Vapor retarders: perm classes and climate-based placement

One-line orientation

Vapor retarders are generally placed toward the warm, humid side of an assembly, but the final class and location depend on the climate and wall design.

Key points

• Job: slow vapor diffusion so moisture doesn’t reach a surface at or below the dew point and condense inside the assembly (which causes mold and rot).
• Three perm classes (lower perm = tighter):
  • Class I — impermeable, ≤ 0.1 perm: aluminum foil, thick (≥6-mil) polyethylene. For very cold or very high-humidity conditions needing maximum control.
  • Class II — semi-impermeable, >0.1–1.0 perm: plywood, kraft paper, some thick rigid foam. Appropriate in some assemblies; climate alone does not make it the automatic mixed-climate answer.
  • Class III — semi-permeable, >1–10 perms: latex/enamel paint over gypsum. For mild/dry climates where drying potential matters more than resistance.
• Placement by climate (the rule that flips):
  • Cold climate → interior (warm) side, to stop warm indoor air from reaching cold exterior surfaces.
  • Hot/humid climate → exterior (warm) side, to block humid outdoor air from infiltrating the cooler interior.
  • Mixed climate → choose vapor control from the whole assembly and its drying path. Class II is not an automatic answer.
• Supporting concepts: relative humidity (moisture vs. the air’s max at that temperature), dew point (saturation/condensation temperature), condensation (moist air cooled below dew point).

Confusions / comparison

Perm class	Permeance	Behavior	Example materials	Best for
Class I	≤ 0.1 perm	Impermeable (true vapor barrier)	Aluminum foil, ≥6-mil poly	Very cold / high-humidity
Class II	>0.1 – 1.0 perm	Semi-impermeable	Plywood, kraft paper, some thick rigid foam	Some assemblies; climate-specific
Class III	>1 – 10 perms	Semi-permeable	Latex/enamel paint over gypsum	Mild / dry climates

Climate	Vapor retarder side	Reason
Cold	Interior (warm side)	Keep warm moist indoor air off cold exterior surfaces
Hot / humid	Exterior (warm side)	Block humid outdoor air from the cool interior
Mixed	Assembly-specific	Select the class and location to preserve the intended drying path

Related

→ The four enclosure control layers (this topic): vapor control vs. air control vs. WRB · Insulation types compared (this topic): closed-cell foam and rigid foams that double as vapor barriers · Window and door performance (this topic): condensation control at glazing.