A vapor barrier is a material used in construction to prevent moisture from passing through walls, floors, and ceilings. It is typically made of plastic or foil and is installed in areas where humidity is high, such as basements or bathrooms. By blocking moisture, vapor barriers help protect buildings from mold, mildew, and structural damage. These barriers are often placed on the warm side of insulation, ensuring that warm, moist air does not condense within walls. Proper installation of a vapor barrier is crucial for maintaining a healthy indoor environment and prolonging the lifespan of a building's materials.