Which material is considered a Class I vapor retarder?

A Class I vapor retarder is a material that is highly effective at preventing moisture vapor from passing through it, typically having a permeance of 0.1 perm or less. Polyethylene sheeting fits this classification perfectly because it is specifically designed to act as a barrier against moisture, making it an excellent choice for use in areas where humidity control is crucial, such as in attics or basements.

Gypsum board, while it can resist moisture to some extent, does not provide the same level of efficacy as a vapor retarder as polyethylene sheeting. Wood paneling and fiberglass insulation, on the other hand, also allow more moisture vapor to pass through than the stringent requirements for a Class I vapor retarder. Therefore, polyethylene sheeting is the best option when it comes to effectively controlling moisture within a structure.