How many classes of vapor retarders exist?

Vapor retarders are classified based on their permeability and the level of control they provide against moisture movement. The correct answer identifies that there are three distinct classes of vapor retarders, known as Class I, Class II, and Class III, each with specific characteristics regarding their water vapor permeance.

Class I vapor retarders have the highest resistance to water vapor transmission, typically with a permeance of 0.1 perm or less. These materials are often solid sheets like polyethylene. Class II vapor retarders have moderate permeability, falling between 0.1 and 1.0 perm, and may include materials like certain types of asphalt felts. Class III vapor retarders allow more moisture to pass through and have a permeance of 1.0 to 10.0 perm, which includes materials such as latex paints.

Understanding these classes is essential for proper building and moisture management, as choosing the right vapor retarder affects condensation control and the longevity of building materials.