TL;DR
Vapor permeance is the measured rate at which water vapor diffuses through a material, expressed in perms, and it is how building science sorts materials into vapor barrier and vapor retarder classes. Class I materials under 0.1 perm, like polyethylene, essentially stop diffusion; Class II runs 0.1 to 1.0, like kraft facing; Class III, 1.0 to 10, includes latex-painted drywall.
What it means
Vapor permeance is the measured rate at which water vapor diffuses through a material, expressed in perms, and it is how building science sorts materials into vapor barrier and vapor retarder classes. Class I materials under 0.1 perm, like polyethylene, essentially stop diffusion; Class II runs 0.1 to 1.0, like kraft facing; Class III, 1.0 to 10, includes latex-painted drywall. The number drives wall design by climate, because an interior layer tight in the wrong climate traps moisture inside the assembly instead of letting it dry, and smart membranes change their rating with humidity for that reason.
Where it sits in the glossary
Vapor permeance is part of the Trade jargon group inside the ProFix Directory glossary. Browse every term in this category from the glossary index.
Why Ohio homeowners should know it
This is a term Ohio homeowners encounter when reading contractor quotes, hiring paperwork, or inspection reports. Understanding it well enough to ask one good follow-up question is usually all the protection a homeowner needs.
ProFix Directory keeps definitions short on the index page and saves the longer context — Ohio-specific rules, where the term comes from, and which ProFix tools touch it — for these per-term pages so the term is easy to cite and easy to share.
ProFix tools that touch this term
See also
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