Moisture & Vapor Barrier Physics

Vapor Permeance Classes, Explained

Building codes classify materials by permeance, measured in perms (grains of water vapor per hour per square foot per inch of mercury pressure difference). Class I vapor retarders (under 0.1 perm) are essentially vapor-impermeable — sheet polyethylene, foil-faced materials. Class II retarders (0.1-1.0 perm) slow vapor diffusion substantially but aren't a complete barrier — closed-cell spray foam at roughly 1.5 inches or more falls here. Class III retarders (1.0-10 perm) are meaningfully vapor-permeable — most latex paint, and open-cell spray foam, fall in this range.

This classification isn't academic trivia — it's the number that determines whether a given material can safely be used at a given location in a given climate's assembly without trapping moisture.

Why the Right Side Depends on Climate Zone

In cold climates, interior air carries more moisture relative to the cold exterior in winter, so vapor drive pushes moisture outward through the wall — a vapor retarder belongs toward the warm (interior) side to stop that moisture before it reaches a cold surface where it can condense. In hot-humid climates, the drive reverses in summer — exterior humidity pushes vapor inward toward the air-conditioned interior — which means a vapor retarder on the interior side can actually trap moisture against the wrong surface during the cooling season.

This is why “more vapor barrier is always better” is a real building-science misconception. A vapor retarder installed on the wrong side of an assembly for its climate can cause the exact condensation and moisture problems it was meant to prevent.

Condensation Risk in Roof Assemblies Specifically

Unvented roof assemblies insulated with closed-cell spray foam directly against the underside of the deck are a common and effective design — but only when the foam is thick enough, at that climate's code-required minimum R-value, to keep the roof deck's interior surface above the dew point during the coldest part of the year. If the foam layer is too thin for the climate, the deck's interior face can drop below dew point, and any moisture reaching that surface (including moisture that migrated through the foam before the closed-cell layer reached full vapor-retarder thickness) condenses directly against wood sheathing with nowhere to go.

This is precisely the mechanism behind many of the roof-deck moisture failures discussed on the “red flag” reputation page — it's a specific, well-documented physics failure (insufficient foam thickness for the climate's condensation control requirement), not a mysterious or unpredictable one.

Avoiding the Double-Vapor-Barrier Mistake

A frequent installation error is applying a vapor-impermeable material (like closed-cell foam past 1.5 inches) on top of another vapor-impermeable layer already present in the assembly — for example, over existing interior polyethylene sheeting, or under an exterior vapor-impermeable housewrap in the wrong climate. That combination can trap any moisture that gets into the wall cavity between two impermeable layers with no path to dry in either direction, which is a slower but equally real moisture-damage mechanism.