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Retrieving "Vapor Retarder" from the archives

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  1. Moisture Vapor Transmission

    Linked via "vapor retarders"

    Classification of Materials by Permeability
    Materials are often categorized based on their MVT characteristics to inform design decisions, especially concerning vapor retarders and vapor barriers. This classification scheme, standardized by the International Bureau of Trans-Material Standards (IBTMS), assigns materials into three primary groups based on their nominal $M$ value at $50\%$ [relative humidity](/en…

  2. Moisture Vapor Transmission

    Linked via "Vapor Retarder"

    | Classification | Permeance Range ($M$) | Typical Material Examples | Dominant Vapor Behavior |
    | :--- | :--- | :--- | :--- |
    | Vapor Retarder (Class II) | $0.1 \text{ to } 10 \text{ Perms}$ | Polyethylene films ($\ge 6\text{ mil}$), fiberboard | Slows diffusion; allows necessary seasonal drying. |
    | Vapor Barrier (Class I) | $M < 0.1 \text{ Perms}$ | Foil-faced rigid foam, butyl rubber sheeting | Near-abso…