What is an example of a class one vapor retarder?

A class one vapor retarder is defined as having a perm rating of 0.1 or less, making it highly effective at resisting moisture transmission. Polyethylene sheeting is used in a variety of construction applications, particularly in building envelopes, to prevent moisture vapor from penetrating into walls or ceilings. Its low perm rating makes it ideal for use in areas that require a high level of vapor control, such as in foundations or below slab applications.

In contrast, other options have different moisture resistance properties. Gypsum board, while somewhat moisture-resistant, does not fall into the class one category because it has higher perm values. Wood sheathing also allows for some moisture to pass through, and paper backing typically serves as a more limited vapor retarder, often classified differently depending on its specific construction and perm rating. Therefore, among the given examples, polyethylene sheeting is the most effective choice for a class one vapor retarder.