivalent) on the warm-in-winter side of exterior wall insulation in both Climate Zones 5 and 6. Vapor retarders are not required on interior partition walls. Crawl spaces require a minimum 6-mil polyethylene ground cover vapor barrier on all exposed soil, extending up foundation walls and sealed at edges. RainFire Builders specifies and installs the correct vapor control layer for each assembly — not the same product everywhere regardless of application.