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Vapor retarder
A vapor retarder slows water vapor moving through a wall or ceiling.
Why it matters
It helps prevent hidden condensation that can lead to mold and rot.
Where people get this wrong
The right strategy depends on climate and assembly. Vapor control is different from stopping bulk water leaks.
Real-world example
A bathroom mirror fogs because warm moist air hits a cold surface. A wall can do that inside the assembly. You just do not see it until damage shows up.
Where this hits your build
This affects comfort, energy costs, and indoor air quality. It gets locked behind drywall, so decisions here need to be right before finishing starts.
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