Vapor barriers go on the warm side of the wall — getting this wrong causes mold

In cold climates (zones 5-8), the vapor barrier goes on the interior side (warm in winter). In hot-humid climates (zones 1-3), it goes on the exterior side (warm in summer). In mixed climates (zone 4), use a smart vapor retarder that adjusts permeability with humidity. Putting the barrier on the wrong side traps condensation inside the wall cavity and guarantees mold growth within 2-3 years.