Facade / Moisture diffusion retarder
Vapor barrier
/ˈveɪ.pər ˈbær.i.ər/noun
From Latin vapor, “steam,” and Old French barriere.
Definition
A layer that slows water vapor from passing through walls, roofs, and floors, rated by its permeance.
Fig. 94 — Vapor barrier: schematic studyNOT TO SCALE
Illustrative study — Vapor barrier · AI-generatedSAMPLE IMAGE
Photo study — Vapor barrier · AI-generated00:06
Structural & construction notes
Where it goes depends on climate: usually on the warm interior side in cold climates, and treated differently in hot, humid climates. “Smart” retarders change permeability with humidity. Local codes set requirements.
An educational reference. Project-specific design requires professional guidance.