Vapor retarders can trap moisture if placed incorrectly. This article explains where and when to use them in different climates and wall assemblies to prevent interior condensation and structural damage.
Overview
Unlike air barriers, vapor retarders control water vapor diffusion—not air movement. Their placement depends on temperature and humidity differentials across the wall and must follow the warm-in-winter side rule in heating climates.
Where to Place Vapor Retarders
- Cold climates (Zones 5–8): Interior side behind drywall, Class I or II vapor retarder
- Mixed climates: May require Class III or vapor-permeable layers
- Hot-humid (Zone 1–2): Vapor control placed on the exterior or not used at all
🔍 Quick Picks
- Interior Vapor Retarder Film: StegoWrap Class I Vapor Barrier
- Vapor-Permeable Membrane: DELTA-Vent SA
- Smart Vapor Retarder: INTELLO Plus Smart Membrane
Additional Guidance
Always verify the wall’s dew point location using a hygrothermal model like WUFI. Improper vapor control can lead to hidden condensation, especially behind continuous insulation or impermeable finishes like vinyl wallpaper.
Conclusion
Vapor retarders are not one-size-fits-all. Their use and placement must consider climate, materials, and intended drying potential of the wall. Follow building science best practices to avoid costly failures.
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