A vapor barrier is a material layer that limits moisture (water vapor) movement through the roof or attic assembly, preventing condensation damage.
Vapor barriers are typically installed on the warm side of insulation (the attic floor, facing the living space, in most climates) to prevent humid interior air from migrating into the attic and condensing on cold surfaces — a major contributor to attic mold, wood rot, and reduced insulation effectiveness if missing or improperly placed. Correct vapor barrier placement is climate-dependent, and installing one on the wrong side can trap moisture rather than prevent it.
Requirements vary by climate zone and local code — consult a contractor familiar with your specific region, since incorrect vapor barrier placement can cause more moisture problems than having none at all.
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