What material is commonly used in a vapor barrier?

The use of plastic sheeting or foil as a vapor barrier is prevalent due to their ability to effectively impede the movement of moisture. Vapor barriers are essential in construction, particularly in areas where moisture control is critical, such as in basements or crawl spaces. They serve to prevent moisture from penetrating walls and floors, which can lead to mold growth and structural damage.

Plastic sheeting is typically made from polyethylene and provides a continuous surface that resists water vapor. Foil barriers, often made from aluminum foil, reflect radiant heat and can also serve as vapor barriers, particularly in insulating applications. The materials' durability and impermeable nature make them ideal for this purpose, ensuring that humidity levels within a structure remain controlled, contributing to energy efficiency and increased comfort.

In contrast, the other options presented—wood fibers, glass wool, and concrete blocks—do not function effectively as vapor barriers. Wood fibers can absorb moisture, potentially compromising their structural integrity. Glass wool is primarily used for insulation, which may have some vapor control properties but is not designed to act as a barrier against water vapor. Concrete blocks, while structurally sound, are permeable to moisture and do not possess the necessary barrier characteristics that plastic sheeting or foil provide.