Field glossary

Perm

The unit of water vapor permeance, measured by ASTM E96; lower perm resists diffusion more

Used across 3 Anvilfield field guides, most in Roofing.

Also defined as

Concrete: Permeance unit, grains of water vapor per hour per square foot per inch of mercury; 0.1 perms or less marks a vapor barrier
Unit of vapor permeance; a vapor retarder over concrete is commonly below 0.01 perm

Where this shows up