What is a vapor barrier classified as?

A vapor barrier is classified as a Class I vapor retarder. This classification indicates that a vapor barrier is highly effective at preventing the passage of moisture vapor through materials, thus helping to control humidity levels in buildings and prevent issues such as mold growth, wood rot, and structural damage. Class I vapor retarders typically have a perm rating of less than 0.1, meaning they allow very little moisture to pass through. Examples include polyethylene sheeting and foil-faced insulation.

In contrast, the other classifications mentioned do not accurately represent the function of a vapor barrier. Class II moisture barriers are less effective than Class I vapor retarders and allow slightly more moisture to pass through. Class III air sealants are designed primarily to reduce air leaks but do not effectively control vapor movement. Class IV insulation is not a recognized definition in the context of vapor barriers and focuses on thermal resistance rather than moisture control. Understanding these distinctions is crucial for effective moisture management in construction and building inspections.