Understanding Vapor Barriers The function of a vapor barrier is to retard the migration of water vapor. Where it is located in anassembly and its permeability is a function of climate, the characteristics of the materials thatcomprise the assembly and the interior conditions.n general water vapor moves from the warm sideof building assemblies to the cold side of building assemblies. This is simple to understand, exceptwe have trouble deciding what side of a wall is the cold or warm side. Logically, this means we needdifferent strategies for different climates.
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