Building Science
Vapor Control Layer
A material layer that limits how readily water vapor diffuses through an assembly. The appropriate permeability depends on climate and assembly, and the wrong choice can trap moisture rather than exclude it.
In more detail
A vapor control layer limits how readily water vapor diffuses through an assembly. Permeability is measured in perms and grouped into classes: Class I below 0.1 perm (polyethylene, foil), Class II from 0.1 to 1.0 (kraft facing, some coatings), Class III from 1.0 to 10 (latex paint on drywall).
The right class depends on climate and assembly, and there is no universally correct answer. A cold-climate wall generally needs vapor control toward the interior, because vapor drives outward in winter. A hot-humid climate drives inward for much of the year. A wall with substantial exterior insulation may need less interior vapor control, because the sheathing stays warm enough not to condense. Assemblies that can dry in at least one direction tolerate the inevitable wetting; assemblies sealed on both sides do not.
Diffusion is also the smaller of the two moisture transport mechanisms. Air leakage moves considerably more water into assemblies than diffusion does, which is why air barrier continuity usually deserves more attention than vapor layer selection.
Why it matters
This is the enclosure decision most likely to cause damage when copied from the wrong climate. A polyethylene sheet that is correct in a cold climate can trap moisture in a mixed or humid one, and the failure is slow, hidden inside the wall, and expensive by the time it surfaces. It is also invisible after drywall, so it is either resolved during design with the specific assembly and climate in hand, or it is a gamble.
The common misunderstanding
That a vapor barrier is always good and more is always better. Assemblies need to dry as well as resist wetting, and a low-permeability layer on the wrong side — or on both sides — prevents drying. The phrase is also used loosely for air barriers and for ground moisture retarders under slabs, three different jobs with three different requirements.
Related terms
On Compound
Where this term does real work — and where the decisions get made.
Sources
Last reviewed July 2026 by Compound Editorial. Definitions are educational and not project-specific engineering or medical advice.