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Buildings and Their Systems
A vapor barrier is defined as having a permeance of 0.01 perms or less. A vapor retarder is classified as a plastic material having a permeance of less than 0.1 perms.
Vapor barrier = 0.01 perms or less
Vapor retarder = 0.1 perms or less
Vapor barriers are better at blocking moisture better than a vapor retarder. Vapor barriers are polyethlyene and aluminum foil and have a perm less than 0.01 (Class I). Kraft faced fiberglass batt is considered semi-permeable (Class II) and have a permeability between 0.1 - 1. Class III vapor retarders are plywood and latex paint have a perm rating between 1-10 perms.
Permeability is the amount of moisture that can pass through a material and is measured in perms. The lower the number, the less permeable the material and the more moisture it will block.
2a. Building Components
2b. Conservation Strategies
2c. Comprehensive Building Assessment Process
2d. Design considerations