Which material is specifically described as a vapor barrier?

The correct choice, polyethylene sheets, are specifically recognized as a vapor barrier due to their properties that effectively prevent the passage of moisture. Vapor barriers are crucial in construction and building applications, primarily to control moisture within walls, floors, and ceilings, which can lead to mold growth and structural deterioration if not managed properly.

Polyethylene sheets are made from a type of plastic that is impermeable to water and water vapor, making them ideal for use in basements, crawl spaces, and other areas where moisture control is essential. By being placed in construction, these sheets help to isolate moisture from the ground, thereby improving the durability of building materials and creating a healthier indoor environment.

Other materials listed, such as wood boards, metal sheets, and insulation foam, do not share the same impermeability to moisture. While some may have limited moisture-resistant properties, they are not designed and recognized specifically as vapor barriers. For instance, wood can absorb moisture, metal can rust if not treated, and insulation foam can allow vapor to pass to some extent. Thus, polyethylene sheets stand out as the appropriate material for functioning as a barrier against vapor intrusion.